CHAPTER III. THE LEAF. 23 



Even where the leaves are compactly clustered or fascicled, as 

 in the Larch, Fig. 45, and some other trees, 

 careful examination reveals the fact that the 

 arrangement is really a spiral one. 



It is not uncommonly the case, of course, 

 that in a mature branch we find some slight 

 deviations from regularity in the arrangement 

 of leaves, but these are of such a character 

 that they may readily be accounted for, either twi^of^Lwdi, show- 

 by the distortions which stems frequently ing fascicled le * ves - 

 undergo during the process of growth, or by the failure of some 

 of the leaves to develop. 



It will be seen that since branches usually spring from axil- 

 lary buds, their arrangement must also be regular and corre- 

 spond in plan with that of the phyllotaxy. 



Duration of Leaves. Leaves differ widely as to their 

 period of duration. They are described as persistent or ever- 

 green, if they remain green and on the tree for a year or more ; 

 they are deciduous, if unfolding in the spring or summer, they fall 

 off in the autumn or the season of frosts ; and if falling off early 

 in the season, as is the case with bud-scales, and often also with 

 other imperfect leaves, they are described as fugacious or cadu- 

 cous. 



As now in tropical regions evergreen trees are much the more 

 common, while in our own climate they are rare, there is good 

 reason to believe that in the warm ages of the world preceding 

 the ice period, all trees were evergreens, and that our northern 

 trees have become deciduous-leaved by gradual adaptation to 

 the vicissitudes of the climate. 



Position. According to their place of insertion on the stem, 

 leaves often differ from each other considerably in form and 

 appearance, and it is often, therefore, convenient to use special 

 terms indicative of their position. Cauline leaves are those 

 which are inserted on the main stem; rameal leaves are those 

 borne on the branches; radical leaves are those which spring 

 from the basal portion of the stem at or just beneath the 

 ground; seminal or primordial leaves are those which are borne 

 by the embryo in the seed ; and floral leaves are the leaves of 

 the flower. 



