Section Vii. Leaves.

118. STEMS bear leaves, at definite points (nodes, 13); and these are produced in a great variety of forms, and subserve various uses. The commonest kind of leaf, which therefore may be taken as the type or pattern, is an expanded green body, by means of which the plant exposes to the air and light the matters which it imbibes, exhales certain portions, and assimilates the residue into vegetable matter for its nourishment and growth.

119. But the fact is already familiar (10-30) that leaves occur under other forms and serve for other uses,--for the storage of food already assimilated, as in thickened seed-leaves and bulb-scales; for covering, as in bud-scales; and still other uses are to be pointed out. Indeed, sometimes they are of no service to the plant, being reduced to mere scales or rudiments, such as those on the rootstocks of Peppermint (Fig. 97) or the tubers of Jerusalem Artichoke (Fig. 101). These may be said to be of service only to the botanist, in explaining to him the plan upon which a plant is constructed.

120. Accordingly, just as a rootstock, or a tuber, or a tendril is a kind of stem, so a bud-scale, or a bulb-scale, or a cotyledon, or a petal of a flower, is a kind of leaf. Even in respect to ordinary leaves, it is natural to use the word either in a wider or in a narrower sense; as when in one sense we say that a leaf consists of blade and petiole or leaf-stalk, and in another sense say that a leaf is petioled, or that the leaf of Hepatica is three-lobed. The connection should make it plain whether by leaf we mean leaf-blade only, or the blade with any other parts it may have. And the student will readily understand that by leaf in its largest or morphological sense, the botanist means the organ which occupies the place of a leaf, whatever be its form or its function.

Sec. 1. LEAVES AS FOLIAGE.

121. This is tautological; for foliage is simply leaves: but it is very convenient to speak of typical leaves, or those which serve the plant for assimilation, as foliage-leaves, or ordinary leaves. These may first be considered.

122. =The Parts of a Leaf.= The ordinary leaf, complete in its parts, consists of blade, foot-stalk, or petiole, and a pair of stipules.

123. First the BLADE or LAMINA, which is the essential part of ordinary leaves, that is, of such as serve the purpose of foliage. In structure it consists of a softer part, the green pulp, called parenchyma, which is traversed and supported by a fibrous frame, the parts of which are called ribs or veins, on account of a certain likeness in arrangement to the veins of animals. The whole surface is covered by a transparent skin, the Epidermis, not unlike that which covers the surface of all fresh shoots.

124. Note that the leaf-blade expands horizontally,--that is, normally presents its faces one to the sky, the other to the ground, or when the leaf is erect the upper face looks toward the stem that bears it, the lower face away from it. Whenever this is not the case there is something to be explained.

125. The framework consists of wood,--a fibrous and tough material which runs from the stem through the leaf-stalk, when there is one, in the form of parallel threads or bundles of fibres; and in the blade these spread out in a horizontal direction, to form the ribs and veins of the leaf. The stout main branches of the framework are called the Ribs. When there is only one, as in Fig. 112, 114, or a middle one decidedly larger than the rest, it is called the Midrib. The smaller divisions are termed Veins; and their still smaller subdivisions, Veinlets. The latter subdivide again and again, until they become so fine that they are invisible to the naked eye. The fibres of which they are composed are hollow; forming tubes by which the sap is brought into the leaves and carried to every part.

126. =Venation= is the name of the mode of veining, that is, of the way in which the veins are distributed in the blade. This is of two principal kinds; namely, the parallel-veined, and the netted-veined.

127. In Netted-veined (also called Reticulated) leaves, the veins branch off from the main rib or ribs, divide into finer and finer veinlets, and the branches unite with each other to form meshes of network. That is, they anastomose, as anatomists say of the veins and arteries of the body. The Quince-leaf, in Fig. 112, shows this kind of veining in a leaf with a single rib. The Maple, Basswood, Plane or Buttonwood (Fig. 74) show it in leaves of several ribs.

128. In parallel-veined leaves, the whole framework consists of slender ribs or veins, which run parallel with each other, or nearly so, from the base to the point of the leaf,--not dividing and subdividing, nor forming meshes, except by minute cross-veinlets. The leaf of any grass, or that of the Lily of the Valley (Fig. 113) will furnish a good illustration. Such parallel veins Linnaeus called Nerves, and parallel-veined leaves are still commonly called nerved leaves, while those of the other kind are said to be veined,--terms which it is convenient to use, although these "nerves" and "veins" are all the same thing, and have no likeness to the nerves and little to the veins of animals.

129. Netted-veined leaves belong to plants which have a pair of seed-leaves or cotyledons, such as the Maple (Fig. 20, 24), Beech (Fig. 33), and the like; while parallel-veined or nerved leaves belong to plants with one cotyledon or true seed-leaf; such as the Iris (Fig. 59), and Indian Corn (Fig. 70). So that a mere glance at the leaves generally tells what the structure of the embryo is, and refers the plant to one or the other of these two grand classes,--which is a great convenience. For when plants differ from each other in some one important respect, they usually differ correspondingly in other respects also.

130. Parallel-veined leaves are of two sorts,--one kind, and the commonest, having the ribs or nerves all running from the base to the point of the leaf, as in the examples already given; while in another kind they run from a midrib to the margin, as in the common Pickerel-weed of our ponds, in the Banana, in Calla (Fig. 114), and many similar plants of warm climates.

131. Netted-veined leaves are also of two sorts, as in the examples already referred to. In one case the veins all rise from a single rib (the midrib), as in Fig. 112, 116-127. Such leaves are called Feather-veined or Penni-veined, i. e. Pinnately-veined; both terms meaning the same thing, namely, that the veins are arranged on the sides of the rib like the plume of a feather on each side of the shaft.

132. In the other case (as in Fig. 74, 129-132), the veins branch off from three, five, seven, or nine ribs, which spread from the top of the leaf-stalk, and run through the blade like the toes of a web-footed bird. Hence these are said to be Palmately or Digitately veined, or (since the ribs diverge like rays from a centre) Radiate-veined.

133. Since the general outline of leaves accords with the framework or skeleton, it is plain that feather-veined (or penni-veined) leaves will incline to elongated shapes, or at least to be longer than broad; while in radiate-veined leaves more rounded forms are to be expected. A glance at the following figures shows this.

134. =Forms of Leaves as to General Outline.= It is necessary to give names to the principal shapes, and to define them rather precisely, since they afford easy marks for distinguishing species. The same terms are used for all other flattened parts as well, such as petals; so that they make up a great part of the descriptive language of Botany. It will be a good exercise for young students to look up leaves answering to these names and definitions. Beginning with the narrower and proceeding to the broadest forms, a leaf is said to be

Linear (Fig. 115), when narrow, several times longer than wide, and of the same breadth throughout.

Lanceolate, or Lance-shaped, when conspicuously longer than wide, and tapering upwards (Fig. 116), or both upwards and downwards.

Oblong (Fig. 117), when nearly twice or thrice as long as broad.

Elliptical (Fig. 118) is oblong with a flowing outline, the two ends alike in width.

Oval is the same as broadly elliptical, or elliptical with the breadth considerably more than half the length.

Ovate (Fig. 119), when the outline is like a section of a hen's egg lengthwise, the broader end downward.

Orbicular, or Rotund (Fig. 132), circular in outline, or nearly so.

135. A leaf which tapers toward the base instead of toward the apex may be

Oblanceolate (Fig. 121) when of the lance-shaped form, only more tapering toward the base than in the opposite direction.

Spatulate (Fig. 122) when more rounded above, but tapering thence to a narrow base, like an old-fashioned spatula.

Obovate (Fig. 123) or inversely ovate, that is, ovate with the narrower end down.

Cuneate or Cuneiform, that is, Wedge-shaped (Fig. 124), broad above and tapering by nearly straight lines to an acute angle at the base.

136. =As to the Base=, its shape characterizes several forms, such as

Cordate or Heart-shaped (Fig. 120, 129), when a leaf of an ovate form, or something like it, has the outline of its rounded base turned in (forming a notch or sinus) where the stalk is attached.

Reniform, or Kidney-shaped (Fig. 131), like the last, only rounder and broader than long.

Auriculate, or Eared, having a pair of small and blunt projections, or ears, at the base, as in one species of Magnolia (Fig. 126).

Sagittate, or arrow-shaped, where such ears are acute and turned downwards, while the main body of the blade tapers upwards to a point, as in the common Sagittaria or Arrow-head, and in the Arrow-leaved Polygonum (Fig. 125).

Hastate, or Halberd-shaped, when such lobes at the base point outwards, giving the shape of the halberd of the olden time, as in another Polygonum (Fig. 127).

Peltate, or Shield-shaped (Fig. 132), is the name applied to a curious modification of the leaf, commonly of a rounded form, where the footstalk is attached to the lower surface, instead of the base, and therefore is naturally likened to a shield borne by the outstretched arm. The common Watershield, the Nelumbium, and the White Water-lily, and also the Mandrake, exhibit this sort of leaf. On comparing the shield-shaped leaf of the common Marsh Pennywort (Fig. 132) with that of another common species (Fig. 130), it is at once seen that a shield-shaped leaf is like a kidney-shaped (Fig. 130, 131) or other rounded leaf, with the margins at the base brought together and united.

137. =As to the Apex=, the following terms express the principal variations:--

Acuminate, Pointed, or Taper-pointed, when the summit is more or less prolonged into a narrowed or tapering point; as in Fig. 133.

Acute, ending in an acute angle or not prolonged point; Fig. 134.

Obtuse, with a blunt or rounded apex; as in Fig. 135, etc.

Truncate, with the end as if cut off square; as in Fig. 136.

Retuse, with rounded summit slightly indented, forming a very shallow notch, as in Fig. 137.

Emarginate, or Notched, indented at the end more decidedly; as in Fig. 138.

Obcordate, that is, inversely heart-shaped, where an obovate leaf is more deeply notched at the end (Fig. 139), as in White Clover and Wood-sorrel; so as to resemble a cordate leaf inverted.

Cuspidate, tipped with a sharp and rigid point; as in Fig. 140.

Mucronate, abruptly tipped with a small and short point, like a mere projection of the midrib; as in Fig. 141.

Aristate, Awn-pointed, and Bristle-pointed, are terms used when this mucronate point is extended into a longer bristle-form or slender appendage.

The first six of these terms can be applied to the lower as well as to the upper end of a leaf or other organ. The others belong to the apex only.

138. =As to degree and nature of Division=, there is first of all the difference between

Simple Leaves, those in which the blade is of one piece, however much it may be cut up, and

Compound Leaves, those in which the blade consists of two or more separate pieces, upon a common leaf-stalk or support. Yet between these two kinds every intermediate gradation is to be met with.

139. =As to Particular Outlines of Simple Leaves= (and the same applies to their separate parts), they are

Entire, when their general outline is completely filled out, so that the margin is an even line, without teeth or notches.

Serrate, or Saw-toothed, when the margin only is cut into sharp teeth, like those of a saw, and pointing forwards; as in Fig. 142.

Dentate, or Toothed, when such teeth point outwards, instead of forwards; as in Fig. 143.

Crenate, or Scalloped, when the teeth are broad and rounded; as in Fig. 144.

Repand, Undulate, or Wavy, when the margin of the leaf forms a wavy line, bending slightly inwards and outwards in succession; as in Fig. 145.

Sinuate, when the margin is more strongly sinuous or turned inwards and outwards; as in Fig. 146.

Incised, Cut, or Jagged, when the margin is cut into sharp, deep, and irregular teeth or incisions; as in Fig. 147.

Lobed, when deeply cut. Then the pieces are in a general way called LOBES. The number of the lobes is briefly expressed by the phrase two-lobed, three-lobed, five-lobed, many-lobed, etc., as the case may be.

140. When the depth and character of the lobing needs to be more particularly specified, the following terms are employed, viz.:--

Lobed, in a special sense, when the incisions do not extend deeper than about half-way between the margin and the centre of the blade, if so far, and are more or less rounded; as in the leaves of the Post-Oak, Fig. 148, and the Hepatica, Fig. 152.

Cleft, when the incisions extend half way down or more, and especially when they are sharp; as in Fig. 149, 153. And the phrases two-cleft, or, in the Latin form, bifid, three-cleft or trifid, four-cleft or quadrifid, five-cleft or quinquefid, etc., or many-cleft, in the Latin form, multifid,--express the number of the Segments, or portions.

Parted, when the incisions are still deeper, but yet do not quite reach to the midrib or the base of the blade; as in Fig. 150, 154. And the terms two-parted, three-parted, etc., express the number of such divisions.

Divided, when the incisions extend quite to the midrib, as in the lower part of Fig. 151, or to the leaf-stalk, as in Fig. 155; which really makes the leaf compound. Here, using the Latin form, the leaf is said to be bisected, trisected (Fig. 155), etc., according to the number of the divisions.

141. =The Mode of Lobing or Division= corresponds to that of the veining, whether pinnately veined or palmately veined. In the former the notches or incisions, or sinuses, coming between the principal veins or ribs are directed toward the midrib: in the latter they are directed toward the apex of the petiole; as the figures show.

142. So degree and mode of division may be tersely expressed in brief phrases. Thus, in the four upper figures of pinnately veined leaves, the first is said to be pinnately lobed (in the special sense), the second pinnately cleft (or pinnatifid in Latin form), the third pinnately parted, the fourth pinnately divided, or pinnatisected.

143. Correspondingly in the lower row, of palmately veined leaves, the first is palmately lobed, the second palmately cleft, the third palmately parted, the fourth palmately divided. Or, in other language of the same meaning (but now less commonly employed), they are said to be digitately lobed, cleft, parted, or divided.

144. The number of the divisions or lobes may come into the phrase. Thus in the four last named figures the leaves are respectively palmately three-lobed, three-cleft (or trifid), three-parted, three-divided, or better (in Latin form), trisected. And so for higher numbers, as five-lobed, five-cleft, etc., up to many-lobed, many-cleft or multifid, etc. The same mode of expression may be used for pinnately lobed leaves, as pinnately 7-lobed, -cleft, -parted, etc.

145. The divisions, lobes, etc., may themselves be entire (without teeth or notches), or serrate, or otherwise toothed or incised; or lobed, cleft, parted, etc.: in the latter cases making twice pinnatifid, twice palmately or pinnately lobed, parted, or divided leaves, etc. From these illustrations one will perceive how the botanist, in two or three words, may describe any one of the almost endlessly diversified shapes of leaves, so as to give a clear and definite idea of it.

146. =Compound Leaves.= A compound leaf is one which has its blade in entirely separate parts, each usually with a stalklet of its own; and the stalklet is often jointed (or articulated) with the main leaf-stalk, just as this is jointed with the stem. When this is the case, there is no doubt that the leaf is compound. But when the pieces have no stalklets, and are not jointed with the main leaf-stalk, it may be considered either as a divided simple leaf, or a compound leaf, according to the circumstances. This is a matter of names where all intermediate forms may be expected.

147. While the pieces or projecting parts of a simple leaf-blade are called Lobes, or in deeply cut leaves, etc., Segments, or Divisions, the separate pieces or blades of a compound leaf are called LEAFLETS.

148. Compound leaves are of two principal kinds, namely, the Pinnate and the Palmate; answering to the two modes of veining in reticulated leaves, and to the two sorts of lobed or divided leaves (141).

149. Pinnate leaves are those in which the leaflets are arranged on the sides of a main leaf-stalk; as in Fig. 156-158. They answer to the feather-veined (i. e. pinnately-veined) simple leaf; as will be seen at once on comparing the forms. The leaflets of the former answer to the lobes or divisions of the latter; and the continuation of the petiole, along which the leaflets are arranged, answers to the midrib of the simple leaf.

150. Three sorts of pinnate leaves are here given. Fig. 156 is pinnate with an odd or end leaflet, as in the Common Locust and the Ash. Fig. 157 is pinnate with a tendril at the end, in place of the odd leaflet, as in the Vetches and the Pea. Fig. 158 is evenly or abruptly pinnate, as in the Honey-Locust.

151. Palmate (also named Digitate) leaves are those in which the leaflets are all borne on the tip of the leaf-stalk, as in the Lupine, the Common Clover, the Virginia Creeper (Fig. 93), and the Horse-chestnut and Buckeye (Fig. 159). They evidently answer to the radiate-veined or palmately-veined simple leaf. That is, the Clover-leaf of three leaflets is the same as a palmately three-ribbed leaf cut into three separate leaflets. And such a simple five-lobed leaf as that of the Sugar Maple, if more cut, so as to separate the parts, would produce a palmate leaf of five leaflets, like that of the Horse-chestnut or Buckeye.

152. Either sort of compound leaf may have any number of leaflets; yet palmate leaves cannot well have a great many, since they are all crowded together on the end of the main leaf-stalk. Some Lupines have nine or eleven; the Horse-chestnut has seven, the Sweet Buckeye more commonly five, the Clover three. A pinnate leaf often has only seven or five leaflets, or only three, as in Beans of the genus Phaseolus, etc.; in some rarer cases only two; in the Orange and Lemon and also in the common Barberry there is only one! The joint at the place where the leaflet is united with the petiole distinguishes this last case from a simple leaf. In other species of these genera the lateral leaflets also are present.

153. The leaflets of a compound leaf may be either entire (as in Fig. 126-128), or serrate, or lobed, cleft, parted, etc.; in fact, may present all the variations of simple leaves, and the same terms equally apply to them.

154. When the division is carried so far as to separate what would be one leaflet into two, three, or several, the leaf becomes doubly or twice compound, either pinnately or palmately, as the case may be. For example, while the clustered leaves of the Honey-Locust are simply pinnate, that is, once pinnate, those on new shoots are bipinnate, or twice pinnate, as in Fig. 160. When these leaflets are again divided in the same way, the leaf becomes thrice pinnate, or tripinnate, as in many Acacias. The first divisions are called Pinnae; the others, Pinnules; and the last, or little blades themselves, Leaflets.

155. So the palmate leaf, if again compounded in the same way, becomes twice palmate, or, as we say when the divisions are in threes, twice ternate (in Latin form biternate); if a third time compounded, thrice ternate or triternate. But if the division goes still further, or if the degree is variable, we simply say that the leaf is decompound; either palmately or pinnately decompound, as the case may be. Thus, Fig. 161 represents a four times ternately compound (in other words a ternately decompound) leaf of a common Meadow Rue.

156. When the botanist, in describing leaves, wishes to express the number of the leaflets, he may use terms like these:--

Unifoliolate, for a compound leaf of a single leaflet; from the Latin unum, one, and foliolum, leaflet.

Bifoliolate, of two leaflets, from the Latin bis, twice, and foliolum, leaflet.

Trifoliolate (or ternate), of three leaflets, as the Clover; and so on.

Palmately bifoliolate, trifoliolate, quadrifoliolate, plurifoliolate (of several leaflets), etc.: or else

Pinnately bi-, tri-, quadri-, or plurifoliolate (that is, of two, three, four, five, or several leaflets), as the case may be: these are terse ways of denoting in single phrases both the number of leaflets and the kind of compounding.

157. Of foliage-leaves having certain peculiarities in structure, the following may be noted:--

158. =Perfoliate Leaves.= In these the stem that bears them seems to run through the blade of the leaf, more or less above its base. A common Bellwort (Uvularia perfoliata, Fig. 162) is a familiar illustration. The lower and earlier leaves show it distinctly. Later, the plant is apt to produce some leaves merely clasping the stem by the sessile and heart-shaped base, and the latest may be merely sessile. So the series explains the peculiarity: in the formation of the leaf the bases, meeting around the stem, grow together there.

159. =Connate-perfoliate.= Such are the upper leaves of true Honeysuckles. Here (Fig. 163) of the opposite and sessile leaves, some pairs, especially the uppermost, in the course of their formation unite around the stem, which thus seems to run through the disk formed by their union.

160. =Equitant Leaves.= While ordinary leaves spread horizontally, and present one face to the sky and the other to the earth, there are some that present their tip to the sky, and their faces right and left to the horizon. Among these are the equitant leaves of the Iris or Flower-de-Luce. Inspection shows that each leaf was formed as if folded together lengthwise, so that what would be the upper surface is within, and all grown together, except next the bottom, where each leaf covers the next younger one. It was from their straddling over each other, like a man on horseback (as is seen in the cross-section, Fig. 165), that Linnaeus, with his lively fancy, called these Equitant leaves.

161. =Leaves with no distinction of Petiole and Blade.= The leaves of Iris just mentioned show one form of this. The flat but narrow leaves of Jonquils, Daffodils, and the cylindrical leaf of Onions are other instances. Needle-shaped leaves, like those of the Pine, Larch, and Spruce, and the awl-shaped as well as the scale-shaped leaves of Junipers, Red Cedar, and Arbor-Vitae (Fig. 166), are examples.

162. =Phyllodia.= Sometimes an expanded petiole takes the place of the blade; as in numerous New Holland Acacias, some of which are now common in greenhouses. Such counterfeit blades are called phyllodia,--meaning leaf-like bodies. They may be known from true blades by their standing edgewise, their margins being directed upwards and downwards; while in true blades the faces look upwards and downwards; excepting in equitant leaves, as already explained.

163. =Falsely Vertical Leaves.= These are apparent exceptions to the rule, the blade standing edgewise instead of flatwise to the stem; but this position comes by a twist of the stalk or the base of the blade. Such leaves present the two faces about equally to the light. The Compass-plant (Silphium laciniatum) is an example. So also the leaves of Boltonia, of Wild Lettuce, and of a vast number of Australian Myrtaceous shrubs and trees, which much resemble the phyllodia of the Acacias of the same country. They are familiar in Callistemon, the Bottle-brush Flower, and in Eucalyptus. But in the latter the leaves of the young tree have the normal structure and position.

164. =Cladophylla=, meaning branch-leaves. The foliage of Ruscus (the Butcher's Broom of Europe) and of Myrsiphyllum of South Africa (cultivated for decoration under the false name of Smilax) is peculiar and puzzling. If these blades (Fig. 167, 168) are really leaves, they are most anomalous in occupying the axil of another leaf, reduced to a little scale. Yet they have an upper and lower face, as leaves should, although they soon twist, so as to stand more or less edgewise. If they are branches which have assumed exactly the form and office of leaves, they are equally extraordinary in not making any further development. But in Ruscus, flowers are borne on one face, in the axil of a little scale: and this would seem to settle that they are branches. In Asparagus just the same things as to position are thread-shaped and branch-like.

Sec. 2. LEAVES OF SPECIAL CONFORMATION AND USE.

165. =Leaves for Storage.= A leaf may at the same time serve both ordinary and special uses. Thus in those leaves of Lilies, such as the common White Lily, which spring from the bulb, the upper and green part serves for foliage and elaborates nourishment, while the thickened portion or bud-scale beneath serves for the storage of this nourishment. The thread-shaped leaf of the Onion fulfils the same office, and the nourishing matter it prepares is deposited in its sheathing base, forming one of the concentric layers of the onion. When these layers, so thick and succulent, have given up their store to the growing parts within, they are left as thin and dry husks. In a Houseleek, an Aloe or an Agave, the green color of the surface of the fleshy leaf indicates that it is doing the work of foliage; the deeper-seated white portion within is the storehouse of the nourishment which the green surface has elaborated. So, also, the seed-leaves or cotyledons are commonly used for storage. Some, as in one of the Maples, the Pea, Horse-chestnut, Oak, etc., are for nothing else. Others, as in Beech and in our common Beans, give faint indications of service as foliage also, chiefly in vain. Still others, as in the Pumpkin and Flax, having served for storage, develop into the first efficient foliage. Compare 11, 22-30, and the accompanying figures.

166. =Leaves as Bud-Scales= serve to protect the forming parts within. Having fulfilled this purpose they commonly fall off when the shoot develops and foliage-leaves appear. Occasionally, as in Fig. 170, there is a transition of bud-scales to leaves, which reveals the nature of the former. The Lilac also shows a gradation from bud-scale to simple leaf. In Cornus florida (the Flowering Dogwood), the four bud-scales which through the winter protect the head of forming flowers remain until blossoming, and then the base of each grows out into a large and very showy petal-like leaf; the original dry scale is apparent in the notch at the apex.

167. =Leaves as Spines= occur in several plants. A familiar instance is that of the common Barberry (Fig. 171). In almost any summer shoot, most of the gradations may be seen between the ordinary leaves, with sharp bristly teeth, and leaves which are reduced to a branching spine or thorn. The fact that the spines of the Barberry produce a leaf-bud in their axil also proves them to be leaves.

168. =Leaves for Climbing= are various in adaptation. True foliage-leaves serve this purpose; as in Gloriosa, where the attenuated tip of a simple leaf (otherwise like that of a Lily) hooks around a supporting object; or in Solanum jasminoides of the gardens (Fig. 172), and in Maurandia, etc., where the leaf-stalk coils round and clings to a support; or in the compound leaves of Clematis and of Adlumia, in which both the leaflets and their stalks hook or coil around the support.

169. Or in a compound leaf, as in the Pea and most Vetches, and in Cobaea, while the lower leaflets serve for foliage, some of the uppermost are developed as tendrils for climbing (Fig. 167). In the common Pea this is so with all but one or two pairs of leaflets.

170. In one European Vetch, the leaflets are wanting and the whole petiole is a tendril, while the stipules become the only foliage (Fig. 173).

171. =Leaves as Pitchers=, or hollow tubes, are familiar in the common Pitcher-plant or Side-saddle Flower (Sarracenia, Fig. 174) of our bogs. These pitchers are generally half full of water, in which flies and other insects are drowned, often in such numbers as to make a rich manure for the plant. More curious are some of the southern species of Sarracenia, which seem to be specially adapted to the capture and destruction of flies and other insects.

172. The leaf of Nepenthes (Fig. 175) combines three structures and uses. The expanded part below is foliage: this tapers into a tendril for climbing; and this bears a pitcher with a lid. Insects are caught, and perhaps digested, in the pitcher.

173. =Leaves as Fly-traps.= Insects are caught in another way, and more expertly, by the most extraordinary of all the plants of this country, the Dionaea or Venus's Fly-trap, which grows in the sandy bogs around Wilmington, North Carolina. Here (Fig. 176) each leaf bears at its summit an appendage which opens and shuts, in shape something like a steel-trap, and operating much like one. For when open, no sooner does a fly alight on its surface, and brush against any one of the two or three bristles that grow there, than the trap suddenly closes, capturing the intruder. If the fly escapes, the trap soon slowly opens, and is ready for another capture. When retained, the insect is after a time moistened by a secretion from minute glands of the inner surface, and is digested. In the various species of Drosera or Sundew, insects are caught by sticking fast to very viscid glands at the tip of strong bristles, aided by adjacent gland-tipped bristles which bend slowly toward the captive. The use of such adaptations and operations may be explained in another place.

Sec. 3. STIPULES.

174. A leaf complete in its parts consists of blade, leaf-stalk or petiole, and a pair of stipules. But most leaves have either fugacious or minute stipules or none at all; many have no petiole (the blade being sessile or stalkless); some have no clear distinction of blade and petiole; and many of these, such as those of the Onion and all phyllodia (166), consist of petiole only.

175. The base of the petiole is apt to be broadened and flattened, sometimes into thin margins, sometimes into a sheath which embraces the stem at the point of attachment.

176. =Stipules= are such appendages, either wholly or partly separated from the petiole. When quite separate they are said to be free, as in Fig. 112. When attached to the base of the petiole, as in the Rose and in Clover (Fig. 177), they are adnate. When the two stipules unite and sheathe the stem above the insertion, as in Polygonum (Fig. 178), this sheath is called an Ocrea from its likeness to a greave or leggin.

177. In Grasses, when the sheathing base of the leaf may answer to petiole, the summit of the sheath commonly projects as a thin and short membrane, like an ocrea: this is called a LIGULA or LIGULE.

178. When stipules are green and leaf-like they act as so much foliage. In the Pea they make up no small part of the actual foliage. In a related plant (Lathyrus Aphaca, Fig. 173), they make the whole of it, the remainder of the leaf being tendril.

179. In many trees the stipules are the bud-scales, as in the Beech, and very conspicuously in the Fig-tree, Tulip-tree, and Magnolia (Fig. 179). These fall off as the leaves unfold.

180. The stipules are spines or prickles in Locust and several other Leguminous trees and shrubs; they are tendrils in Smilax or Greenbrier.

Sec. 4. THE ARRANGEMENT OF LEAVES.

181. =Phyllotaxy=, meaning leaf-arrangement, is the study of the position of leaves, or parts answering to leaves, upon the stem.

182. The technical name for the attachment of leaves to the stem is the insertion. Leaves (as already noticed, 54) are inserted in three modes. They are

Alternate (Fig. 181), that is, one after another, or in other words, with only a single leaf to each node;

Opposite (Fig. 182), when there is a pair to each node, the two leaves in this case being always on opposite sides of the stem;

Whorled or Verticillate (Fig. 183) when there are more than two leaves on a node, in which case they divide the circle equally between them, forming a Verticel or whorl. When there are three leaves in the whorl, the leaves are one third of the circumference apart; when four, one quarter, and so on. So the plan of opposite leaves, which is very common, is merely that of whorled leaves, with the fewest leaves to the whorl, namely, two.

183. In both modes and in all their modifications, the arrangement is such as to distribute the leaves systematically and in a way to give them a good exposure to the light.

184. No two or more leaves ever grow from the same point. The so-called Fascicled or Clustered leaves are the leaves of a branch the nodes of which are very close, just as they are in the bud, so keeping the leaves in a cluster. This is evident in the Larch (Fig. 184), in which examination shows each cluster to be made up of numerous leaves crowded on a spur or short axis. In spring there are only such clusters; but in summer some of them lengthen into ordinary shoots with scattered alternate leaves. So, likewise, each cluster of two or three needle-shaped leaves in Pitch Pines (as in Fig. 185), or of five leaves in White Pine, answers to a similar extremely short branch, springing from the axil of a thin and slender scale, which represents a leaf of the main shoot. For Pines produce two kinds of leaves,--1. primary, the proper leaves of the shoots, not as foliage, but in the shape of delicate scales in spring, which soon fall away; and 2. secondary, the fascicled leaves, from buds in the axils of the former, and these form the actual foliage.

185. =Phyllotaxy of Alternate Leaves.= Alternate leaves are distributed along the stem in an order which is uniform for each species. The arrangement in all its modifications is said to be spiral, because, if we draw a line from the insertion (i. e. the point of attachment) of one leaf to that of the next, and so on, this line will wind spirally around the stem as it rises, and in the same species will always bear the same number of leaves for each turn round the stem. That is, any two successive leaves will always be separated from each other by an equal portion of the circumference of the stem. The distance in height between any two leaves may vary greatly, even on the same shoot, for that depends upon the length of the internodes, or spaces between the leaves; but the distance as measured around the circumference (in other words, the Angular Divergence, or angle formed by any two successive leaves) is uniformly the same.

186. =Two-ranked.= The greatest possible divergence is, of course, where the second leaf stands on exactly the opposite side of the stem from the first, the third on the side opposite the second, and therefore over the first, and the fourth over the second. This brings all the leaves into two ranks, one on one side of the stem and one on the other, and is therefore called the Two-ranked arrangement. It occurs in all Grasses,--in Indian Corn, for instance; also, in the Basswood (Fig. 181). This is the simplest of all arrangements, and the one which most widely distributes successive leaves, but which therefore gives the fewest vertical ranks. Next is the

187. =Three-ranked= arrangement,--that of all Sedges, and of White Hellebore. Here the second leaf is placed one third of the way round the stem, the third leaf two thirds of the way round, the fourth leaf accordingly directly over the first, the fifth over the second, and so on. That is, three leaves occur in each turn round the stem, and they are separated from each other by one third of the circumference. (Fig. 186, 187.)

188. =Five-ranked= is the next in the series, and the most common. It is seen in the Apple (Fig. 188), Cherry, Poplar, and the greater number of trees and shrubs. In this case the line traced from leaf to leaf will pass twice round the stem before it reaches a leaf situated directly over any below (Fig. 189). Here the sixth leaf is over the first; the leaves stand in five perpendicular ranks, with equal angular distance from each other; and this distance between any two successive leaves is just two fifths of the circumference of the stem.

189. The five-ranked arrangement is expressed by the fraction 2/5. This fraction denotes the divergence of the successive leaves, i. e. the angle they form with each other: the numerator also expresses the number of turns made round the stem by the spiral line in completing one cycle or set of leaves, namely, two; and the denominator gives the number of leaves in each cycle, or the number of perpendicular ranks, namely, five. In the same way the fraction 1/2 stands for the two-ranked mode, and 1/3 for the three-ranked: and so these different sorts are expressed by the series of fractions 1/2, 1/3, 2/5. Other cases follow in the same numerical progression, the next being the

190. =Eight-ranked= arrangement. In this the ninth leaf stands over the first, and three turns are made around the stem to reach it; so it is expressed by the fraction 3/8. This is seen in the Holly, and in the common Plantain. Then comes the

191. =Thirteen-ranked= arrangement, in which the fourteenth leaf is over the first, after five turns around the stem. The common Houseleek (Fig. 191) is a good example.

192. The series so far, then, is 1/2, 1/3, 2/5, 3/8, 5/13; the numerator and the denominator of each fraction being those of the two next preceding ones added together. At this rate the next higher should be 8/21, then 13/34, and so on; and in fact just such cases are met with, and (commonly) no others. These higher sorts are found in the Pine Family, both in the leaves and the cones and in many other plants with small and crowded leaves. But in those the number of the ranks, or of leaves in each cycle, can only rarely be made out by direct inspection. They may be indirectly ascertained, however, by studying the secondary spirals, as they are called, which usually become conspicuous, at least two series of them, one turning to the right and one to the left, as shown in Fig. 191. For an account of the way in which the character of the phyllotaxy may be deduced from the secondary spirals, see Structural Botany, Chapter IV.

193. =Phyllotaxy of Opposite and whorled Leaves.= This is simple and comparatively uniform. The leaves of each pair or whorl are placed over the intervals between those of the preceding, and therefore under the intervals of the pair or whorl next above. The whorls or pairs alternate or cross each other, usually at right angles, that is, they decussate. Opposite leaves, that is, whorls of two leaves only, are far commoner than whorls of three or four or more members. This arrangement in successive decussating pairs gives an advantageous distribution on the stem in four vertical ranks. Whorls of three give six vertical ranks, and so on. Note that in descriptive botany leaves in whorls of two are simply called opposite leaves; and that the term verticillate or whorled, is employed only for cases of more than two, unless the latter number is specified.

194. =Vernation or Praefoliation=, the disposition of the leaf-blades in the bud, comprises two things; 1st, the way in which each separate leaf is folded, coiled, or packed up in the bud; and 2d, the arrangement of the leaves in the bud with respect to one another. The latter of course depends very much upon the phyllotaxy, i. e. the position and order of the leaves upon the stem. The same terms are used for it as for the arrangement of the leaves of the flower in the flower-bud. See, therefore, "AEstivation, or Praefloration."

195. As to each leaf separately, it is sometimes straight and open in vernation, but more commonly it is either bent, folded, or rolled up. When the upper part is bent down upon the lower, as the young blade in the Tulip-tree is bent upon the leaf-stalk, it is said to be Inflexed or Reclined in vernation. When folded by the midrib so that the two halves are placed face to face, it is Conduplicate (Fig. 193), as in the Magnolia, the Cherry, and the Oak. When folded back and forth like the plaits of a fan, it is Plicate or Plaited (Fig. 194), as in the Maple and Currant. If rolled, it may be so either from the tip downwards, as in Ferns and the Sundew (Fig. 197), when in unrolling it resembles the head of a crosier, and is said to be Circinate; or it may be rolled up parallel with the axis, either from one edge into a coil, when it is Convolute (Fig. 195), as in the Apricot and Plum; or rolled from both edges towards the midrib,--sometimes inwards, when it is Involute (Fig. 198), as in the Violet and Water-Lily; sometimes outwards, when it is Revolute (Fig. 196), in the Rosemary and Azalea. The figures are diagrams, representing sections through the leaf, in the way they were represented by Linnaeus.