254 



A TEXTBOOK OF BOTANY 



[Ch. V, 8 



Orchids (Fig. 126), familiar in all greenhouses, while some of 

 the roots penetrate the frevices of decaying bark and there- 

 from absorb both water and mineral matters, others hang 

 down free in the air. The latter display a distinctive, 

 swollen, whitish aspect, the thickness being due to the pres- 

 ence of many epidermal layers of loose, empty cells. Into 

 these the water from rain is easily absorbed and thence 



Fig. 178. — The Banyan, Ficus religiosa, of India. 

 All degrees of development of the descending aerial roots appear in the 

 view. The tree is a small and very open one. (From Balfour.) 



transferred to the fibro-vascular system, though the popular 

 belief that such roots can ab.sorb water as vapor from the air 

 seems unfounded. In some kinds of Orchids the aerial roots, 

 hanging in the light, show traces of chlorophyll, while 

 in a few tropical kinds (Fig. 179) the roots become fully 

 green, flatten to almost leaf-like thinness, and completely 

 assume the photosynthetic function in place of the leaves, 

 which are reduced to mere rudiments. Herein we have 

 indeed a remarkable case of a complete substitute function 

 in roots, and the one we would least expect, — that of foliage. 

 Aerial roots, of other forms, act as supports to climl^ing 

 plants. Thus the true Ivies (e.g. the English Ivy and, of 

 course, the Boston Ivy) jnit forth from the shaded sides of 



