Page:EB1911 - Volume 16.djvu/348

pass either from right to left or from left to right. It sometimes follows a different direction in the branches from that pursued in the stem. When it follows the same course in the stem and branches, they are homodromous; when the direction differs, they are heterodromous. In different species of the same genus the phyllotaxis frequently varies.

All modifications of leaves follow the same laws of arrangement as true leaves—a fact which is of importance in a morphological point of view. In dicotyledonous plants the first leaves produced (the cotyledons) are opposite. This arrangement often continues during the life of the plant, but at other times it changes, passing into distichous and spiral forms. Some tribes of plants are distinguished Fig. 33.—Diagram of a phyllotaxis represented by the fraction 2/5. by their opposite or verticillate, others by their alternate, leaves. Labiate plants have decussate leaves, while Boraginaceae have alternate leaves, and Tiliaceae usually have distichous leaves; Rubiaceae have opposite leaves. Such arrangements as 2/5, 3/8, 5/13 and 8/21 are common in Dicotyledons. The first of these, called a quincunx, is met with in the apple, pear and cherry (fig. 32); the second, in the bay, holly, Plantago media; the third, in the cones of Picea alba (fig. 35); and the fourth in those of the silver fir. In monocotyledonous plants there is only one seed-leaf or cotyledon, and hence the arrangement is at first alternate; and it generally continues so more or less, rarely being verticillate. Such arrangements as 1/2, 1/3 and 2/3 are common in Monocotyledons, as in grasses, sedges and lilies. It has been found in general that, while the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in that of Monocotyledons.

Fig. 34.—Cycle of thirteen leaves placed closely together so as to form a rosette, as in Sempervivum. A is the very short axis to which the leaves are attached. The leaves are numbered in their order, from below upwards. The circles in the centre indicate the five turns of the spiral, and show the insertion of each of the leaves. The divergence is expressed by the fraction 5/13ths.

Fig. 35.—Cone of Picea alba with the scales or modified leaves numbered in the order of their arrangement on the axis of the cone. The lines indicate a rectilinear series of scales and two lateral secondary spirals, one turning from left to right, the other from right to left.

In the axil of previously formed leaves leaf-buds arise. These leaf-buds contain the rudiments of a shoot, and consist of leaves covering a growing point. The buds of trees of temperate climates, which lie dormant during the winter, are protected by scale leaves. These scales or protective appendages of the bud consist either of the altered laminae or of the enlarged petiolary sheath, or of stipules, as in the fig and magnolia, or of one or two of these parts combined. These are often of a coarse nature, serving a temporary purpose, and then falling off when the leaf is expanded. They are frequently covered with a resinous matter, as in balsam-poplar and horse-chestnut, or by a thick downy covering as in the willow. In plants of warm climates the buds have often no protective appendages, and are then said to be naked.

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Fig. 36.		Fig. 37.		Fig. 38.
Fig. 39.		Fig. 40.		Fig. 41.

Fig. 36.—Circinate vernation.
Fig. 37.—Transverse section of a conduplicate leaf.
Fig. 38.—Transverse section of a plicate or plaited leaf.
Fig. 39.—Transverse section of a convolute leaf.
Fig. 40.—Transverse section of an involute leaf.
Fig. 41.—Transverse section of a revolute leaf.

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Fig. 42.		Fig. 43.		Fig. 44.		Fig. 45.

Fig. 42.—Transverse section of a bud, in which the leaves are arranged in an accumbent manner.

Fig. 43.—Transverse section of a bud, in which the leaves are arranged in an equitant manner.

Fig. 44.—Transverse section of a bud, showing two leaves folded in an obvolute manner. Each is conduplicate, and one embraces the edge of the other.

Fig. 45.—Transverse section of a bud, showing two leaves arranged in a supervolute manner.

The arrangement of the leaves in the bud is termed vernation or prefoliation. In considering vernation we must take into account both the manner in which each individual leaf is folded and also the arrangement of the leaves in relation to each other. These vary in different plants, but in each species they follow a regular law. The leaves in the bud are either placed simply in apposition, as in the mistletoe, or they are folded or rolled up longitudinally or laterally, giving rise to different kinds of vernation, as delineated in figs. 36 to 45, where the folded or curved lines represent the leaves, the thickened part being the midrib. The leaf taken individually is either folded longitudinally from apex to base, as in the tulip-tree, and called reclinate or replicate; or rolled up in a circular manner from apex to base, as in ferns (fig. 36), and called circinate; or folded laterally, conduplicate (fig. 37), as in oak; or it has several folds like a fan, plicate or plaited (fig. 38), as in vine and sycamore, and in leaves with radiating vernation, where the ribs mark the foldings; or it is rolled upon itself, convolute (fig. 39), as in banana and apricot; or its edges are rolled inwards, involute (fig. 40), as in violet; or outwards, revolute (fig. 41), as in rosemary. The different divisions of a cut leaf may be folded or rolled up separately, as in ferns, while the entire leaf may have either the same or a different kind of vernation. The leaves have a definite relation to each other in the bud, being either opposite, alternate or verticillate; and thus different kinds of vernation are produced. Sometimes they are nearly in a circle at the same level, remaining flat or only slightly convex externally, and placed so as to touch each other by their edges, thus giving rise to valvate vernation. At other times they are at different levels, and are applied over each other, so as to be imbricated, as in lilac, and in the outer scales of sycamore; and occasionally the margin of one leaf overlaps that of another, while it in its turn is overlapped by a third, so as to be twisted, spiral or contortive. When leaves are applied to each other face to face, without being folded or rolled together, they are appressed. When the leaves are more completely folded they either touch at their extremities and are accumbent or opposite (fig. 42), or are folded inwards by their margin and become induplicate; or a conduplicate leaf covers another similarly folded, which in turn covers a third, and thus the vernation is equitant (fig. 43), as in privet; or conduplicate leaves are placed so that the half of the one covers the half of another, and thus they become half-equitant or obvolute (fig. 44), as in sage. When in the case of convolute leaves one leaf is rolled up within the other, it is supervolute (fig. 45). The scales of a bud sometimes exhibit one kind of vernation and the leaves another. The same modes of arrangement occur in the flower-buds.

Leaves, after performing their functions for a certain time, wither and die. In doing so they frequently change colour, and hence arise the beautiful and varied tints of the autumnal foliage. This change