86 MORPHOLOGY OF LEAVES. 



different uses. Form and function, therefore, are not sure indi- 

 cations of the true nature of organs. 



160. Morphologically, and in the most comprehensive sense, 

 leaves are special lateral outgrowths from the stem, definitely 

 and symmetrically arranged upon it ; in ordinary vegetation and 

 in the most general form constituting the assimilating apparatus 

 (or foliage) , but also occurring in other forms and subserving 

 various uses. Sometimes these uses are combined with or sub- 

 sidiary to the general function of foliage ; sometimes the leaf is 

 adapted to special uses only. So the botanist recognizing the 

 essential identity of organs, whatever their form, which appear 

 in the position and conform to the arrangement of leaves 

 discerns the leaf in the cotyledons of a bean or acorn, the scale of 

 a lily-bulb or the coat of an onion, the scale of a winter bud, and 

 the petal of a blossom. Therefore, while expanded green leaves 

 (which may be tautologically termed foliage-leaves) are taken as 

 the proper t}*pe, the common name of leaves, in the lack of any 

 available generic word, is in morphological language extended to 

 these special forms, whenever it becomes needful to express their 

 phylline or foliar nature. 



161. In the morphological view, all the plant's organs except- 

 ing roots (and excepting mere superficial productions, such as 

 hairs, prickles, &c.), belong either to stem or to leaves, are 

 either cauline or phylline in nature. To the latter belong all the 

 primary outgrowths from nodes, all lateral productions which 

 are not axillary. 1 Whatever is produced in the axil of a leaf is 

 cauline, and when developed is a branch. 



162. The Duration of Leaves is transient, compared with that 

 of the stem. They may be fugacious, when they fall off soon 

 after their appearance ; deciduous, when they last only for a 

 single season ; and persistent, when the}' remain through the cold 

 season, or other interval during which vegetation is interrupted, 

 and until after the appearance of new leaves, so that the stem is 

 never leafless, as in Erergreens. In many evergreens, the leaves 

 have onh' an annual duration ; the old leaves falling soon after 

 those of the ensuing season are expanded, or, if they remain 

 longer, ceasing to bear any active part in the economy of the 

 vegetable, and soon losing their vitality altogether. In Pines 

 and Firs, however, although there is an annual fall of leaves 

 either in autumn or spring, }*et these were the produce of some 



1 There are cases in which this rule is of difficult application, or is seem- 

 ingly violated, sometimes by the suppression of the subtending leaf, as in the 

 inflorescence of Cruciferse, rarely in other ways, to be explained in the 

 proper places. 



