Chapter Ii. Germination and Growth

I. PROCESSES ACCOMPANYING GERMINATION

MATERIAL.—A pint or two of corn, peas, beans, or any quickly germinating seed.

APPLIANCES.—Matches; wood splinters; gas jet or alcohol lamp; test tubes; a small quantity of mercuric oxide; a thermometer; a couple of two-quart preserve jars, and a smaller wide-mouthed bottle that can be put into one of them; some limewater; a glass tube (the straws used by druggists for soft drinks will answer).

=26. Preliminary exercises.=—Before taking up the study of germinating seeds, it is important to learn from what sources the organic substances used by the growing plant are derived, and some of the processes that accompany growth and development.

EXPERIMENT 21. TO SHOW THE CHANGES THAT ACCOMPANY OXIDATION.—Strike a match and let it burn out. Examine the burnt portion remaining in your hand; what changes do you notice? These changes have been caused by the union of some substance in the match with something outside of it, in the act of burning; let us see if we can find out what this outside substance is.

EXPERIMENT 22. TO SHOW THE ACTIVE AGENT IN OXIDATION.—Heat some mercuric oxide in a test tube over the flame of a burner. The heat will cause the oxygen to separate from the mercury, and in a short time the tube will be filled with the gas. Extinguish the flame from a lighted splinter and thrust the glowing end into the tube; what happens? The oxygen unites with something in the wood and causes it to burn just as the match did. Compare your burnt splinter with the burnt end of the match; what resemblance do you notice between them?

EXPERIMENT 23. TO SHOW THAT CARBON DIOXIDE IS A PRODUCT OF OXIDATION.—Your experiment with the match showed that ignition is accompanied by heat, and if active enough, by light, and also that it left behind a solid substance in the form of charcoal. But how about the part that united with the oxygen to produce these results? Let us see what became of it. Hold a lighted candle under the open end of a test tube, or under the mouth of a small glass jar. Does any vapor collect on the inside? After two or three minutes quickly invert the jar or the tube, and thrust in a lighted match: what happens? Can the substance now in the jar be ordinary air? Why not? (Exps. 21, 22.) Pour in a small quantity of limewater, holding your hand over the mouth of the tube to prevent the air from getting in; the gas inside, being heavier than air, will not escape immediately unless agitated. What change do you notice in the limewater?

It has been proved by experiment that the kind of gas formed by the burning candle has the property of turning limewater milky; hence, whenever you see this effect produced in limewater, you may conclude that this gas, known as carbon dioxide, is present; and conversely, the presence of carbon dioxide, especially if accompanied by some of the other effects observed, as the giving out of heat and moisture, may be taken as evidence that some process similar to that going on in the burning candle is, or has been, at work.

EXPERIMENT 24. DO THESE EFFECTS ACCOMPANY ANY OF THE LIFE PROCESSES OF ANIMALS?—Blow your breath against the palm of your hand; what sensation do you feel? Blow it against a mirror, or a piece of common glass; what do you see? Blow through a tube into the bottom of a glass containing limewater; how is the water affected? How do these facts correspond with the results of Exp. 23?

EXPERIMENT 25. IS THERE ANY EVIDENCE THAT A SIMILAR PROCESS GOES ON IN PLANTS?—(1) Half fill a small, wide-mouthed jar with limewater, place it inside a larger one (Fig. 46), and fill the space between them, up to the neck of the smaller vessel, with well-soaked peas, beans, or barleycorns, on a bed of moist cotton or blotting paper. Cover with a piece of glass and keep at a moderately warm temperature. (2) As a control experiment, place beside this another jar arranged in precisely the same way, except that seeds must be used whose vitality has been destroyed by heat. To prevent the entrance of germs among the dead seeds, which might cause fermentation and thus interfere with the experiment, set the jar containing them in a vessel of water and boil an hour or two before the experiment begins. Otherwise, treat precisely as in (1).

After germination has taken place in (1), what change do you notice in the limewater? If the effect is not apparent, gently stir with a straw or a glass rod to mix it with the gas in the larger jar. Has the limewater in the control experiment undergone the same change? (It may show a slight milkiness due to the carbon dioxide in the air.) Insert a thermometer among the seeds in both of the larger jars, and compare their temperature with that of the outside air; which shows the greater rise? From this experiment and the last one, what process, common to animals, would you conclude has been going on in the germinating seeds?

NOTE.—Heat in germinating seeds is not always due to this cause alone, but is sometimes increased by the presence of minute organisms called bacteria. Germinating barley and rye in breweries sometimes show an increase in temperature of 40 to 70 degrees, due to these organisms, and spontaneous combustion in seed cotton has been reported from the same cause.

=27. Oxidation.=—The process that brought about the results observed in the foregoing experiments, and popularly known as combustion, is more accurately defined by chemists as oxidation. It takes place whenever substances enter into new combinations with oxygen. The most familiar examples of it are when oxygen enters into combination with substances containing carbon. It was the union of a portion of the oxygen of the air in Exp. 21, and of that in the tube in Exp. 22, with some of the carbon in the wood, that caused the burning. The effect was more marked in the second case because the oxygen in the tube was pure, while in the air it is mixed with other substances.

=28. Carbon.=—The black substance left in your hand after oxidation of the wood in Exps. 21 and 22 is carbon. It composes the greater part of most plant bodies, and, in fact, is the most important element in the realm of organic nature. There is not a living thing known, from the smallest microscopic germ to the most gigantic tree in existence, that does not contain carbon as one of its essential constituents.

=29. Carbon dioxide.=—The gas produced by the burning candle in Exp. 23, by the germinating seeds in Exp. 25, and expelled from your own lungs in Exp. 24, is carbon dioxide. Chemists designate it by the symbol CO₂, which means that it consists of one part carbon to two parts oxygen. It is an invariable product wherever the oxidation of substances containing carbon goes on. Heat and moisture are evolved at the same time, and if oxidation is very active, as in Exps. 21 and 22, light also. When the process takes place very slowly, no light is evolved, and so little heat as to be imperceptible without special observation. Hence, oxidation may go on around us and even in our own bodies without our being conscious of the fact.

Carbon dioxide is of prime importance to the well-being of plants. It furnishes the material from which the greater part of their organic food is derived, as will be seen when we take up the study of the leaf and its work. To animals, on the contrary, its presence is so injurious that if the proportion of it in the air we breathe ever rises much above 1 part to 1000, the ill effects become painfully sensible. It is not, however, as was formerly supposed, a poison, the harm it does being to decrease the proportion of oxygen in the atmosphere so that animals cannot get enough of it to breathe, and die of suffocation.

=30. Respiration in plants and in animals.=—It was shown in Exp. 24 that respiration in animals is accompanied by the products of oxidation; hence we conclude that respiration is a form of oxidation. And since these same products are given off by plants (Exp. 25), the inference is clear that the same process goes on in them. But in plants the life functions are so much more sluggish than in animals that it is only in their most active state, during germination and flowering, that evidence of it is to be looked for.

=31. Respiration and energy.=—In plants, as in animals, respiration is the expression or measure of energy. Sleeping animals breathe more slowly than waking ones, snakes and tortoises more slowly than hares and hawks. The more we exert ourselves and the more vital force we expend, the harder we breathe; hence, respiration is more active in children than in older persons and in working people than in those at rest. It is the same with plants; respiration is most perceptible in germinating seeds and young leaves, in buds and flowers, where active work is going on. Hence, in this condition they consume proportionately larger quantities of oxygen and liberate correspondingly larger quantities of carbon dioxide, with a proportionate increase of heat. In some of the arums,—calla lily, Jack-in-the-pulpit, colocasia, etc.,—and in large heads of compositæ, like the sunflower, where a great number of small flowers are brought together within the same protecting envelope, the rise of temperature is sometimes so marked that it may be perceived by placing a flower cluster against the cheek.

Practical Questions

1. What is charcoal? (28.)

2. Is any of this substance contained in the seed? in the flour and meal made from seed? (28; Exp. 25.)

3. What combination takes place when the cook lets the stove get too hot and burns the biscuits? (27, 28.)

4. Of what does the burned part consist? (28.) What was it before it was burned? (27, 28).

5. Which burns the more readily, an oily seed or a starchy one? Which leaves the more solid matter behind? (Suggestion: test by putting a bean, or a large grain of corn, and an equal quantity of the kernel of a Brazil nut on the end of a piece of wire and thrusting into a flame.)

6. Is there any rational ground for the statement that the wooden buildings formerly used on Southern plantations as cotton ginneries were sometimes destroyed through spontaneous combustion due to the heat generated by piles of decaying cotton seed? (Exp. 25, Note.)

II. CONDITIONS OF GERMINATION

MATERIAL.—Several ounces each of various kinds of seed. For the softer kinds, pea, bean, corn, oats, wheat are recommended; for those with harder coverings, squash, castor bean, apple, pear, or, where obtainable, cotton; for still harder kinds, persimmon and date seeds, or the stones of plum and cherry.

APPLIANCES.—1 dozen common earthenware plates for germinators; 1 dozen two-ounce wide-mouthed bottles; 2 common glass tumblers; clean sand, sawdust, or cotton batting, for bedding; a double boiler; a gas burner, or a lamp stove.

=32. Recording observations.=—For this purpose a page should be ruled off in the notebook of each student, after the model here given, and the facts brought out by the different experiments set down as observed.

NUMBER OF SEEDS GERMINATED

==============+=+==+==+==+====+====+====+====+====+=====+==== No. of hours | |24|48|72|4 d.|5 d.|6 d.|7 d.|8 d.|10 d.|2 w. +-+--+--+--+----+----+----+----+----+-----+---- No. of vessel|1| | | | | | | | | | +-+--+--+--+----+----+----+----+----+-----+---- No. of vessel|2| | | | | | | | | | +-+--+--+--+----+----+----+----+----+-----+---- No. of vessel|3| | | | | | | | | | +-+--+--+--+----+----+----+----+----+-----+---- No. of vessel|4| | | | | | | | | | +-+--+--+--+----+----+----+----+----+-----+---- No. of vessel|5| | | | | | | | | | +-+--+--+--+----+----+----+----+----+-----+---- No. of vessel|6| | | | | | | | | | ==============+=+==+==+==+====+====+====+====+====+=====+====

EXPERIMENT 26. CAN SEEDS HAVE TOO MUCH MOISTURE?—Drop a number of dry beans or grains of corn, oats, or other convenient seed, into a vessel with a bedding of cotton or paper that is barely moistened, and an equal number of soaked seeds of the same kind into another vessel with a saturated bedding of the same material. In a third vessel place the same number of soaked seed, covering them partially with water, and in a fourth cover the same number entirely. Label them 1, 2, 3, and 4; keep all together in a warm, even temperature, and observe at intervals of twenty-four hours for a week. What condition as to moisture do you find most favorable to germination? Would seeds germinate in the entire absence of moisture? How do you know?

EXPERIMENT 27. WAS IT THE PRESENCE OF TOO MUCH WATER, OR THE LACK OF AIR CAUSED BY IT, THAT INTERFERED WITH GERMINATION IN THE LAST EXPERIMENT?—To answer this question experimentally is not easy, since it is difficult to obtain a complete vacuum without special appliances. The simplest way is to fill with mercury a glass tube 30 inches long, closed at one end, and invert it over a small vessel—a teacup, or an egg cup will answer—containing mercury enough to cover the bottom to a depth of two or three centimeters (see Appendix, Weights and Measures, for English equivalents.) The tube must be supported in such a way that its lower end will dip into the mercury without touching the bottom of the vessel. With a pair of forceps insert under the mouth of the tube two or three seeds that have been well soaked in water deprived of air by previous boiling. Being lighter than mercury, they will float to the top, where there is a complete absence of air while other conditions favorable to germination are present. Before releasing, they should be well shaken under the mercury to free them from air bubbles, and if the coats are loose fitting so that they can be removed without injury to the parts inclosed in them, they should be slipped off in order to get rid of any imprisoned air they may contain. Additional moisture may be supplied, if necessary, by injecting, by means of a medicine dropper inserted under the mouth of the tube, a drop or two of water that has been previously boiled. Keep in a warm, even temperature, under conditions favorable to germination, and compare the behavior of the seeds with those placed in the different vessels in Exp. 26.

If appliances for this experiment are lacking, a rough approximation can be made by using the seeds of aquatic plants, such as the lotus, water lily, and the so-called Chinese sacred bean, sold in the variety stores, which we know are capable of germinating in the limited amount of air contained in ordinary soil water. Place an equal number of such seeds, of about the same size and weight, on a bedding of common garden soil in two glass tumblers. Fill one vessel a little over half full of ordinary soil water and the other to the same height with water from which the air has been expelled by boiling. Pour over the liquid a film of sweet oil or castor oil, to prevent the access of air, leaving the surface of the water in the other vessel exposed. In which do the seeds come up most freely?

Some seeds, especially those rich in proteins, as peas and beans, will germinate in a vacuum, because oxygen is supplied for a time by the chemical decomposition of substances in their tissues which contain it, but when these are exhausted, respiration ceases and death ensues.

EXPERIMENT 28. DOES THE DEPTH AT WHICH SEEDS ARE PLANTED AFFECT THEIR GERMINATION?—Plant a number of peas or grains of corn at different depths in a wide-mouthed glass jar filled with moist sand, as shown in Fig. 47, the lowest ones at the bottom, the top ones barely covered. Try different kinds of seed and grain,—radish, squash, cotton, or wheat,—and watch them make their way to the surface. Do you notice any difference in this respect between large seed and small ones? Between those with thick cotyledons and thin ones? At what depth do you find, from your recorded observations, that seed germinate best?

EXPERIMENT 29. WHAT TEMPERATURE IS MOST FAVORABLE TO GERMINATION?—Put half a dozen soaked beans on moist cotton or sawdust in three wide-mouthed bottles of the same size or in germinators arranged as in Figs. 48, 49, the seed also being selected with a view to similarity of size and weight. Keep one at a freezing temperature; the second in a temperature of 15° to 20° C. (see Appendix for Fahrenheit equivalents); and the third, at 30° C. If a place can be found near a stove or a register, where an even temperature of about 125° F. is maintained, place a fourth receptacle there. Observe at intervals of twenty-four hours for a week or ten days, keeping the temperature as even as possible, and maintaining an equal quantity of moisture in each vessel. Make a daily record of your observations. What temperature do you find most favorable to germination?

EXPERIMENT 30. AT WHAT TEMPERATURE DO SEEDS LOSE THEIR VITALITY?—Place about two dozen each of grains of corn, beans, squash seed, and castor beans, with an equal number of plum or cherry stones, in water, and heat to a temperature of 150° F. After an exposure of ten minutes, take out six of each kind and place in germinators made of two plates with moist sand or damp cloth between them, as shown in Figs. 48, 49. Raise the temperature to 175° F., and after ten minutes take out six more of each kind of seed and place in another germinator. Raise the water in the vessel to 200°, take out another batch of seeds; raise to the boiling point for ten minutes more, and plant the remaining six of each lot. Number the four germinators, and observe at intervals of twenty-four hours for two weeks. The harder kinds should be kept under observation for three or four weeks, as they germinate slowly.

Try the same experiments with the same kinds of seeds at a dry heat, using a double boiler to prevent scorching, and record observations as before.

EXPERIMENT 31. TIME REQUIRED FOR GERMINATION.—Arrange in germinators seeds of various kinds, such as corn, wheat, peas, turnip, apple, orange, grape, castor bean, etc. “Clip” some of the harder ones and keep all the kinds experimented with under similar conditions as to moisture, temperature, etc., and record the time required for each to sprout. What is the effect of clipping, and why?

EXPERIMENT 32. ARE VERY YOUNG OR IMMATURE SEEDS CAPABLE OF GERMINATING?—Plant some seeds from half-grown tomatoes, and grains of wheat, oats, or barley before they are ready for harvesting. Try as many kinds as you like, and see how many will come up. Notice whether there is any difference in the health and vigor of plants raised from seeds in different stages of maturity.

EXPERIMENT 33. THE RELATIVE VALUE OF PERFECT AND INFERIOR SEED.—From a number of seeds of the same species select half a dozen of the largest, heaviest, and most perfect, and an equal number of small, inferior ones. If a pair of scales is at hand, the different sets should be weighed and a record kept for comparison with the seedlings at the end of the experiment. Plant the two sets in pots containing exactly the same kind of soil, and keep under identical conditions as to light, temperature, and moisture. Keep the seedlings under observation for two or three weeks, making daily notes and occasional drawings of the height and size of the stems, and the number of leaves produced by each.

=33. Resistance to heat and cold.=—In making experiments with regard to temperature, notice how the extremes tolerated are influenced, first, by the length of time the seeds are exposed; second, by the amount of water contained in them; and third, by the nature of the seed coats. Every farmer knows that the effect of freezing is much more injurious to plants or parts of plants when full of sap (water) than when dry. This, in the opinion of the most recent investigators, is because the water in the spaces outside the cells freezes first and as moisture is gradually withdrawn from the inside to take its place, the soluble salts which may be present in the cell sap become more concentrated, and by their chemical action on the contained proteins cause them to be precipitated, or “salted out,” as we see sugar or salt precipitated from solutions of those substances when water is withdrawn by evaporation. In this way, it is believed, the fundamental protoplasm of the cell may be so disorganized that death ensues if the freezing is continued long enough, since the protein precipitates become “denatured” and cannot be reabsorbed if kept in a solid state too long. The length of time necessary to produce death from this cause is, of course, different in different plants, according to the kind of salts dissolved in the sap and the nature of the proteins acted on by them. The proteins in the sap of Begonia, or Pelargonium, plants which are very sensitive to cold, yield a denatured precipitate at, or a little below the freezing point of water, while those of winter rye withstand a temperature of -15° C., and of pine needles, -40° C.

Mechanical injury through rupture of parts by freezing is not apt to cause serious damage except in cases of sudden and violent cold at a time when the tissues are gorged with sap, as not infrequently happens during the abrupt changes of temperature which sometimes occur in spring after the trees have put forth their leaves. In an extreme case of this kind, the writer has seen the trunk of an oak a foot or more in diameter split in deep seams from the effects of freezing.

=34. The length of time during which seeds may retain their vitality.=—No direct experiment can be made to test this point, since it would require months, or even years, covering in some instances more than the lifetime of a generation. It has been stated on good authority that seeds of the water chinquapin (Nelumbo) have germinated after more than a hundred years, and moss spores preserved in herbariums, after fifty. But the records in such cases are not always trustworthy, and there is absolutely no foundation for the statements sometimes made about the germination of wheat grains found preserved with mummies over two thousand years old. If kept perfectly dry, however, seed may sometimes be preserved for months, or even years. Peas have been known to sprout after ten years, red clover after twelve, and tobacco after twenty. Ordinarily, however, the vitality of seeds diminishes with age, and in making experiments it is best to select fresh ones. Those used for comparison should also, as far as possible, be of the same size and weight.

=35. Effect of precocious germination.=—It has been found by experiment that plants raised from immature seed, when they will germinate at all (Exp. 32), yield earlier and larger crops than the same kinds from mature seed. Early tomatoes and some other vegetables are produced in this way. The majority of seeds, however, require a period of rest before beginning their life work. Those that are forced to take up the burden of “child labor” show the effect of such abnormal condition by yielding fruits that are smaller and less firm than those raised from mature seed, so that they do not keep well and have to be marketed quickly. Under what circumstances does it pay to cultivate such fruits?

Practical Questions

1. What are the principal external conditions that affect germination? (Exps. 26-29.)

2. What effect has cold? want of air? too much water?

3. Is light necessary to germination?

4. What is the use of clipping seeds? (Exps. 12, 13, 14, and Material, p. 12.)

5. In what cases should it be resorted to? (Exp. 31.)

6. Why will seed not germinate in hard, sunbaked land without abundant tillage? Why not on undrained or badly drained land? (Exps. 26, 27.)

7. Will seeds that have lost their vitality swell when soaked? (Exp. 16.)

8. Are there any grounds for the statement that the seeds of plums boiled into jam have sometimes been known to germinate? (33; Exp. 30.)

9. Could such a thing happen in the case of apple or sunflower seed, and why or why not? (33.)

10. Does it make any difference in the health and vigor of a plant whether it is grown from a large and well-developed seed or from a weak and puny one? (Exp. 33.)

11. Would a farmer be wise who should market all his best grain and keep only the inferior for seed?

12. What would be the result of repeated plantings from the worst seed?

13. Of constantly replanting the best and most vigorous?

14. Suppose seed would germinate without moisture; would this be an advantage, or a disadvantage to agriculturists?

15. Why is a cool, dry place best for keeping seeds? (Exps. 26, 29.)

16. Why are the earliest tomatoes found in the market usually smaller than those offered later? (35.)

17. Why is continued rain so injurious to wheat, oats, and other grains before they are mature enough to be harvested? (35; Exp. 32.)

18. Would the same effect be likely to occur in the case of very oily seeds, such as flax and castor beans? Why? (Suggestion: try the effect of putting water on a piece of oiled paper.)

19. Explain why many seeds cannot germinate successfully without air. (30, 31; Exp. 25.)

20. Mention some of the practical advantages that a farmer, a gardener, or a careful housewife might gain from experiments like those made in this section.

21. Explain why seeds can endure so much greater extremes of temperature than growing plants. (23, 33.)

III. DEVELOPMENT OF THE SEEDLING

MATERIAL.—Seedlings of various kinds in different stages of growth. It is recommended that the same species be used that were studied in Section III, Chapter I, or such equivalents as may have been substituted for them. Enough should be provided to give each pupil three or four specimens in different stages of development. Seeds, even of the same kind, develop at such different rates that it will probably not be necessary to make more than two plantings of each sort, from 2 to 5 days apart. Soaked seeds of corn and wheat will germinate in from 3 to 7 days, according to the temperature; oats in 1 to 4; beans in 4 to 6; squash and castor beans in from 8 to 10. Very obdurate ones may be hastened by clipping. Keep the germinators in an even temperature, at about 70° to 80° F.

Pine is a very difficult seed to germinate, requiring usually from 18 to 21 days. By soaking the mast for twenty-four hours and planting in damp sand or sawdust kept at an even temperature of 23° C. or about 75° F., specimens may be obtained.

=36. Seedlings of monocotyls.=—Examine a seedling of corn that has just begun to sprout; from which side does the seedling spring, the plain or the grooved one? Refer to your sketch of the dry grain and see if this agrees with the position of the embryo as observed in the seed. Make sketches of four or five seedlings in different stages of advancement, until you reach one with a well-developed blade. From what part of the embryo has each part of the seedling developed? Which part first appeared above ground? Is it straight, or bent in any way? In what direction does the plumule grow? The hypocotyl? Does the cotyledon appear above ground at all? Slip off the husk and see if there is any difference in the size and appearance of the contents as you proceed from the younger to the older plants. How would you account for the difference?

=37. The root.=—Examine the lower end of the hypocotyl and find where the roots originate; would you say that they are an outgrowth from the stem, or the stem from the root? Observe that the root of the corn does not continue to grow in a single main axis like that of the castor bean, but that numerous adventitious and secondary roots spring from various points near the base of the hypocotyl and spread out in every direction, thus giving rise to the fibrous roots of grains and grasses.

=38. Root hairs.=—Notice the grains of sand or sawdust that cling to the rootlets of plants grown in a bedding of that kind. Examine with a lens and see if you can account for their presence. Lay the root in water on a bit of glass, hold up to the light and look for root hairs; on what part are they most abundant?

The hairs are the chief agents in absorbing moisture from the soil. They do not last very long, but are constantly dying and being renewed in the younger and tenderer parts of the root. These are usually broken away in tearing the roots from the soil, so that it is not easy to detect the hairs except in seedlings, even with a microscope. In oat, maple, and radish seedlings they are very abundant and clearly visible to the naked eye. The amount of absorbing surface on a root is greatly increased by their presence.

=39. The root cap.=—Look at the tip of the root through your lens and notice the soft, transparent crescent or horseshoe-shaped mass in which it terminates. This is the root cap and serves to protect the tender parts behind it as the roots burrow their way through the soil. Being soft and yielding, it is not so likely to be injured by the hard substances with which it comes in contact as would be the more compact tissue of the roots. It is composed of loose cells out of which the solid root substance is being formed; the growing point of the root, g, is at the extremity of the tip just behind the cap, c (Fig. 57). The cap is very apparent in a seedling of corn, and can easily be seen with the naked eye, especially if a thin longitudinal section is made. It is also well seen in the water roots of the common duckweed (Lemna), and on those developed by a cutting of the wandering Jew, when placed in water. Are there any hairs on the root cap? Can you account for their absence?

NOTE.—For a minute study of the structure of roots, see =67=.

=40. Organs of vegetation.=—The three parts, root, stem, and leaf, are called organs of vegetation in contradistinction to the flower and fruit, which constitute the organs of reproduction. The former serve to maintain the plant’s individual existence, the latter to produce seed for the propagation of the species, so we find that the seed is both the beginning and the end of vegetable life.

=41. Definitions.=—Organ is a general name for any part of a living thing, whether animal or vegetable, set apart to do a certain work, as the heart for pumping blood, or the stem and leaves of a plant for conveying and digesting sap. By “function” is meant the particular work or office that an organ has to perform.

=42. Seedlings of dicotyls. The bean.=—Sketch, without removing it, a bean seedling that has just begun to show itself above ground; what part is it that protrudes first? Sketch in succession four or five others in different stages of advancement. Notice how the hypocotyl is arched where it breaks through the soil. Does this occur in the monocotyls examined? Do the cotyledons of the bean appear above ground? How do they get out? Can you perceive any advantage in their being dragged out of the ground backwards in this way rather than pushed up tip foremost? What changes have the cotyledons undergone in the successive seedlings? Remove from the earth a seedling just beginning to sprout and sketch it. From what point does the hypocotyl protrude through the coats? Does this agree with its position as sketched in your study of the seed? In which part of the embryo does the first growth take place?

Remove in succession the several seedlings you have sketched and note their changes. How does the root differ from that of the corn and oats? The first root formed by the extension of the hypocotyl is the primary root and should be so labeled in your drawings; the branches that spring from it are secondary roots. Look for root hairs; if there are any, where do they occur?

=43. Germination of the squash.=—How does the manner of breaking through the soil compare with that of the bean? With the corn? From which end of the seed, the large or the small one, does the hypocotyl spring? Do the cotyledons come above ground? How do they get out of the seed coat? Notice the thick protuberance developed by the hypocotyl and pressing against the lower half of the coat at the point where the hypocotyl breaks through. This is called the “peg”; can you tell its use? Could the cotyledons get out of their hard covering without it? Slip the peg below the coat in one of your growing specimens, leave it in the soil, and see what will happen. How do the cotyledons of the squash differ from those of the bean as they come out of the seed cover? Do they act as foliage leaves? Do you see any difference in the development of the plumule in the two seeds (Figs. 19, 25) to account for the different behavior of the cotyledons? Sketch three seedlings in different stages, labeling correctly the parts observed. Make a similar study of the castor bean, or other seedling selected by your teacher, and illustrate by drawings.

=44. Arched and straight hypocotyls.=—This difference in the manner of getting above ground is an important one. That by means of the arched hypocotyl is, in general, characteristic of the process of germination in which the cotyledons come above ground, while the straight kind, which was illustrated in the corn and wheat, is the prevailing method when the cotyledons remain below ground. Can you give a reason for the difference?

=45. Polycotyledons; germination of the pine.=—Examine a pine seedling just beginning to sprout. What part emerges first from the seed coat? Where does it break through? Where did you find the micropyle in the pine seed? (15.) Can you give a reason why the hypocotyl in seeds should break through the coats at this point? How do the cotyledons get out of the testa? Is the hypocotyl arched or straight in germination? How does it compare with the bean and squash in this respect? With the corn? Is any endosperm left in the testa after the cotyledons have come out? What has become of it? Do the cotyledons function as leaves? How many of them have the specimen you are studying? Notice the little knob or button at the upper end of the hypocotyl, just above the point where the cotyledons are attached; this is the epicotyl, or part above the cotyledons, here identical with the plumule; does it develop as rapidly as in the other seedlings you have examined?

=46. Relation of parts in the seedling.=—Before leaving this subject, it is important to fix clearly in mind the different parts of the germinating seedling and their relation to both the embryo from which they originated and the plant into which they are to develop. The part labeled “hypocotyl” in your sketches is all that portion of the embryo below the point of attachment of the cotyledons. In germination its upper part will become the stem, and in the embryo constitutes the caulicle, or stemlet, while its lower part, from which the root will develop, is the radicle, or rootlet; hence the term “hypocotyl” includes both the future root and stem. The plumule is that part of the embryo between the cotyledons and above their point of attachment to the caulicle. It is the upward growing point of the young plant, and hence the place of attachment of the cotyledon is the first node, or point of leaf origin, on the stem.

The epicotyl, in contradistinction to the hypocotyl, is all that part of the plant above the insertion of the cotyledons. Before germination it is identical with the plumule. As the seedling grows, the epicotyl advances its growing point by adding new nodes and internodes, as the spaces between the successive points of leaf insertion are called.

=47. Botanical terms.=—As the prefixes hypo and epi are of frequent occurrence in botanical works, it will aid in understanding their various compounds if you will remember that hypo always refers to something below or beneath, and epi, to something over or above. With this idea in mind you will see that botanical terms are a labor-saving device, since it is much easier, in making notes, to use a single descriptive word than to write out the long English equivalent, such as “the part under (or over) the cotyledons.”

Practical Questions

1. Do the cotyledons, as a general thing, resemble the mature leaves of the same plants?

2. Name some plants in which you have observed differences, and account for them; could convenience of packing in the seed coats, for instance, or of getting out of them, have any bearing on the matter?

3. Does the position in which seeds are planted in the ground have anything to do with the position of the seedlings as they appear above the surface?

4. Is this fact of any importance to the farmer?

5. Will grain that has begun to germinate make good meal or flour? Why? (27, 36; Exp. 25.)

IV. GROWTH

MATERIAL.—Two young potted plants; some lily or hyacinth bulbs; seedlings of different kinds,—some with well-developed taproots,—apple, cotton, and maple are good examples.

APPLIANCES.—A small flat dish, some mercury, and a piece of cork.

EXPERIMENT 34. HOW DOES THE ROOT INCREASE IN LENGTH?—Mark off the root of a very young corn seedling into sections by moistening a piece of sewing thread with indelible ink and applying it to the surface of the root at intervals of about two millimeters (⅒ of an inch), or by tying a thread lightly around it at the same intervals. Lay the seedling on a moist bedding between two panes of glass kept apart by a sliver of wood to prevent their injuring the root by pressure. Watch for a day or two, and you will see that growth takes place from a point just back of the tip (Figs. 61, 62).

Mark off a seedling of the bean in the same way and watch to see whether it increases in the same manner as the corn.

EXPERIMENT 35. HOW DOES THE STEM INCREASE IN LENGTH?—Mark off a portion of the stem of a bean seedling as explained in the last experiment, and find out how it grows. Allow a seedling to develop until it has put forth several leaves and measure daily the spaces between them. Label these spaces in your drawings, “internodes,” and the points where the leaves are attached, “nodes.” Does an internode stop growing when the one next above it has formed? When is growth most rapid? Reverse the position of a number of seedlings that have just begun to sprout and watch what will happen. After a few days reverse again and note the effect.

EXPERIMENT 36. CAN PLANTS GROW AND LOSE WEIGHT AT THE SAME TIME?—Remove the scales from a white lily bulb, weigh them, and lay in a warm, but not too damp place, away from the light. After a time bulblets will form at the bases of the scales. Weigh them again, and if there has been any loss, account for it. The experiment may be tried by allowing a potato tuber or a hyacinth bulb to germinate without absorbing moisture enough to affect its weight.

EXPERIMENT 37. IS THE DIRECTION OF GROWTH A MATTER OF ANY IMPORTANCE?—Plant in a pot suspended as shown in Fig. 67, a healthy seedling of some kind, two or three inches high, so that the plumule shall point downward through the drain hole and the root upward into the soil. Watch the action of the stem for six or eight days, and sketch it at successive intervals. After the stem has directed itself well upward, invert the pot again, and watch the growth. After a week remove the plant and notice the direction of the root. Sketch it entire, showing the changes in direction of growth.

At the same time that this experiment is arranged, lay another pot with a rapidly growing plant on one side, and every forty-eight hours reverse the position of the pot, laying it on the opposite side. At the end of ten or twelve days remove the plant and examine. How has the growth of root and stem been affected?

What do we learn from these experiments and from Exp. 35 as to the normal direction of growth in these two organs respectively? Can you think of any natural force that might influence this direction?

EXPERIMENT 38. TO SHOW THAT PLANTS WILL EXERT FORCE RATHER THAN CHANGE THEIR DIRECTION OF GROWTH.—Pin a sprouted bean to a cork and fasten the cork to the side of a flat dish, as shown in Fig. 69. Cover the bottom of the dish with mercury at least half an inch deep, and over the mercury pour a layer of water. Cover the whole with a pane of glass to keep the moisture in, and leave for several days. The root will force its way downward into the mercury, although the latter is fourteen times heavier than an equal bulk of the bean root substance, and the root must thus overcome a resistance equal to at least fourteen times its own weight.

=48. What growth is.=—With the seedling begins the growth of the plant. Most people understand by this word mere increase in size; but growth is something more than this. It involves a change of form, usually, but not necessarily, accompanied by increase in bulk. Mere mechanical change is not growth, as when we bend or stretch an organ by force, though if it can be kept in the altered position till such position becomes permanent, or as we say in common speech, “till it grows that way,” the change may become growth. To constitute true growth, the change of form must be permanent, and brought about, or maintained, by forces within the plant itself.

=49. Conditions of growth.=—The internal conditions depend upon the organization of the plant. The essential external conditions are the same as those required for germination: food material, water, oxygen, and a sufficient degree of warmth. It may be greatly influenced by other circumstances, such as light, gravitation, pressure, and (probably) electricity; but the four first named are the essential conditions without which no growth is possible.

=50. Cycle of growth.=—When an organ becomes rigid and its form fixed, there is no further growth, but only nutrition and repair,—processes which must not be confounded with it. Every plant and part of a plant has its period of beginning, maximum, decline, and cessation of growth. The cycle may extend over a few hours, as in some of the fungi, or, in the case of large trees, over thousands of years.

=51. Geotropism.=—The general tendency of the growing axes of plants to take an upward and downward course as shown in Exp. 37—in other words, to point to and from the center of the earth—is called geotropism. It is positive when the growing organs point downward, as most primary roots do; negative when they point upward, as in most primary stems; and transverse, or lateral, when they extend horizontally, as is the case with most secondary roots and branches.

=52. Gravity and growth.=—It cannot be proved directly that geotropism is due to gravity, because it is not possible to remove plants from its influence so as to see how they would behave in its absence. The effect of gravity may be neutralized, however, by arranging a number of sprouting seeds on the vertical disk of a clinostat, an instrument fitted with a clockwork movement by means of which they may be kept revolving steadily for several days. By this constant change of position gravity is made to act on them in all directions alike, which is the same in some respects as if it did not act at all. If the disk is made to revolve rapidly, the growing root tips turn toward the axis of motion, without showing a tendency to grow downward. We may then conclude that geotropism is a reaction to gravity.

=53. Geotropism an active force.=—It must be noted, however, that the force here alluded to is not the mere mechanical effect of gravity, due to weight of parts, as when the bough of a fruit tree is bent under the load of its crop, but a certain stimulus to which the plant reacts by a spontaneous adjustment of its growing parts. In other words, geotropism is an active, not a passive function, and the plant will overcome considerable resistance in response to it. (Exp. 38).

=54. Other factors.=—The direction of growth is influenced by many other factors, such as light, heat, moisture, contact with other bodies, and perhaps by electricity. The result of all these forces is an endless variety in the forms and growth of organs that seems to defy all law.

Heat, unless excessive, generally stimulates growth; contact sometimes stimulates it, causing the stem to curve away from the disturbing object, and sometimes retards it, causing the stem to curve toward the object of contact by growing more rapidly on the opposite side, as in the stems of twining vines. Light stimulates nutrition, but generally retards growth. The movements of plants toward the light are effected in this way; growth being checked on that side, the plant bends toward the light.

Practical Questions

1. Why do stems of corn, wheat, rye, etc., straighten themselves after being prostrated by the wind? (51, 54.)

2. Do plants grow more rapidly in the daytime, or at night? (54.)

3. Reconcile this with the fact that green plants will die if deprived of light.

4. Which grows more rapidly, a young shoot or an old one? (31, 50.)

5. Which, as a general thing, are the more rapid growers, annuals or perennials? Herbaceous or woody-stemmed plants?

6. Name some of the most rapid growers you know.

7. Of what advantage is this habit to them?

8. Why do roots form only on the under side of subterraneous stems? (51.)

9. Why do new twigs develop most freely on the upper side of horizontal branches? (51.)

Field Work

(1) Notice the various seedlings met with in your walks and see how many you can recognize by their resemblance to the mature plants. Account for any differences you may observe between seedlings and older plants of the same species. Observe the cotyledons as they come up and their manner of getting out of the ground, and notice the ways in which this is influenced by moisture, light, and the nature of the soil. Where the cotyledons do not appear, dig into the ground and find out the reason. Notice which method of emergence occurs in each case, the arched, or straight, and account for it. Observe particularly the behavior of seedlings in hard, sunbaked soil. If you see any of them lifting cakes of earth, compare the size and weight of the cake with that of the seed; if there is any disparity, what does this imply? What is the force called which the plant exercises in lifting the weight? (51.)

(2) Notice if there are any seeds germinating successfully on top of the ground, and find out by what means their roots get into the soil. Observe what effect sun and shade, moisture and drought, and the nature of the soil have on the process. Find out whether roots exercise force in penetrating the soil; what kinds they penetrate most readily, and what kinds, if any, they fail to penetrate at all. Notice whether seedlings with taproots, like the turnip and castor bean, or those with fibrous roots, like corn and wheat, are more successful in working their way downward.

(3) Look for tree seedlings. Explain why seedlings of fruit trees are so much more widely distributed in cultivated districts, and so much easier to find than those of forest trees. Where do the latter occur, as a general thing? Account for the fact that seedling trees are so much more rare than germinating herbs, and why trees like the oak and chestnut and black walnut propagate so much more slowly, in a state of nature, than the pine, cedar, ash, and maple.

(4) Observe the direction of growth in plants on the sides of gullies and ravines, and tell how it is influenced by geotropism. Notice whether there are other influences at work; for instance, light, or in the case of roots, the attraction of moisture.