THE EPIPHYTES 151 



but others are able, owing to certain peculi- 

 arities of structure and habit, to endure 

 recurrent periods of drought provided that 

 they do not suffer too much in this respect 

 during their season of active vegetative growth. 



Disregarding, then, the less highly specialised 

 epiphytes, which respond to a dry season 

 by simply closing down their vital processes, 

 we will turn our attention to the more highly 

 adapted types amongst the flowering plants. 



The orchids will serve as our first examples. 

 A large proportion of the members of this 

 family are not epiphytic at all, but grow in 

 the ground. Even there they exhibit many 

 deviations from the typical structure of roots, 

 but in a number of the epiphytic species, so 

 common in the tropical forests, the root- 

 system undergoes a remarkable and adaptive 

 change of structure. Whilst some of the 

 roots may depart but slightly from the form 

 commonly met with in these organs, and 

 serve to fasten the plant to its arboreal perch, 

 others are thicker, often green, and when 

 dry are of a white or lustrous grey colour 

 The whiteness is due to the presence of air 

 in the outer layers of cells which form a very 

 peculiar sheathing mantle on the root. In 

 ordinary roots there is but one well-defined 

 layer sheathing the rind and giving rise to the 

 root- hairs; but in these orchids it divides 

 and forms many layers, whilst the root- hairs 

 are usually suppressed. The illustration will 

 better explain what is meant, and will serve 



