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.
Fig. 184. A piece of stem of Larch with two clusters (fascicles) of numerous leaves.
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 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.