BOTANY [NUTRITIVE ORGANS. verticillate leaves. When the interruption to development takes place at the end of a branch, the leaves become fasci culate or clustered, as in the Larch. The primitive or generating spiral may pass either from right to left or from left to right. It sometimes follows a different direc tion in the branches from that pursued in the stem. When it follows the same course in the stem and branches, they are komodromous ; 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 arrange ment as true leaves a fact which is of importance in a mor phological point of view. In Dicotyledonous plants the first leaves produced (the cotyledons) are opposite, in some cases verticillate. 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 dis tinguished by their opposite or verticillate, others by their alternate, leaves. Labiate plants have decussate leaves, while Boraginacea) have alternate leaves, and Tiliacece usually have distichous leaves ; Cinchonacese have opposite leaves ; Galiaceae, verticillate. Such arrangements as f, -| , -~ s , and TT 8 T are common in Dicotyledons. The first of these, called a quincunx, is met with in the Apple, Pear, and Cherry (fig. 130) ; the second, in the Bay, Holly, Plantago media; the third, in the cones of Pinus (Abies) alba (fig. 133); and the fourth in those of the Pinus (Abies) Picea. In Mono- cotyledonous plants there is only one seed-leaf or cotyledon produced, and hence the arrangement is at first alternate ; and it generally continues so more or less, rarely being ver ticillate. Such arrangements as , -|, and -| are common in Monocotyledons, as in Grasses, Sedges, and Lilies. In Aco- tyledons the leaves assume all kinds of arrangement, being opposite, alternate, and verticillate. It has been found in general that, while the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in that of Monocotyledons. 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 cataphyllary leaves constituting the tegmenta- or peruke. These scales or protective append ages 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. The arrangement of the leaves in the bud has been denominated vernation, prefoliation, and gemmation. 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 Mistleto, or they are folded or rolled up longitudinally or laterally, giving rise to different kinds of vernation, as delineated m figs. 135 to 144, where the folded or curved lines represent the leaves, the thickened part being the midrib. The leaf taken individually is cither 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. 135), and called circinate ; or folded laterally, conduplicate (fig. 136), as in Oak ; or it has several folds like a fan, plicate or plaited (fig. 137), 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. 138), as in Banana and Apricot; or its edges are rolled inwards, involute (fig. 139), as in Violet; or outwards, revolute (fig. 140), as in Kosemary. The different divisions of a cut Fig. 136. Fig. 138. Fig. 139. FIG. 135. Circinate vernation. FIG. 13fi. Transverse section of a conduplicate leaf. FIG. 137. Transverse section of a plicate or plaited leaf. FIG. 138. Transverse section of a convolute leaf. FIG. 139. Transverse section of an involute leaf. FIG. 140. Transverse section of a revolute leaf. 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 pro duced. Sometimes they are nearly in a circle at the samo 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 (fig. 63); 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. 141), 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. 142), as in Privet; or conduplicate leaves are placed so that the half of the one covers the half Fig. 141. Fig. 142. Fig. 143. Fig. 144. FIG. 141. Transverse section of a bud, in which the leaves are arranged in an accumbent manner. FIG. 142. Transverse section of a bud, in which the leaves are arranged in an equitant manner. FIG. 143. Transverse section of a bud, showing two leaves folded in an obvolute manner. Each is condnplicate, and one embraces the edge of the other. FIG. 144. Transverse section of a bud, showing two leaves arranged in a super- volute manner. of another, and thus they become half-equitant or obvo lute (fig. 143), as in Sage. When in the case of con volute leaves one leaf is rolled up within the other, it is supervolute (fig. 144). 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, as will be after wards shown.
Leaves expose the fluids of plants to the influence of air