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 
