Exercise XII 



THE STEM 



I. Introduction. — In this and the two following exercises, a 

 brief study will be made of the comparative anatomy of the three 

 principal vegetative "organs" of the sporophyte of seed plants, 

 viz. : the stem, leaf, and root. Paleobotanical evidence shows 

 clearly, however, that this conventional subdivision of the plant 

 body cannot be applied to the Psilophytales which are generally 

 regarded as the most primitive of all tracheophytes. Indeed, in 

 the Psilotales, which are the living representatives of this ancient 

 group, roots are absent and the aerial portion of the sporophyte 

 is not clearly demarcated into stem and leaves. Furthermore, it 

 is clear that even in seed plants the boundary between stem and 

 leaf can only be made rather arbitrarily. Both of these "organs" 

 arise from a common terminal meristem (i.e., the shoot apex) and 

 their further differentiation and growth is reciprocal and inter- 

 dependent to a large extent. For these reasons, it seems prefer- 

 able, from a morphological standpoint, to include both the axis 

 (i.e.. the stem) and its foliar appendages under the broader con- 

 cept of the shaot. This concept has found application not only 

 to the vegetative region but has also been widely adopted in the 

 anatomical interpretation of the flower of angiosperms (cf. 

 Eames, 1931, and Foster, 1939). Hence, in this book, the sepa- 

 rate treatment given to the stem and the leaf is largely a matter 

 of practical convenience and its limitations on morphological 

 grounds should be constantly borne in mind. (Cf. Arber, 1941, 

 for a penetrating discussion of the problem.) The root, which is 

 axis-like in form, clearly deserves separate discussion and study 

 and its anatomical features will be outlined briefly in Exer- 

 cise XIV. 



In the followng resume of the basic aspects of stem anatomy, 

 the histology of the iniernodal regions of this structure is the 

 principal consideration. The nodal regions of stems, because of 



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