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 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. |
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| 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.