1NEXTENS1BLE ORGANS 



185 



with roots in having their mechanical tissues united to form a stout 

 axile tube or a solid central strand; this centralisation of the 

 mechanical system is very marked in the rhizomes of Grasses, Sedges 

 and Eushes, which accordingly when regarded from an anatomico- 

 physiological standpoint approximate more closely to roots than 

 to the aerial stems of which they are the morphological equivalents 

 (Fig. 65 a). 



The anatomical contrast between inflexible aerial and inextensible 

 subterranean structures is sometimes even exemplified by different por- 

 tions of the same stem. This point is excellently illustrated by certain 

 Cypekaceae and Juncaceae. The lower portion of the haulm of 



A Fig. 65. 



A. T.S. through a rhizome of Carex tjlauca. The mechanical system is in the form 

 of a thick-walled axile tube of stereome with numerous mestome-bundles embedded 

 therein. B. Lateral root of Ze a Mays in T.S. The mechanical system comprises (1 a 

 thick-walled axile tube of stereome (to ensure inextensibility), (2) a subepidermal 

 fibrous cylinder (to ensure longitudinal incompressibility). 



Juncus glaucus extends several centimetres below the surface of the 

 soil ; like a tap-root it is exposed solely to longitudinal tensions, when 

 the aerial portion is bent. It is a priori improbable that this 

 subterranean prolongation of the haulm should possess the structure 

 of a rhizome. Anatomical investigation does in fact show that the 

 disposition of the fibrous strands and mestome-bundles is almost 

 identical with that which prevails in the aerial region of the haulm. 

 But the two portions of the stem differ appreciably as regards 

 the quantitative development of the bast in the individual strands. 

 A glance at the two transverse sections (Fig. 59 a and b) at once 

 reveals the centripetal tendency of the mechanical tissue in the 

 subterranean, and its predominantly centrifugal disposition in the 

 aerial, region of the haulm. The most obvious point of contrast is 

 provided by the development of the subepidermal girders ; these 



