MECHANICAL CONSTRUCTION OF PLANT-BODY 159 



stele is compact as compared with that of tlic axis. Underground 

 rhizomes show a similar construction. Their stele is contracted, 

 and their cortex widened, as is seen in the Marram Grass, and still 

 more clearly in the Sand Sedge, where the cortex is very weak and 



n 



Fig. 120. 



Sections of stems of two Sedges. .4, Rfiizome of Qarex arenaria with mechanical 

 tissue condensed centrally, as resistant to the rope-requirement. ( ■. 14.} B, Carex 

 vulgaris, aerial stem constructed to meet the columnar requirement. { ;: 25.) 



lacunar, while the stele is compactly cemented together with sclerotic 

 tissue, so as to form a sohd core (Fig. 120, A). This is in sharp con- 

 trast to the aerial stems of most Sedges (Fig. 120, B). Stems support- 

 ing heavy, pendent fruits show a like structure ; also submerged and 

 some climbing stems, all of which are liable to longitudinal tension. The 

 similar modification of plants of such various habit, when subjected 

 to the same mechanical demand, indicates 

 that the rope-like concentration is adaptive. 



The problem for strut-roots, such as are 

 seen at the base of the stem of the Maize, is a 

 mixed one ; for the roots on the windward 

 side of the plant suffer longitudinal tension, 

 while those on the lee side act as oblique 

 struts, and are subject to columnar pressure. 

 The structural requirements are thus opposed. 

 The mechanical tissues in such roots are found 

 to form two systems (Fig. 121). The stele is 

 compact, and cemented together in a hollow 

 ring, which is, however, wider than in the underground roots. It is 

 suited to resist longitudinal tension, but is not highly specialised to 

 meet it. The cortex is, however, sclerotic, the thickening being 

 greatest at the periphery. This is suited to meet the columnar 



Fig. 121. 

 Transverse section of strut- 

 root of Zc-a A/ai's. Themechani- 

 cal tissue of the cortex is dotted. 

 ( < 12.) F. O. B. 



