The Elements of Botany Part 8

[Ill.u.s.tration: Fig. 175. Leaf of Nepenthes; foliage, tendril, and pitcher combined.]

[Ill.u.s.tration: Fig. 176. Leaves of Dionaea; the trap in one of them open, in the others closed.]

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.

-- 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.

[Ill.u.s.tration: Fig. 177. Leaf of Red Clover: _st_, stipules, adhering to the base of _p_, the petiole; _b_, blade of three leaflets.]

[Ill.u.s.tration: Fig. 178. Part of stem and leaf of Prince's-Feather (Polygonum orientale) with the united sheathing stipules forming a sheath or _ocrea_.]

[Ill.u.s.tration: Fig. 179. Terminal winter bud of Magnolia Umbrella, natural size. 180. Outermost bud-scale (pair of stipules) detached.]

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 Gra.s.ses, 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 p.r.i.c.kles in Locust and several other Leguminous trees and shrubs; they are tendrils in Smilax or Greenbrier.

-- 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.

[Ill.u.s.tration: Fig. 181. Alternate leaves, in Linden, Lime-tree, or Ba.s.swood.]

[Ill.u.s.tration: Fig. 182. Opposite leaves, in Red Maple.]

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 circ.u.mference 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.

[Ill.u.s.tration: Fig. 183. Whorled leaves of Galium.]

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.

[Ill.u.s.tration: Fig. 184. A piece of stem of Larch with two cl.u.s.ters (fascicles) of numerous leaves.]

[Ill.u.s.tration: Fig. 185. Piece of a branch of Pitch Pine, with three leaves in a fascicle or bundle, in the axil of a thin scale which answers to a primary leaf. The bundle is surrounded at the base by a short sheath, formed of the delicate scales of the axillary bud.]

184. No two or more leaves ever grow from the same point. The so-called _Fascicled_ or _Cl.u.s.tered_ 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 cl.u.s.ter. This is evident in the Larch (Fig. 184), in which examination shows each cl.u.s.ter to be made up of numerous leaves crowded on a spur or short axis. In spring there are only such cl.u.s.ters; but in summer some of them lengthen into ordinary shoots with scattered alternate leaves. So, likewise, each cl.u.s.ter 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 circ.u.mference 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 s.p.a.ces between the leaves; but the distance as measured around the circ.u.mference (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 Gra.s.ses,--in Indian Corn, for instance; also, in the Ba.s.swood (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 h.e.l.lebore. 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 circ.u.mference. (Fig.

186, 187.)

[Ill.u.s.tration: Fig. 186. Two-ranked arrangement, shown in a piece of the stalk of a Sedge, with the leaves cut off above their bases; the leaves are numbered in order, from 1 to 6. 187. Diagram or cross-section of the same, in one plane; the leaves similarly numbered; showing two cycles of three.]

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 pa.s.s 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 circ.u.mference of the stem.

[Ill.u.s.tration: Fig. 188. Shoot with its leaves 5-ranked, the sixth leaf over the first; as in the Apple-tree.]

[Ill.u.s.tration: Fig. 189. Diagram of this arrangement, with a spiral line drawn from the attachment of one leaf to the next, and so on; the parts on the side turned from the eye are fainter.]

[Ill.u.s.tration: Fig. 190. A ground-plan of the same; the section of the leaves similarly numbered; a dotted line drawn from the edge of one leaf to that of the next marks out the spiral.]

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.

[Ill.u.s.tration: Fig. 191. A young plant of the Houseleek, with the leaves (not yet expanded) numbered, and exhibiting the 13-ranked arrangement; and showing secondary spirals.]

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.

[Ill.u.s.tration: Fig. 192. Opposite leaves of Euonymus, or Spindle-tree, showing the successive pairs crossing each other at right angles.]

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.