822 



A TEXTBOOK OF THEORETICAL BOTANY 



The foregoing modifications of the normal root in structure or function 

 are for the most part shown by members of the primary root system. There 

 are some further modifications which are characteristic of adventitious roots. 



(h) Climbing Roots- 



Many tropical climbers, especially of the family Araceae, make use of 

 clasping roots, which are non-geotropic but respond negatively to light 

 and are markedly sensitive to touch. These reactions cause them to cling 

 closely to the bark of the supporting tree, which they envelop with a stout 

 network (Fig. 819). Absorbing roots are produced separately in some cases. 

 The latter grow directly downwards and eventually enter the soil. Both 



Fig. 820. — Hedera helix (Ivy). Shoot with 

 adventitious climbing roots. 



types are evidently modifications of one common root-form and difi^er from 

 one another in little except in the larger vessels of the absorbing root and 

 the thicker sclerenchyma of the clasping roots. 



A similar modification is familiar in the common Ivy {Hedera helix). 

 Adventitious roots are formed in great numbers on the side of the stem 

 next to the support (Fig. 820). Normally these remain short and unbranched 

 and become precociously corky. They attach themselves to the support by 

 the formation of mucilage from the surface cells of the apex (Fig. 821). The 



