THE LEAF. 



Fig. 52. 



the leaves overlap more or less along the developed axis ; such 

 leaves are termed imbricate ; and this condition is verv common in 

 the leaf-scale forms of the leaf. A great number of well-developed 

 leaves are often crowded together by the non-development of inter- 

 nodes at the base of the flowering stems of perennial herbs, such as 

 the various Saxifrages, the Turnip, Dandelion, 

 &c. ; and where these so-called " radical " leaves 

 are arranged with some regularity, and spread 

 out horizontally as in the House-leeks, they are 

 said to be tufted, cwspitose or rosulate (fig. 50). 



A somewhat similar condition occurs upon 

 branches of some trees, on which a number of 

 leaves appear to spring from one point, as in the 

 Larch (tig. 52) and the Berberry; the collections 

 of fasciculate leaves really belong to a branch 

 the internodes of which are not developed, so 

 that they all spring at once from the leaf -axil 

 in which the branch-bud was formed. 



Fasciculate leaves of 

 the Larch. 



Fig. 53. 



In other Conifers the number of leaves in these bundles is smaller and 

 very regular and characteristic ; e. (/., in Pinns sylvestris two leaves are 

 thus associated, in P. Cembra three, in P. Strobus five, &c. In those buds 

 of the Larch which afterwards unfold into shoots, the transition from a 

 fasciculate into a regular spiral arrangement becomes evident. 



Opposite and wJiorled leaves likewise exhibit great regularity. 

 The number of leaves in a whorl is here also 

 sometimes expressed by a fraction, which is 

 enclosed in a parenthesis ; the denominator 

 in this case indicates the number of leaves in 

 one circle. 



Examples of those in true leaves are furnished 

 by the following plants : 



plan (opposite leaves). Pinks, Labiatse. 

 Lysimachia vulgaris, Trillium. 

 Paris qiiadrifolia. 

 () Myriophyllum pectinatum. 

 Sometimes the numbers vary on different parts of 

 the same stem, as in TLippuris. 



When leaves are opposite, the pairs are 

 almost invariably alternate * ; that is, they 

 cross at right angles, the third pair standing over the first. Such 

 leaves are called decussate (fig. 53). With whorls of three leaves, 



(I) 

 Q-) 



Diagram of decussating 

 pairs of leaves. 



An exception is seen in Potamogcton. 



