Exercise XIII 



THE LEAF 



I. Introduction.— As stated in the previous exercise, it is diffi- 

 cult, on both theoretical as well as practical grounds, to de- 

 marcate rigidly the leaf from the stem. If, as. several mor- 

 phologists have suggested, the "leaves" of higher plants arose 

 phylogenetically from determinate branch systems, this difficulty 

 at once becomes understandable. Indeed, perhaps the most useful 

 character which distinguishes the leaf from the stem, apart from 

 its origin at the shoot apex, is the early cessation of apical growth. 

 The leaves of ferns retain an apical meristem for a relatively long 

 period in their development, but in seed plants the final size and 

 form of the leaf is largely determined by intercalary growth. 

 Leaves are without much question the most diversified of all the 

 "organs" produced by higher tracheophytes (cf. Troll 1938- 

 1939. and Arber 1941). The foliage leaf, which is the most 

 familiar type, varies from the small scale-like structures found in 

 certain gymnosperms and angiosperms to the enormous and com- 

 plex leaves of palms. In addition to foliage leaves, other types 

 of foliar organs must be considered under the morphological con- 

 cept of "leaf." As illustrations may be mentioned cotyledons, 

 bud scales, bracts, and according to classical theory, the appen- 

 dages of the flower. In view of such morphological and functional 

 diversity, it is obviously impossible to generalize with respect 

 to the histological structure of "leaves." From an anatomical 

 standpoint, the leaf may be regarded as an "expansion" of the 

 axis in which all the fundamental primary tissue regions (i.e., 

 epidermis, cortex and stele) may be recognized. But the ar- 

 rangement and structure of the photosynthetic parenchyma (i.e., 

 the 7nesophyll), the vascular system (i.e., the major and minor 

 veins) and the mechanical tissues (e.g.. collenchyma, sclereides 

 and fibers) vary within extremely wide limits. 



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