Chapter Vi. the Leaf

I. THE TYPICAL LEAF AND ITS PARTS

MATERIAL.—Leaves of different kinds showing the various modes of attachment, shapes, texture, etc. For stipules, leaves on very young twigs should be selected, as these bodies often fall away soon after the leaves expand. The rose, Japan quince, willow, strawberry, pea, pansy, and young leaves of beech, apple, elm, tulip tree, India rubber tree, magnolia, knotweed, furnish good examples of stipules. For the different orders of leaf arrangement, lilac, maple, spurge, trillium, cleavers (Galium) show the opposite and whorled kinds. Elm, basswood, grasses; alder, birch, sedges; peach, apple, cherry, show respectively for each group the three principal orders of alternate arrangement.

=165. Parts of the leaf.=—Examine a young, healthy leaf of apple, quince, or elm, as it stands upon the stem, and notice that it consists of three parts: a broad expansion called the blade; a leaf stalk or petiole that attaches it to the stem; and two little leaflike or bristle-like bodies at the base, known as stipules. Make a sketch of any leaf provided with all these parts, and label them, respectively, blade, petiole, and stipules. These three parts make up a perfect or typical leaf, but as a matter of fact, one or more of them is usually wanting.

=166. Stipules.=—The office of stipules, when present, is generally to subserve in some way the purposes of protection. In many cases, as in the fig, elm, beech, oak, magnolia, etc., they appear only as protective scales that cover the bud during winter, and fall away as soon as the leaf expands. When persistent, that is, enduring, they take various forms according to the purposes they serve. But under whatever guise they occur, their true nature may be recognized by their position on each side of the base of the petiole, and not in the axil, or angle formed by the leaf with the stem. (149.)

=167. Leaf attachment.=—The normal use of the petiole is to secure a better light exposure for the leaves, but, like other parts, it is subject to modifications, and is often wanting altogether. In this case the leaf is said to be sessile, that is, seated, on the stem, and the leaf bases are designated by various terms descriptive of their mode of attachment. The meaning of these terms, when not self-explanatory, can best be learned by a comparison of living specimens with Figs. 184-187.

=168. Arrangement of leaves on the stem.=—The mode of attachment is something quite distinct from the mode of leaf arrangement on the stem, or phyllotaxy, as it is termed by botanists. It was seen in 148 that this takes place in two different ways, the alternate and opposite. These two kinds of arrangement represent the principal forms of leaf disposition on the stem, the different varieties of each depending on the manner in which the leaves are distributed.

Where three or more occur at a node, as in the trillium and cleavers (Galium), they constitute a whorl, which is only a variant of the opposite arrangement. There is no limit to the number of leaves that may be in a whorl except the space around the stem to accommodate them.

The phyllotaxy of alternate leaves is more complicated. The different forms are characterized by the angular distance between the points of leaf insertion around the stem. In the elm, basswood, and most grasses, they are distributed in two rows or ranks on opposite sides of the stem, each just half way round the circumference from the one next in succession (Fig. 189), the third in vertical order standing directly over the first. In most of our common trees and shrubs five leaves are passed in making two turns round the stem, the sixth leaf in vertical order standing over the first. This is called the five-ranked arrangement, and is the most common order among dicotyls.

=169. Relation between the shape and arrangement of leaves.=—Phyllotaxy is of importance chiefly on account of its influence on the light relation of leaves. A compact, close-ranked arrangement tends to shut off the light from the lower nodes, and hence, in plants where it prevails, the leaves are apt to be long and narrow in proportion to the frequency of the vertical rows. The yucca, oleander, Canada fleabane and bitterweed (Helenium tenuifolium), illustrate this relation.

On the other hand, when the leaves are large and rounded in outline, as those of the sunflower, hollyhock, and catalpa, they are usually separated by longer internodes, or their blades are cut and incised so that the sunlight easily strikes through to the lower ones.

=170. Other external characteristics= to be observed in leaves are:—

(1) General Outline: whether round, oval, heart-shaped, etc. (Figs. 191-197).

(2) Margins: whether unbroken (entire), or variously toothed and indented. (Figs. 198-202.)

(3) Texture: whether thick, thin, soft, hard, fleshy, leathery, brittle.

(4) Surface: smooth, shining, dull, wrinkled, hairy, or otherwise roughened.

Not only do leaves of different kinds exhibit these characteristics in varying degrees, but young and old leaves, or those on young and old plants of the same kind, often differ from each other in color, size, shape, texture, mode of attachment, and the like, to such a degree (Figs. 203, 204) that one not familiar with them in both stages would hardly recognize them as belonging to the same species. The young leaves of eucalyptus, mulberry, and some oaks afford conspicuous examples of such differences, and they exist between the cotyledons and mature leaves of most plants.

Can you see any benefit, in the case of the plant whose leaves you are studying, that could be derived from such of the characteristics named above as they may exhibit?

Practical Questions

1. Tell the nature and use of the stipules in such of the following plants as you can find: tulip tree; fig; beech; apple; willow; pansy; garden pea; Japan quince (Pyrus Japonica); sycamore; rose; paper mulberry (Broussonetia).

2. How would you distinguish between a chinquapin, a chestnut, a chestnut oak, and a horse-chestnut tree by their leaves alone? By their bark and branches? Between a hickory, ash, common elder, box elder, ailanthus, sumach? Between beech, birch, elm, hackberry, alder?

(Any other sets of leaves may be substituted for those named, the object being merely to form the habit of distinguishing readily the differences and resemblances among those that bear some general likeness to one another.)

3. From the study of these or similar specimens, would you conclude that resemblances in leaves are confined to those of closely related kinds?

4. Name some causes independent of botanical relationship that might influence them. (169, 170; Exps. 48, 57.)

5. Do you find, as a general thing, more leaves with stipules or without?

6. Is their absence from a mature leaf always a sign that it is really exstipulate? (166.)

7. Can you trace any line of development through intervening forms from a merely sessile leaf, like that of the pimpernel or specularia, to a peltate one? (Figs. 184-187, and observation of living specimens.)

8. Does the leaf determine the position of the node, or the node the position of the leaf?

9. Strip the leaves from a twig of one order of arrangement and replace them with foliage from a twig of a different order; for instance, place basswood upon white oak, birch upon lilac, elm upon pear, honeysuckle upon barberry, etc. Is the same amount of surface exposed as in the natural order?

10. What disadvantage would it be to a plant if the leaves were arranged so that they stood directly over one another? (169.)

11. Why are the internodes of vigorous young shoots, or scions, generally so long? (150.)

12. If the upward growth of a stem or branch is stopped by pruning, what effect is produced upon the parts below, and why? (152, 153.)

13. Give some of the reasons why corn grows so small and stunted when sown broadcast for forage? (60, 63, 169.)

14. What is the use of “chopping” (i.e. thinning out) cotton?

II. THE VEINING AND LOBING OF LEAVES

MATERIAL.—Leaves of any monocotyl and dicotyl will show the difference between parallel and net-veining. To illustrate the palmate and pinnate kinds, the leaves of grasses and arums may be used for monocotyls, and for dicotyls, those of ivy, maple, grape, elm, peach, cherry, etc.; for division, examine lobed and compound leaves of as many kinds as are attainable. A specimen showing each kind of veining should be placed in coloring fluid a short time before the lesson begins. The leafstalks of celery and plantain are excellent for showing the relation between the leaf veins and vascular system of the plant.

=171. Parallel and net veining.=—Compare a leaf of the wandering Jew, lily, or any kind of grass, with one of grape, ivy, or willow. Hold each up to the light, and note the veins or little threads of woody substance that run through it. Make a drawing of each so as to show plainly the direction and manner of veining. Write under the first, parallel-veined, and under the second, net-veined. This distinction of leaves into parallel and net-veined corresponds with the two great classes into which seed-bearing plants are divided, monocotyls, as a general thing, being characterized by the first kind, and dicotyls by the second.

=172. Pinnate and palmate veining.=—Next, compare a leaf of the canna, calla lily, or any kind of arum, with one of the elm, peach, cherry, etc. What resemblances do you notice between the two? What differences? Which is parallel-veined and which is net-veined? Make a drawing of each, and compare with the first two. Notice that in leaves of this kind, the petiole is continued in a large central vein, called the midrib, from which the secondary veins branch off on either side like the pinnæ of a feather; whence such leaves are said to be pinnately, or feather veined, as in Figs. 206, 207. In the cotton, maple, ivy, etc., on the other hand, the petiole breaks up at the base of the leaf (Fig. 208) into a number of primary veins or ribs, which radiate in all directions like the fingers from the palm of the hand; hence, such a leaf is said to be palmately veined. Net-veined leaves—the plantain (Fig. 209), wild smilax, beech, dogwood—are sometimes ribbed in a way that might lead an inexperienced observer to confound them with parallel-veined ones, but the reticulations between the ribs show that they belong to the net-veined class.

=173. Veins as a mechanical support.=—Hold up a stiff, firm leaf of any kind, like the magnolia, holly, or India rubber, to the light, having first scraped away a little of the under surface, and examine it with a lens. Compare it with one of softer texture, like the peach, maple, or clover. In which are the veins the closer and stronger? Which is the more easily torn and wilted? Tear a blade of grass longitudinally and then cross-wise; in which direction does it give way the more readily? Tear apart gently a leaf of maple, or ivy, and one of elm or other pinnately veined plant; in which direction does each give way with least resistance? What would you judge from these facts as to the mechanical use of the veins?

=174. Effect upon shape.=—By comparing a number of leaves of each kind it will be seen that the feather-veined ones tend to assume elongated outlines (Figs. 197, 207); the palmate-veined ones, broad and rounded forms (Figs. 195, 208). Notice also that the straight, unbroken venation of parallel-veined leaves is generally accompanied by smooth, unbroken margins, while the irregular, open meshes of net-veined leaves are favorable to breaks and indentations.

=175. Veins as water carriers.=—Examine a leaf from a stem that has stood in red ink for an hour or two. Do you see evidence that it has absorbed any of the liquid? Cut across the blade and examine with a lens. What course has the absorbed liquid followed? What use does this indicate for the veins, besides the one already noted? Observe the point of insertion on the stem, and examine the scar with a lens: do you see any evidence of a connection between the leaf veins and the fibrovascular bundles of the stem? (111, 125, 126.) Notice where and how the veins end. Are they of the same size all the way, or do they grow smaller toward the tip? Are they separate and distinct, or are they connected throughout their ramifications, like the veins and arteries of the human body? How do you know? Do you see any of the coloring fluid in the small reticulations between the veins? How did it get there?

=176. The nature and office of veins.=—We learn from 173 and 175 that the veining serves two important purposes in the economy of the leaf: first, as a skeleton or framework, to support the expanded blade; and second, as a system of water pipes, for conveying the sap out of which its food is manufactured. In other words the veins are a continuation of the fibrovascular bundles into the leaves, by means of which the latter are put in communication with the body of the plant.

=177. The relation between veining and lobing.=—Compare the outline of a leaf of maple or ivy with one of oak or chrysanthemum. Do you perceive any correspondence between the manner of lobing or indentation of their margins, and the direction of the veins? (Figs. 210, 211.) To what class would you refer each one?

The lobes themselves may be variously cut, as in the fennel and rose geranium, thus giving rise to twice-cleft, thrice-cleft (Fig. 212), four-cleft, or even still more intricately divided blades.

=178. Compound leaves.=—Compare with the specimens just examined a leaf of horse-chestnut, clover, or Virginia creeper, and one of rose, black locust, or vetch. Notice that each of these last is made up of entirely separate divisions or leaflets, thus forming a compound leaf. Notice also that the two kinds of compound leaves correspond to the two kinds of veining and lobing, so that we have palmately and pinnately compound ones. In pinnate leaves the continuation of the common petiole along which the leaflets are ranged is called the rhachis.

Practical Questions

1. In selecting leaves for decorations that are to remain several hours without water, which of the following would you prefer, and why: smilax or Madeira vine (Boussingaultia); ivy or Virginia creeper; magnolia or maple; maidenhair or shield fern (Aspidium)? (173.)

2. Would you select very young leaves, or more mature ones, and why?

3. Can you name any parallel-veined leaves that have their margins lobed, or indented in any way?

4. Which are the more common, parallel-veined or net-veined leaves?

5. Why do the leaves of corn and other grains not shrivel lengthwise in withering, but roll inward from side to side? (173.)

6. Can you name any palmately veined leaves in which the secondary veins are pinnate? Any pinnately veined ones in which the secondary veins are palmate?

7. Lay one of each kind before you; try to draw a pinnate leaf with palmate divisions. Do you see any reason now why these so seldom occur in nature?

8. Name some advantages to a plant in having its leaves cut-lobed or compound. (169.)

9. Mention some circumstances under which it might be advantageous for a plant to have large, entire leaves. (169; Plate 9.)

10. How would the floating qualities of the leaves of the pond lily be affected if their blades were cut-lobed or compound?

11. Do the leaves of the red cedar and arbor vitæ contribute to their value as shade trees?

12. Name some of the favorite shade trees of your neighborhood; do they, as a general thing, have their leaves entire, or lobed and compound?

13. Which of the following are the best shade trees, and why: pine, white oak, mimosa (Albizzia), sycamore, locust, horse-chestnut, fir, maple, linden, China tree, cedar, ash?

14. Which would shade your porch best, and why: cypress vine, grape, gourd, morning-glory, wistaria, clematis, smilax, kidney bean, Madeira vine, rose, yellow jasmine, passion flower?

III. TRANSPIRATION

MATERIAL.—Leafy twigs of actively growing young plants. Sunflower, corn, peach, grape, calla, and arums in general transpire rapidly; thick-leaved evergreens and hairy or rough species, like mullein and horehound more slowly. For Exp. 63, small-leaved, large-leaved, and thick-leaved kinds will be needed.

APPLIANCES.—Glass jars and bottles with air-tight stoppers; a little vaseline, oil, gardener’s wax, thread, cardboard, and a pair of scales.

EXPERIMENT 62. TO SHOW WHY LEAVES WITHER.—Dry two self-sealing jars thoroughly, by holding them over a stove or a lighted lamp for a short time to prevent “sweating.” Place in one a freshly cut leafy sprig of any kind, leaving the other empty. Seal both jars and set them in the shade. Place beside them, but without covering of any kind, a twig similar to the one in the jar. Both twigs should have been cut at the same time, and their cut ends covered with wax or vaseline, to prevent access of air. Look at intervals to see if there is any moisture deposited on the inside of either jar. If there is none, set them both in a refrigerator or cover with a wet cloth and allow to cool for half an hour, and then examine again. In which jar is there a greater deposit of dew? How do you account for it? Take the twig out of the jar and compare its leaves with those of the one left outside; which have withered the more, and why?

EXPERIMENT 63. TO MEASURE THE RATE AT WHICH WATER IS GIVEN OFF BY LEAVES OF DIFFERENT KINDS.—Fill three glass vessels of the same size with water and cover with oil to prevent evaporation. Insert into one the end of a healthy twig of peach or cherry; into the second a twig of catalpa, grape, or any large-leaved plant, and into the third, one of magnolia, holly, or other thick-leaved evergreen, letting the stems of all reach well down into the water. Care must be taken to select twigs of approximately the same size and age, since the absorbent properties of very young stems are more injured by cutting and exposure than those of older ones. All specimens should be cut under water as directed in Exp. 58. Weigh all three vessels, and at the end of twenty-four hours, weigh again, taking note of the quantity of liquid that has disappeared from each glass. This will represent approximately the amount absorbed by the leaves from the twigs to replace that given off. Which twig has lost most? Which least? Note the condition of the leaves on the different twigs; have they all absorbed water about as rapidly as they have lost it? How do you know this? Pluck the leaves from each twig, one by one, lay them on a flat surface that has been previously measured off, into square inches or centimeters, and thus form a rough estimate of the area covered by each specimen. Make the best estimate you can of the number of leaves on each tree, and calculate the number of kilograms of water it would give off at that rate in a day.

EXPERIMENT 64. THROUGH WHAT PART OF THE LEAF DOES THE WATER GET OUT?—Take some healthy leaves of tulip tree, grape, tropæolum, or any large, soft kind attainable. Cover with vaseline the leafstalk and upper surface of one; the stalk and under surface of a second; the stalk and both surfaces of a third, and leave a fourth one untreated. Suspend all four in a dry place by means of a thread attached to the petioles so that both surfaces may be equally exposed. The leaves must be all of the same species, and as nearly as possible of the same age, size, and vigor, and care must be taken that none of the vaseline is rubbed off in handling. Examine at intervals of a few hours. Which of the leaves withers soonest? Which keeps fresh longest? From what part would you conclude, judging by this experiment, that the water escapes most rapidly?

=179. Transpiration, nutrition, and growth.=—We learn from the foregoing, and from Exps. 58 and 59, that plants give off moisture very much as animals do by perspiration. The two processes must not be classed together, however, for they are physiologically different. The action, in plants, is called transpiration. It is usually assumed that a large amount of water must pass through the plant in order to bring to it the necessary supply of food material; but since the entrance of mineral salts is brought about by osmosis, conditioned by the living cells of the root; and since osmosis of salts may take place in a direction opposite to that of the greater movement of water, it follows that the entrance of salts is independent of transpiration.

Inasmuch, however, as a certain amount of water is necessary to bring the living cells into a condition of turgor (7) so that they may grow, it follows that there is a relation between transpiration and growth. If transpiration exceeds absorption for any length of time, the tissues will be depleted of their moisture, as is shown by the wilting of crops in dry, hot weather; and if the unequal movement continues long enough, the plant will die. Hence, a knowledge of the laws governing this important function is necessary to all who are interested in cultivating agricultural products.

=180. Magnitude of the work of transpiration.=—Few people have any idea of the enormous quantities of water given off by leaves. It has been calculated that a healthy oak may have as many as 700,000 leaves, and that 111,225 kilograms of water—equal to about 244,700 pounds—may pass from its surface in the five active months from June to October. At this rate 226 times its own weight may pass through it in a year, and it would transpire water enough during that time to cover the ground shaded by it to a depth of 20 feet! Lawn grass gives off water at such a rate that a vacant lot of 150 × 50 feet, if well turfed, would be capable of transpiring over a ton of water a day. Compare these figures with the average yearly rainfall in our Gulf States—53 inches, approximately—and you can form some estimate of the injury done to a growing crop from this cause alone. The moisture is drawn from the surface by shallow rooted weeds (81) and dissipated through the leaves. In the case of forest trees the effect is different. Their roots, striking deep into the soil, draw up water from the lower strata and distribute it to the thirsty air in summer.

As the water given off by transpiration is in the form of vapor, it must draw from the plant the amount of heat necessary for its vaporization, and thus has the effect of making the leaves and the air in contact with them cooler than the surrounding medium. At the same time the coolness and moisture of the air tend to check the loss by evaporation from the surface soil. It is partly to this cause, and not alone to their shade, that the coolness of forests is due. Measurements at various weather bureau stations in the United States show that in summer the temperature of oak woods is 4° C. lower during the day than in the open, and as much higher at night. In a beech wood in Germany the difference between the forest and the general temperature amounted to as much as 7° C.

Practical Questions

1. Is there any foundation in fact for the accounts of “weeping trees” and “rain trees” that we sometimes read about in the papers? (180; Exp. 48.)

2. Can you explain the fact, sometimes noticed by farmers, that in wooded districts, springs which have failed or run low during a dry spell sometimes begin to flow again in autumn when the trees drop their leaves, even though there has been no rain? (180; Exp. 63.)

3. Other things being equal, which would have the cooler, pleasanter atmosphere in summer, a well-wooded region or a treeless one? (180.)

4. Could you keep a bouquet fresh by giving it plenty of fresh air? (Exp. 62.)

5. Why does a withered leaf become soft and flabby, and a dried one hard and brittle? (7; Exp. 62.)

6. Why do large-leaved plants, as a general thing, wither more quickly than those with small leaves? (Exp. 63.)

7. Is the amount of water absorbed always a correct indication of the amount transpired? Explain. (179.)

8. Explain the difference between the withering caused by excessive transpiration and the shrinkage of cells due to plasmolysis. Are both of these physiological processes?

9. Why is it best to trim a tree close when it is transplanted? (179, 180.)

10. Why should transplanting be done in winter or very early spring, before the leaves appear? (180.)

IV. ANATOMY OF THE LEAF

MATERIAL.—For study of the epidermis, leaves of the white garden lily (Lilium album) are best, as the stomata can be seen on their lower surface with the naked eye. Wandering Jew, Spanish bayonet (Yucca aloifolia), anemone, narcissus, iris, canna, show them under a hand lens, but less distinctly. For sections, beet, mustard, and beech leaves may be used, or ready-mounted specimens obtained of a dealer.

A compound microscope is needed for a minute study of the leaf structure.

=181. Stomata.=—It was shown in Exp. 64 that the water of transpiration escapes most rapidly, as a general thing, from the under surface of leaves. To find out why this is so, a careful study of the epidermis will be necessary. For this purpose procure, if possible, the leaf of a white garden lily (Lilium album), wandering Jew, Spanish bayonet (Yucca aloifolia), anemone, narcissus, iris, or canna. The first-named is preferable, as the transpiration pores can be seen on it with the naked eye. Examine the under surface with a hand lens, and you will see that it is covered with small eye-shaped dots like those shown in Figs. 218 and 219. Strip off a portion of the epidermis, hold it up to the light on a piece of moistened glass, and they can be seen quite clearly with a lens. These dots are the pores through which the water vapor escapes in transpiration, and through which air finds its way into the tissues of the leaf. They are called stomata (sing., stoma), from a Greek word meaning “a mouth.” Look for stomata on the upper epidermis; do you find any, and if so, are there as many as on the under surface? Do you see any relation between this fact and the results obtained from Exp. 64? Can you see any good reasons why the stomata should be placed on the under side in preference to the upper? Are they as much exposed to excessive light and heat, or as liable to be choked by dust, rain, and dew here as on the upper side?

=182. Distribution of stomata.=—While stomata are generally more abundant on the under side of leaves, this is not always the case. In vertical leaves, like those of the iris, which have both sides equally exposed to the sun, they are distributed equally on both sides. In plants like the water lily, where the under surface lies upon the water, they occur only on the upper side. Succulent leaves, as a general thing, have very few, because they need to conserve all their moisture. Submerged leaves have none at all; why?

=183. Minute study of a leaf epidermis.=—Place a bit of the lower epidermis of a leaf under the microscope, and examine with a high power. It will appear, if a monocotyl, to be composed of long, flat, rectangular spaces (Fig. 221); if the leaf of a dicotyl is used, they will be more or less irregular (Fig. 220), with the outlines fitting into each other like the tiling of a floor or the blocks of a Chinese puzzle. These spaces are the cells of the epidermis, and the lines are the cell walls. Can you find any of the cell contents? The cell sap is not often visible; do you see the nuclei? Can you give a reason why the epidermal cells are so thin and flat? Between some of the cells you will see two kidney-shaped bodies placed with their concave sides together so as to leave a lenticular opening between them. This is a stoma, and the kidney-shaped bodies (Figs. 218, 219) are guard cells. They are given this name because they open or close the mouth of the stoma. If you will imagine a toy balloon made in the form of a hollow ring, like the tire of a bicycle, you can easily see, from Figs. 218, 219, that when the ring is strongly inflated, it will expand, and in enlarging its own circumference, will at the same time increase the diameter of the opening in the center. When the expansive force is removed, it collapses, thus closing, or greatly reducing, the aperture.

In the same way the guard cells, when there is abundance of water in them, expand, thus opening the stoma so that the water vapor passes out more readily. But when there is a dearth of moisture, or when, by reason of chemical action in the soil, the roots fail to supply it, the leaves wilt, the guard cells, losing their water, collapse, closing the pore, and transpiration is thus prevented or greatly retarded. (Fig. 222.)

Sketch a portion of the epidermis as it appears under the microscope, labeling the parts. If stomata can be found in both conditions, make sketches showing them both open and closed.

=184. Internal structure of a leaf.=—Roll a leaf blade, or fold it tightly to facilitate cutting, and with a scalpel, or a very sharp razor, cut the thinnest possible slice through the roll. This will give a section at right angles to the epidermis. It should be so thin as to appear almost transparent. Put a small bit of a section in a drop of water on a slide, place under the microscope, using a high power, and look for the parts shown in Fig. 223. Notice the horizontally flattened cells of the upper epidermis, e, and of the lower epidermis, e′; also the vertically elongated palisade cells, p, filled with particles of green coloring matter. These particles are the chlorophyll bodies, to which the green color of the leaf is due. They are the active agents in the manufacture of plant food, and in a leaf removed from the plant during the day time and viewed under a high power, the chlorophyll bodies, on treatment with iodine, will be seen to contain granules of starch which they are in the act of elaborating. The collecting cells, t, receive the assimilated product from the palisade cells and pass it on through the spongy parenchyma, sch, to the fibrovascular bundles. Notice how much more abundant the green matter is in the upper part of the leaf than in the lower; has this anything to do with the deeper color of the upper surfaces of leaves? Notice the opening, st, lower epidermis; do you recognize it? (See Fig. 222.) It is a stoma, seen in vertical section. Notice the intercellular air spaces, i, i, in the spongy parenchyma, and the much larger one, a, just behind the stoma. Why is this last so much larger?

Sketch the section of your specimen as it appears under the microscope. It will perhaps differ in some details from the one shown in the figure, but you can recognize and label the corresponding parts. Be sure that your drawing represents accurately the relative size and shapes of the different kinds of cells.

It is in the upper surface, where the chlorophyll particles abound, that the manufacture of food goes on most actively, and from the under surface, where the stomata are situated, that transpiration takes place and air and other gases pass to and from the interior. These facts have important bearings on the growth and external characters of leaves.

Practical Questions

1. Explain why a plant cannot thrive if its stomata are clogged with foreign matter. (179; Exp. 64; 184.)

2. Mention some of the ways in which this might happen. (181.)

3. Why must the leaves of house plants be washed occasionally to keep them healthy? (179, 181.)

4. Why is it so hard for trees and hedges to remain healthy in a large manufacturing town?

V. FOOD MAKING

MATERIAL.—A sprig of pondweed, mare’s-tail (Hippuris), hornwort (Ceratophyllum), marsh St.-John’s-wort (Elodea), or other green aquatic plant; bean or tropæolum, or other green leaves gathered from plants growing in the sunshine; a healthy potted plant; a small, fresh cutting.

APPLIANCES.—A shallow dish of water and two glass tumblers or wide-mouthed jars; a bent glass or rubber tube; a piece of black cloth or paper; a half pint of alcohol; iodine solution; a glass funnel or a long-necked bottle from which the bottom has been removed.

EXPERIMENT 65. IS THERE ANY RELATION BETWEEN SUNLIGHT AND THE GREEN COLOR OF LEAVES?—Place a seedling of oats, or other rapidly growing shoot, in the dark for a few days, and note its loss of color. Leave it in the dark indefinitely, and it will lose all color and die. Hence we may conclude that there is some intimate connection between the action of light and the green coloring matter of leaves.

EXPERIMENT 66. DO LEAVES GIVE OFF ANYTHING ELSE BESIDES WATER?—Submerge a green water plant, with the cut end uppermost, in a glass vessel full of water, and invert over it a glass funnel, or a long-necked bottle from which the bottom has been removed as directed in Exp. 53. Expel the air from the neck of the funnel—or bottle—by submerging and corking under water so as to make it air-tight. Place in the sunlight and notice the bubbles that begin to rise from the cut end of the plant. When they have partly filled the neck of the funnel, remove the stopper and thrust in a glowing splinter. If it bursts into flame, or glows more brightly, what is the gas that was given off? (Exp. 22.)

As oxygen is not a product of respiration, some other process must be at work here, during which oxygen is set free, and some other substance used up. (Exps. 24 and 25.)

EXPERIMENT 67. WHAT IS THE SUBSTANCE TAKEN IN WHEN OXYGEN IS GIVEN OFF?—Fill two glass jars, or two tumblers, with water, to expel the air, and invert in a shallow dish of water, having first introduced a freshly cut sprig of some healthy green plant into one of them. Then, by means of a bent tube, blow into the mouth of each tumbler till all the water is expelled by the impure air from the lungs. Set the dish in the sunshine and leave it, taking care that the end of the cutting is in the water of the dish. After forty-eight hours remove the tumblers by running under the mouth of each, before lifting from the dish, a piece of glass well coated with vaseline (lard will answer), and pressing it down tight so that no air can enter. Place the tumblers in an upright position, keeping them securely covered. Fasten a lighted taper or match to the end of a wire, plunge it quickly first into one tumbler, then into the other, and note the result. What was the gas blown from your lungs into the jars? (Exps. 23, 24.) Why did the taper not go out in the second jar? What had become of the carbon dioxide?

EXPERIMENT 68. TO SHOW THAT LIGHT IS NECESSARY FOR A PLANT TO ABSORB CARBON DIOXIDE AND GIVE OFF OXYGEN.—Repeat Exp. 66, keeping the plant in a dark or shady place; do you see any bubbles? Test with a glowing match; is any oxygen formed in the tube of the funnel? Move back into the sunlight and leave for a few hours; what happens when you thrust a glowing splinter into the tube?

EXPERIMENT 69. IS ANY FOOD PRODUCT FOUND IN LEAVES?—Crush a few leaves of bean, sunflower, or tropæolum, and soak in alcohol until all the chlorophyll is dissolved out. Rinse them in water, and soak the leaves thus treated in a weak solution of iodine for a few minutes, then wash them and hold them up to the light. If there are any blue spots on the leaves, what are you to conclude? If a test for sugar is to be made, use sap pressed from fresh leaves; for oils and fats, leaves should be dried without being placed in alcohol.

EXPERIMENT 70. HAS THE PRESENCE OR ABSENCE OF LIGHT ANYTHING TO DO WITH THE OCCURRENCE OF STARCH IN LEAVES?—Exclude the light from parts of healthy leaves on a growing plant of tropæolum, bean, etc., by placing patches of black cloth or paper over them. Leave in a bright window, or preferably out of doors, for several hours, and then test for starch as in the last experiment; do you find any in the shaded spots?

EXPERIMENT 71. IS THE PRESENCE OF AIR NECESSARY FOR THE PRODUCTION OF STARCH?—Cover the blades and the petioles of several leaves with vaseline or other oily substance so as to exclude the air, and after a day or two test as before.

=185. Influence of plants on the atmosphere.=—These experiments show that leaves cannot do their work without light and air. The particular element of the atmosphere used by them in the process of food making is carbon dioxide. Their action in absorbing this gas and giving off oxygen tends to counterbalance the opposite action of respiration, decomposition, and combustion of all kinds, by which the proportion of it in the atmosphere tends to be constantly increased. In this way they help to regulate the quantity of it present and have a beneficial effect in ridding the air of one source of impurity.

=186. Photosynthesis.=—In our examination of the internal structure of the leaf, the chlorophyll bodies (184) were found to contain small granules of starch which the chlorophyll, under the stimulus of light, had elaborated as a nutriment for the plant tissues. Hence, the leaf may be regarded as a factory in which vegetable food, mainly starch, is manufactured out of the water brought up from the soil, and the carbon dioxide derived through the stomata from the atmosphere. In this process carbon dioxide (CO{2}) is combined with water (H{2}O) in such proportions that part of the oxygen is returned to the surrounding air. This is a fundamental food-forming process characteristic of green plants, and can take place only in the light. For this reason it has been named Photosynthesis, a word which means “building up by means of light,” just as photography means “drawing or engraving by means of light.”

In carrying on the operation of photosynthesis, sunshine is the power, the chlorophyll bodies the working machinery, carbon dioxide and water the raw materials, and starch or oil the finished product, while oxygen and the water of transpiration represent the waste or by-products.

=187. How the new combination is effected.=—It may seem strange that a gas and a liquid should combine to make something so different from either as starch, but their chemical constituents are the same in different proportions. Water is made up of 2 parts hydrogen and 1 part oxygen; carbon dioxide, of 1 part carbon and 2 parts oxygen, while starch contains carbon, hydrogen, and oxygen, in the ratios of 6, 10, and 5, respectively. Hence, by taking sufficient quantities of water and carbon dioxide and combining them in the proper proportions, the leaf factory can turn them into starch. If we use the letters C, H, and O, to represent Carbon, Hydrogen, and Oxygen, respectively, the new combination of materials can be expressed by an equation; thus:—

water carbon dioxide starch by-products 5(H{2}O) + 6(CO{2}) = (C{6}H{10}O{5}) + 6(O{2}) = 12(O).

The water not used up in the process is given off as a waste product in transpiration, while the oxygen is returned to the air, as shown by Exp. 66. This equation is not to be understood as representing the chemical changes that actually take place in the leaf. These are too complicated, and at present too imperfectly known, to be considered here. It will serve, however, to give a fair idea of the final result from the process of photosynthesis, however brought about.

Simple as the operation appears, the chemist has not, as yet, been able to imitate it. He can analyze starch into its original constituents, but while he has the ingredients at hand in abundance, and knows the exact proportions of their combination, it is beyond his power, in the present state of our knowledge, to put them together. Hence, both man and the lower animals are dependent on plants for this most important food element. The so-called factories that supply the starch of commerce do not make starch any more than the miller makes wheat, but merely separate and render available for use that already elaborated by plants.

=188. Proteins.=—Foods of this class are mainly instrumental in furnishing material for the growth and repair of the tissues out of which the bodies of both plants and animals are built up. They embrace a great variety of substances, but their chemical nature is very complex and very imperfectly understood. Nitrogen is an important element in their composition, whence they are commonly distinguished as “nitrogenous foods.” Besides nitrogen, there are present carbon, hydrogen, oxygen, and sulphur, and traces of the mineral salts absorbed from the soil are found in varying quantities in the ash of different proteins. The percentages in which these ingredients are combined and the processes concerned in their formation are at present a matter of pure hypothesis. Botanists are not agreed even as to whether they are made in the leaf or in some other part or parts of the plant, though the weight of opinion inclines to the view that their construction takes place in the leaf.

=189. The activities of leaves.=—As there are only 4 parts of CO{2} to every 10,000 parts of ordinary free air, it has been estimated that in order to supply the leaf factory with the raw material it needs, an active leaf surface of one square meter—a little over one square yard—uses up, during every hour of sunshine, the CO{2} contained in 1000 liters (1000 quarts, approximately) of air. Suppose an oak tree to bear 500,000 leaves, each having a surface of 16 sq. cm., or 4 sq. in., and working 12 hours a day for 6 months in the year; you will then have some idea of the enormous quantity of air that passes each season through its leaf system. Add to this the almost incredible volume of water transpired in the same time (180), and we may well stand amazed at the tremendous activities of these silent workers that we are in the habit of regarding as mere passive elements in the general landscape.

=190. The economic value of leaves.=—Besides their importance as sanitary and food-making agencies, leaves have a direct commercial value as food products in the hay and fodder they supply for our domestic animals, the tea and salads with which they provide our tables, the aromatic flavors and seasonings contained in them, and the drugs, medicines, and dyes of various kinds for which they furnish the ingredients.

Practical Questions

1. Why do gardeners “bank” celery? (Exp. 65.)

2. Why are the buds that sprout on potatoes in the cellar, white? (Exp. 65.)

3. Why does young cotton look pale and sickly in long-continued wet or cloudy weather? (Exp. 65.)

4. Why do parasitic plants generally have either no leaves or very small, scalelike ones? (85, 186, 187.)

5. The mistletoe is an exception to this; explain why, in the light of your answer to question 4.

6. Could an ordinary nonparasitic plant live without green leaves? (186, 187.)

7. Are abundance and color of foliage any indication of the health of a plant? (186, 187; Exp. 65.)

8. Is the practice of lopping and pruning very closely, as in the process called “pollarding,” beneficial to a tree under ordinary conditions? (186, 189; Exp. 63.)

9. Name some plants of your neighborhood that grow well in the shade.

10. Compare in this respect Bermuda grass and Kentucky blue grass; cotton and maize; horse nettle (Solanun Carolinense) and dandelion; beech, oak, red maple, dogwood, pine, cedar, holly, magnolia, etc.

11. Name all the aromatic leaves you can think of; all that are used as food, beverages, drugs, and dyes.

12. What is the use of aromatic and medicinal leaves to the plant itself? (Suggestion: Why does the housewife put lavender or tobacco leaves in her woolen chest?)

13. Which would be richer in nourishment, hay cut in the evening or in the morning, and why? (54, 186; Exp. 70.)

14. Mention three important sanitary services that are rendered by a tree like that shown in plate 6 or 8. (180, 185, 189.)

15. Name some of the plants employed in the manufacture of starch.

VI. THE LEAF AN ORGAN OF RESPIRATION

MATERIAL.—A number of vigorous, freshly cut green leaves; a liter or two (one or two quarts) of expanding flower or leaf buds.

APPLIANCES.—Some wide-mouthed jars of one or two liters’ capacity; two small open vials of limewater.

EXPERIMENT 72. DO LEAVES GIVE OFF CARBON DIOXIDE?—Cover the bottoms of two wide-mouthed jars with water about two centimeters (1 inch) deep. Place in one a number of healthy green leaves with their stalks in the water, and insert among them a small open vial containing limewater. In the other jar place only a vial of limewater in the clear water at the bottom, this last being merely to make the conditions in both vessels the same. Seal both tight and keep together in the dark for about 48 hours, and then examine. In which jar does the limewater indicate the greater accumulation of CO_{2}? (It may show a slight milkiness in the other vessel due to gas derived from the inclosed air and water.) From this experiment, what process would you conclude has been going on among the leaves in jar No. 1? (Exp. 25.)

EXPERIMENT 73. IS THE EXHALATION OF CARBON DIOXIDE ACCOMPANIED BY ANY OTHER CONCOMITANT OF RESPIRATION?—In Exps. 24, 25, it was shown that respiration is accompanied by heat; hence, if the production of carbon dioxide by the leaf is due to this cause, it should be attended by the evolution of heat. To find out whether this is the case, partly fill a glass jar of two liters’ capacity with unfolding leaf buds arranged in layers alternating with damp cotton batting or blotting paper (Fig. 228); close the jar tightly and leave from 12 to 24 hours in the dark to prevent the action of photosynthesis. Then insert a thermometer and note the rise in temperature. If a lighted taper is plunged in, it will quickly be extinguished, showing that respiration has been going on.

=191. Respiration in leaves.=—We see from experiments like the foregoing that the leaf, besides carrying on the functions of digestion, photosynthesis, and transpiration, is also an active agent in the work of respiration. In this function oxygen is used up and carbon dioxide given off, just as in the respiration of animals; but the process is so slow in plants that it is much more difficult to detect than the contrary action in photosynthesis, and is, in fact, not perceptible at all while the latter is going on, though it does not cease even then.

But while the leaf is the principal organ of respiration, the process is carried on in other parts of the plant as well, else it could not survive during the leafless months of winter. It appears to be most active at night, but this is only because it is not obscured then, as during the day, by the more active function of photosynthesis. Indeed, it was for a long time supposed that plants “breathed” only at night, and it was thought to be unwholesome to keep them in a bedroom. It is now known, however, that respiration goes on at all times and in all living parts of the plant, but the quantity of oxygen taken in is so small from a hygienic point of view that it may be disregarded.

=192. Distinctions between respiration and photosynthesis.=—While these two functions are contrasting and antipodal, so to speak, in their action, they are mutually complementary and interdependent, the one manufacturing food and the other using it up, or rather marking the activity of those life processes by which it is used up. The difference between them will be made clear by a comparison of the two processes as summarized in the following statement:

PHOTOSYNTHESIS RESPIRATION

Goes on only in sunlight and in Goes on at all times and in all the green parts of plants. parts of the plant.

Produces starch and sugar. Releases energy (heat and working power).

Gives off, as by-product, oxygen. Gives off, as by-products, CO₂ and water.

A constructive process, in which A destructive, or consumptive energy is used up to make food. process, in which food is used up in expending energy.

=193. Metabolism.=—The total of all the life processes of plants, including growth, waste, repair, etc., is summed up under the general term metabolism. It is a constructive or building-up process when it results in the making of new tissues out of food material absorbed from the earth and air, and the consequent increase of the plant in size or numbers. But, as in the case of animals, so with plants, not all the food provided is converted into new tissue, part being used as a source of energy, and part decomposed and excreted as waste. In this sense, metabolism is said to be destructive. The waste in healthy growing plants is always, of course, less than the gain, and a portion of the food material is laid by as a reserve store. For this reason, photosynthesis, being a constructive process, is usually more energetic than respiration, which is the measure of the destructive change of materials that attends all life processes.

It is evident also, from what has been said, that growth and repair of tissues can take place only so long as the plant has sufficient oxygen for respiration, since the energy liberated by it is necessary for the assimilation of nourishment by the tissues.

Thus we see that plants are dependent on air not only for respiration, but for nutrition, and none of their life processes can be carried on without it.

Practical Questions

1. Can a plant be suffocated, and if so, in what ways? (87, 193; Exps. 26, 27.)

2. The roots on the palm shown in plate 3 are not drawing any sap from it as parasites; why does their continued growth bring about the death of the tree? (87, 193.)

3. Is it unwholesome to keep flowering plants in a bedroom? Leafy ones? Why, in each case? (191.)

4. Would there be any more reason for objecting to the presence of flowers by night than by day? Explain. (191.)

5. Why is respiration much less marked in plants than in animals? (30, 31.)

VII. THE ADJUSTMENT OF LEAVES TO EXTERNAL RELATIONS

MATERIAL.—A potted plant of oxalis, spotted medick, white clover, or other sensitive species. The subject is better suited for outdoor observation than for laboratory work.

EXPERIMENT 74. TO SHOW THAT LEAVES ADJUST THEMSELVES TO CHANGES IN INTENSITY OF LIGHT.—Keep a healthy potted plant of oxalis, white clover, or spotted medick in your room for observation. Note the daily changes of position the leaves undergo. Sketch one as it appears at night and in the morning.

In order to determine whether these changes are due to want of light or of warmth, put your plant in a dark closet in the middle of the day, without change of temperature. After several hours note results. Transfer to a refrigerator, or in winter place outside a window where it will be exposed to a temperature of about 5° C. (40° F.) for several hours, and see if any change takes place. Next put it at night in a well-lighted room and note the effect. If practicable, keep a specimen for several weeks in some place where electric lights are burning continuously all night, and watch the results.

EXPERIMENT 75. TO SHOW THAT THE FALL OF THE LEAF MAY RESULT FROM OTHER CAUSES THAN COLD OR FROST.—Wrap some leaves of ailanthus, Kentucky coffee tree, ash, walnut, or hickory in a damp towel and keep them in the dark for several days; the leaflets will fall away, leaving a clear scar like those on winter twigs.

EXPERIMENT 76. TO SHOW THAT ADJUSTMENTS TO TEMPERATURE MAY BE MADE BY CHEMICAL MEANS.—Place a small twig of oleander, laurestinus, or other broad-leaved evergreen in a 5 to 10 per cent solution of sugar, and transfer it at the end of a few days to a temperature of 6° to 8° below freezing. On comparison with a similar twig that has stood for the same length of time in pure water, it will be found to possess a greater power of resistance to cold.

=194. The light relation.=—The principal external conditions to which leaves have to adjust themselves are light, air, moisture, gravity, temperature, and the attacks of animals. From the knowledge of their work and function gained in the preceding sections, it will be clear that the primary relation of the leaf is a light relation, and to this, first of all, it must adjust itself.

It was shown in Exps. 56 and 57 how promptly leaves respond to changes in the direction of light, and a little observation (Exp. 74) will convince us that they are equally sensitive to changes in intensity and periodicity of illumination.

=195. Phototropism.=—The movement of plants in response to light is called phototropism—a word that means “turning toward or away from light.” It includes all kinds of light adjustments, and examples of it are to be met with everywhere in the disposition of leaves with reference to their light exposure.

=196. Horizontal and vertical adjustment.=—Take two sprigs, one upright, the other horizontal, from any convenient shrub or tree—and notice the difference in the position of the leaves. Examine their points of attachment and see how this is brought about, whether by a twist of the petiole or of the base of the leaf blades, or by a half twist of the stem between two consecutive leaves, or by some other means.

(From Mo. Botanical Garden Rep’t.)]

Observe both branches in their natural position; what part of the leaf is turned upward, the edge or the surface of the blade? Change the position of the two sprigs, placing the vertically growing one horizontal, and the horizontal one vertical. What part of the leaves is turned upward in each?

=197. Leaf mosaics.=—Trees with horizontal or drooping branches, like the elm and beech, and vines growing along walls or trailing on the ground, generally display their foliage in flat, spreading layers, each leaf fitting in between the interstices of the others like the stones in a mosaic, whence this has been called the mosaic arrangement. (Plate 10.) In plants of more upright or bunchy habit, the leaves are placed at all angles, giving the appearance of a rosette when viewed from above, whence this is called the rosette arrangement.

A variety of the same disposition is seen in the pyramidal shape assumed by plants with large, undivided leaves like the mullein and burdock (Fig. 237), in which access of light is secured by a mutual adjustment between the size and position of leaves, the upper ones becoming successively smaller.

=198. Heliotropism=—“turning with the sun”—is the name given to the daily movement of plants like the cotton and sunflower in turning their leaves or their blossoms to face the sun. If you live where cotton is grown, notice the leaves in a field about ten o’clock on a bright sunny morning, and again from the same point of view at about four or five in the afternoon. Do you perceive any difference in their general disposition? Watch on a cloudy day and see if any change takes place. Find out by observation whether the “heliotrope” of the hothouses is really heliotropic.

=199. Adjustment against too great intensity of light.=—Plants frequently have to protect themselves against excess of light and heat. An interesting example of this kind of adjustment is furnished by the rosinweed, or compass plant (Silphium laciniatum, Figs. 238, 239), which grows in the prairies of Alabama and westward, where it is exposed to intense sunlight. The leaves not only stand vertical, but have a tendency to turn their edges north and south so that the blades are exposed only to the gentler morning and evening rays. The prickly lettuce manifests the same habit in a less marked degree.

=200. Night and day adjustments.=—These are movements in response to changes in the degree of illumination and temperature, as evidenced by the fact that they become feeble and soon cease altogether if the plant is kept a sufficient time under uniform conditions as to these two factors. (Exp. 74.) They are called “nyctitropic” or sleep movements, because they are most obvious in certain plants that undergo periodic adjustments to the alternations of day and night suggestive of an imaginary likeness to the sleep of animals. Examples are most frequently met with among members of the pea family (Leguminosæ), the spurges (Euphorbiaceæ), and the sorrel (Oxalis) family. They are found among other species also, and indeed are much more general than is usually supposed, most plants showing signs of them if carefully tested. A simple way of doing this is by attaching bristles about two inches long to the tips of two leaves on opposite sides of the stem, as in Figs. 240, 241, and comparing the divergence of the bristles during the day and at nightfall. In this way a change of position in the leaves, too slight to attract attention otherwise, will be made apparent. The positions assumed vary in different plants, and even in the parts of the same compound leaf; in the kidney bean, for instance, the common petiole turns up at night, while the individual leaflets turn down. One of the common pigweeds (Amaranthus Palmeri, Figs. 242-244) is heliotropic in the day time and nyctitropic at night.

The very striking nyctitropic adjustments of the wild senna (Cassia tora) photographed by Professor Francis E. Lloyd of the Alabama Polytechnic Institute (Figs. 245-250), though obviously influenced by the sun, are not directed toward it as in those of truly heliotropic plants.

These movements are common also among flowers, many of them having regular hours for opening and closing, as indicated by such names as “morning-glory” and “four-o’clock.” In these cases, however, other causes (277, 280) than the light relation must be taken into account.

=201. Irritability= is a general term applied to the power in plants of receiving and responding by spontaneous movements to impressions from without. In its widest acceptation, irritability includes, besides the various forms of adjustment described in this section and the next, all movements due to geotropism, those of roots seeking air and moisture, the revolution of twining stems and tendrils, the circulation of protoplasm in the cell—any movement, in short, that is made in response to an impression from the environment is a manifestation of irritability. It may be of various degrees, but is possessed to some extent by every living vegetable organism.

The term is usually applied, however, more especially to those obvious and pronounced responses made by plants to their surroundings, as exemplified in the cases just given. Still more marked instances are to be found in the movements of the tentacles of insectivorous plants, and the sensitive leaflets of the mimosa that close at the slightest touch. The tendrils of the passion flower are said to appreciate and respond to a pressure that cannot be distinguished even by the human tongue, and many plants will detect and respond to the ultra-violet rays of light, which are entirely invisible to man.

This faculty of irritability among plants corresponds, in an imperfect, rudimentary way, to what we recognize in animals as nervous excitability. By this it is not meant to imply that the two things are identical in their ultimate manifestations, though we may regard them as fundamentally the same in that they are both to be referred to the property inherent in protoplasm of responding to stimuli. There is no indication, however, that irritability in the vegetable kingdom is accompanied by anything like consciousness or volition, or that plants possess any power of initiative. While the movements in response to stimuli are in many cases eminently adapted to a purpose, we have no evidence of a controlling power behind them. The movement comes automatically in response to the stimulus, whether the effect at the moment be advantageous or the reverse.

=202. Adjustments in relation to moisture.=—These adjustments may be—(1) To guard against excess of moisture; e.g. glands for excreting water and salts; scales, wax, down, etc., on the surface of leaves. These may serve also for protection against cold, insects, excess of light and heat. (2) For the conservation of moisture; e.g. the revolute leaf margins of grasses and sand plants growing along the seashore; the fleshy leaves of stonecrops and purselanes; the hard epidermis of yuccas and aloes; the scales, scurf, and down, by which the moisture absorbed from the soil by plants growing in dry and barren places is prevented from escaping too rapidly through the stomata; the leaf cups and holders sometimes formed by winged petioles and clasping leaf bases for retaining dew or rain water. (3) For leaf drainage, or the conduction of moisture, by means of grooves, channels, and taper-pointed leaves, which act as natural gutters and drain pipes.

=203. The fall of the leaf.=—This is, in effect, an adjustment to change of temperature, but that it is not directly due to cold is shown by Exp. 75, and also by the fact that leaves in the tropics and those of evergreens, while they do not fall at stated periods like the bulk of the foliage in the temperate zones, are cut off just the same and replaced by new ones, whenever, for any reason, they are unable to perform their function. In cold climates they fall at the approach of winter, not because the frost loosens them, but because the roots are not able to absorb enough moisture to supply them with material for making food. The needles and the scale-leaves characteristic of evergreens in cold regions are enabled to persist indefinitely by reason of their contracted surface. This prevents the dissipation of moisture and affords no lodging for the accumulations of sleet and snow that would otherwise cumber and perhaps break the boughs with their weight. Trees and shrubs that shed their leaves in winter are said to be deciduous, from a Latin word meaning “to fall.” Can you mention some advantages of the deciduous habit to a plant with broad, expanded leaves, growing in a cold climate?

The mechanical means by which the leaf fall is accomplished is through the growth of a corky layer of loose cells that forms at the base of the petiole and cuts it away from the stem, leaving a smooth, clean scar. Tear some fresh young leaves from a growing twig and compare the scars with those on a winter bough. Do you see any difference? This corky layer can be made to form in some plants artificially, by depriving them of working material. (Exp. 75.)

=204. The protection of wintergreen leaves.=—A great many, perhaps the majority of broad-leaved evergreens, bear no obvious protection against cold, while a large proportion, such as chickweed, violet, fumitory, groundsel (Senecio), and dead nettle (Lamium), would seem peculiarly unfitted, by their delicate structure, to withstand it. But recent investigations by the Swedish botanist, Lidforss, have shown that all wintergreen leaves, with the exception of those on submerged water plants, which are sufficiently protected by the medium in which they live, lose their starch in winter and contain instead an increased percentage of sugar. The same is true of other vegetable structures also, where starch is present, such as roots, stems, tubers, and winter fruits—nuts, haws, persimmons, and the like, which, as every schoolboy knows, become perceptibly sweeter after frost.

The presence of certain substances, of which sugar is the most frequent, enables plants to withstand a greater degree of cold than they could otherwise endure (Exp. 76). This effect, as shown by Lidforss’s experiments, is due to the action of sugar in counteracting, or retarding, the “salting out” of proteins by cold, as explained in 33.

As sugar is readily reconverted into starch by exposure to a moderately high temperature for even a few days, we may find here an explanation of the fact that plants which have survived the prolonged cold of winter are often killed by a single sharp night frost following a few warm days in early spring, before the tender new growth has appeared. The plant suffers, not from the direct effects of cold, but from the warmth preceding it, which stimulated the transformation into starch of the sugar that would have prevented the loss of proteins. On the same principle we may account for the puzzling fact that the sunny southern side of trees and shrubs usually suffers more from the effects of sudden frost than the shaded and colder northern face.

In apparent conflict with this reasoning is the fact that sugar cane and the sugar beet are peculiarly susceptible to cold. This, however, does not invalidate the premises established by Lidforss’s researches, but merely emphasizes the need of further investigation, which may either reconcile all the facts, or modify their interpretation.

=205. The colors of autumn leaves.=—These are due to the breaking up and disappearance of the chlorophyll when the leaf factory has to “shut down” for want of raw material to work with (203). It is closely connected with the appearance of frost, since the same changes of temperature which produce frost cause the cessation of sap flow that brings about the disorganization of the chlorophyll and the formation of various pigments derived from it. Besides these, leaves may contain other coloring matters that are perceptible only when the chlorophyll disappears; and in the sap there is a reddish pigment which becomes either a very bright red, or a dark purplish maroon, from the effect of chemicals that combine with it in the leaves. With these coloring materials at command it is easy to see how the autumn woods can assume such splendid hues.

Practical Questions

1. How would you explain the fact that the outer twigs of trees generally are the most leafy? (99, 194; Exps. 57, 74.)

2. Is the common sunflower a compass plant? Is cotton?

3. Are there any such plants in your neighborhood?

4. Compare the leaves of half a dozen shade-loving plants of your neighborhood with those of as many sun-loving ones; which, as a general thing, are the larger and less incised?

5. Give a reason for the difference. (169.)

6. Why do most leaves—notably grasses—curl their edges backward in withering? (182.)

7. What advantage is gained by doing this? (202.)

8. Observe such of the following plants as are found in your neighborhood, and report any changes of position that may take place in their leaves and the causes to which such changes should be ascribed: wood sorrel, mimosa, honey locust, wild senna, partridge pea, wild sensitive plant, redbud, bush clover, Japan clover, Kentucky coffee tree, sensitive brier (Schrankia), peanut, kidney bean.

9. Which of the trees named below shed their leaves from base to tip of the bough (centripetally), and which in the reverse order: ash, beech, hazel, hornbeam, lime, willow, poplar, pear, peach, sweet gum, elm, sycamore, mulberry, China tree, sumac, chinquapin?

10. Account for the fact that evergreen trees and shrubs have generally thick, hard, and shiny leaves, like those of the holly and magnolia, or scales and needles, as the cedar and pine. (203.)

11. Why do many plants which are deciduous at the North tend to become evergreen at the South? (203.)

12. Why are evergreens more abundant in cold than in warm climates? (203.)

13. There is an apparent inconsistency between questions 11 and 12; can you reconcile it? (203.)

14. Why is it more important to protect the south side of trees against exposure to frost than the northern side? (33, 204.)

15. Explain why peach orchards on the tops and northern slopes of elevated areas are less liable to have their fruit destroyed by late frost than those in the valleys and on the southern slopes. (33, 204.)

VIII. MODIFIED LEAVES

MATERIAL.—Get from a florist a potted plant of sundew, Venus’s-flytrap, sarracenia, or, if possible, one of all three, and keep in the schoolroom for observation. The subject can be studied best in a well-stocked greenhouse, if one is accessible.

=206. Modification and adaptation.=—Modification is structural adjustment, or adaptation, carried so far as to obscure the original form of an organ. Its true nature, however, can generally be determined by some of the tests mentioned in 100.

Examples of the modification of leaves to do the work of other organs have already been noticed, as also their entire disappearance in certain cases (97, 101, 149) and replacement by other parts; it is unnecessary, therefore, to revert to this branch of the subject here.

=207. Protective modifications.=—The most general protective modifications that leaves undergo are (1) for the conservation of moisture, as explained in 202, and (2) for protection against animals. Many of the adaptations for the former purpose serve incidentally for defense against animals also. Spines, hairs, scales, sticky exudations, water holders, clasping and perfoliate leaves bar the way to crawling insects; horny cuticles, as well as offensive odors, bitter secretions, and poisonous juices warn leaf-eating cattle and bugs away. These devices are merely protective, however, and adapted to a passive attitude of self-defense.

=208. Insectivorous leaves.=—But sometimes a plant becomes the aggressor, and instead of standing on the defensive or suffering itself to be quietly devoured, proceeds to capture and devour small game on its own account, and in this case, the leaf sometimes becomes a deadly weapon of destruction.

=209. Pitcher plants.=—The sarracenia, or trumpet leaf, is a familiar example of this class. The lower part of the leaf blade is transformed into a hollow vessel for holding water, and the top is rounded into a broad flap called the lamina. Sometimes the lamina stands erect, as in the common yellow trumpets of our coast regions, and when this is the case, it is brilliantly colored and attracts insects (Fig. 259). Sometimes, as in the parrot-beaked and the spotted trumpet leaf, it is bent over the top of the water vessel like a lid, and the back of the leaf, near the foot of the lamina, is dotted with transparent specks that serve to decoy foolish flies away from the true opening and tempt them to wear themselves out in futile efforts to escape, as we often see them do against a window pane.

If the contents of one of these leaves are examined with a lens, there will generally be found mixed with the water at the bottom the remains of the bodies of a large number of insects. The hairs on the outside all point up, toward the rim of the pitcher, while those on the inside turn down, thus smoothing the way to destruction, but making return impossible to a small insect when once it is ensnared. When we remember that these plants are generally found in poor, barren soil, we can appreciate the value to them of the animal diet thus obtained.

=210. Flytraps.=—The most remarkable examples of insect-catching leaves are the Venus’s-flytrap, found in the seacoast region of North Carolina, and the sundew (Drosera rotundifolia), common on the margins of sandy bogs and ponds. The latter is a delicate, innocent-looking little plant, and owes its poetic name to the dewlike appearance of a shining, sticky fluid exuded from glands on its leaves, which glitter in the sun like dewdrops. It is, however, a most voracious carnivorous plant, the sticky leaves acting as so many bits of fly paper by means of which it catches its prey. When a fly has been trapped, the tentacles close upon it, the edges of the leaf curve inward, making a sort of stomach, from the glands of which an acid juice exudes and digests the meal. After a number of days, varying according to the digestibility of the diet, the blades slowly unfold again and are ready for another capture.

The bladderwort, common in pools and still waters nearly everywhere, has its petioles transformed into floats, while the finely dissected, rootlike blades bear little bladders which, when examined under the microscope, are found to contain the decomposed remains of captured animalculæ.

Practical Questions

1. Can you find any kind of leaf that is not preyed upon by something? If so, how do you account for its immunity?

2. Make a list of some of the most striking of the protected leaves of your neighborhood.

3. What is the nature of the protective organ in each case?

4. For protection against what does it seem to be specially adapted?

5. Are the plants in your list for the most part useful ones, or troublesome weeds?

6. Examine the leaves of the worst weeds that you know of and see if these will help in any way to account for their persistency.

Field Work

(1) In connection with Sections I and II, observe the effect of the lobing and branching of leaves in letting the sunlight through. Notice any general differences that may appear as to shape, margin, and texture in the leaves of sun plants, shade plants, and water plants, and account for them. Study the arrangement of leaves on stems of various kinds, with reference to the size and shapes of leaves and their light relations. Consider the value of the various kinds of foliage for shade; for ornament; as producers of moisture; as food; as insect destroyers, etc.

Make a special study of the twelve principal deciduous trees of your neighborhood. Compare the leaves, bark, and branches of the same trees so that you will be able to recognize them by any one of these means alone.

(2) In connection with Sections III and V, consider the effects upon soil moisture of transpiration from the leaves of forest trees and from those of shallow-rooted herbs and weeds that draw their water supply from the surface. Consider the value of forests in protecting crops from excessive evaporation by acting as wind breaks. Study the effect of the fall of leaves upon the formation of soil. In any undisturbed forest tract turn up a few inches of soil with a garden trowel and see what it is composed of. Notice what kind of plants grow in it. Note the absence of weeds and account for it. Compare the appearance of trees scattered along windy hillsides, where the fallen leaves are constantly blown away, or in any position where the soil is unrenewed, with those in an undisturbed forest, and then give an opinion as to the wisdom of hauling away the leaves every year from a timber lot.

(3) In Section VII, observe, in different kinds of leaf mosaics, the means by which the adjustment has been brought about and the purpose it subserves. Make a list of plants illustrating the two habits. Notice the form and position of petioles of different leaves, and their effect upon light exposure, drainage, etc., and the behavior of the different kinds in the wind. Look for compass plants in your neighborhood, and for other examples of adjustment to heat and light. Study the position of leaves at different times of day and in different kinds of weather and note what changes occur and to what they are due.

Make a list of ten plants that seem to you to have best worked out the problem of leaf adjustment, giving the reasons for your opinion.

Study the drainage system of different plants and observe whether there is any general correspondence between the leaf drainage and the root systems. This will lead to interesting questions in regard to irrigation and manuring. Where plants are crowded, the growth of both roots and leaves is complicated with so many other factors that it is best to select for observations of this sort specimens growing in more or less isolated situations.

Notice the time of the expansion and shedding of the leaves of different plants, and whether the early leafers, as a general thing, shed early or late; in other words, whether there seems to be any general time relation between the two acts of leaf expansion and leaf fall.

(4) Under Section VIII, look for instances of modified leaves; study the nature of the different modifications you find, and try to understand their meaning and object. Make a collection (a) of all the leaves you can find modified to serve other than their normal purposes; (b) of all the organs of other kinds that have been modified to serve as leaves; (c) of all the modified parts of leaves—stipules and petioles—that you can find. Keep the collections separate, labeling each specimen with the name of the plant it belongs to, what part it is, what use it serves, when and where found. These collections need not be made individually, but by the class as a whole and kept for the use of the school.

Observe also (d) the differences between young and old leaves of the same kind, and the leaves of young and old plants or parts of plants of the same kind; (e) resemblances between young leaves belonging to plants of different species; (f) between young leaves of one species and mature ones of one or more different species. Make a collection of all the specimens you can find illustrating the three points mentioned, referring each to its proper head, and giving the name and relative age—old or young—of all specimens collected.