CHAPTER VII 



The Structure of the Young Stem 



The comparatively slight variation in the anatomical construc- 

 tion of roots can be related to their very uniform environment 

 (the soil) ; in fact, the only marked departures from the normal 

 are associated with special needs, as in aerial roots, prop-roots, 

 etc. Stems, on the other hand, develop under very diverse 

 conditions as regards mechanical strains, illumination, supply 

 of moisture, etc., and consequently display much more varied 

 structure. Moreover, the general plan of construction of the 

 stem differs from that of the root in several important respects. 

 The organisation of a Dicotyledonous stem can be studied 

 in a cross-section through the uppermost internode of a young 

 Sunflower (Fig. 34). The most obvious feature is that the 

 vascular tissue is a ring broken up into distinct vascular bundles. 

 In each the thick-walled xylcm {xy.} is towards the inside and 

 the thin-walled phloem [ph.) towards the outside, the two thus 

 lying on the same radius (constituting a collateral bundle), in 

 contrast to the alternate arrangement of these tissues obtaining 

 in the root. By the disposition of the vascular bundles in a 

 ring the ground tissue is marked out into three regions, namely 

 the cor/cx (Fig. 34, c.) on the outside, the pitli (p.) in the centre, 

 and the rays (often called medullary rays, r.) connecting the 

 two and separating adjacent bundles. The terms pith, cortex, 

 and rays are, however, purely topographical, and do not neces- 

 sarily imply any differentiation between the component ceUs, 

 which in fact, in most cases, are largely thin-walled and paren- 

 chjmiatous in all three regions (cf. Fig. ;6, Co. and p.). The 

 edge of the section is bounded by an epidermis 1 which differs 



1 l'"(jr a detailed coiisideralion ni epidermis and stomata, see the next 

 chapter. 



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