RELATION OF STRUCTURE AXD FUNXTION 73 



fibres are developed in relation to the phloem. When roots serve 

 not only for anchoring, but also for the support of the plant, a 

 modified structure may obtain. Thus, a cross-section of the 

 prop-root of the Maize (Fig. 32) contrasts ^\^th one of an ordinary 

 root of this plant in the presence of a special cortical ring of 

 mechanical elements and the larger size of the central cylinder, 



Fig. 31. — Transverse section of the outer part of one of the underground 

 branches of the Bird's Nest Orchid (Neottia nidus-avis), showing the 

 vascular strand (T'.S.), the cortex (C.) and the mycorrhizal zone (.1/.). 



so that the xylem also is more peripheral. In both these 

 respects the mechanical construction of such a prop-root ap- 

 proaches that of a stem (cf. p. 87). 



The aerial roots of many tropical epiphytes {e.g. Orchids) 

 also exhibit a modification of structure in correspondence with 

 their special method of absorption and their function as assimi- 

 latory organs. The cortical cells frequently contain chlorophj-il, 



