RELATION OF STRUCTURE AND FUNCTION 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 with 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, 



V.S. 



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 (V.S.}, the cortex (C.) and the mycorrhizal zone (M.). 



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 chlorophyll, 



