154 PLANT LIFE 
epiphytic members of a very different family, 
the aroids, have also, and independently, 
acquired the faculty of developing a velamen 
which is closely similar to that formed by 
the orchids. 
It is very easy to see that the presence of 
velamen is of great use to a plant growing 
as an epiphyte, but that is not at al! the same 
thing as accounting for its presence. It 
certainly enables the plant to take advantage 
of such positions as the trunks of trees, where 
it becomes lifted up to the light, and enjoys 
various other advantages. But how has it 
come about that it is just developed in these 
orchids (and aroids) in response to their 
particular needs, whilst the innumerable 
epiphytes belonging to other families of 
plants have not altered their roots in this 
striking manner? We cannot tell—at any 
rate at present. 
The same difficulty in giving a real explana- 
tion is inherent in every problem of plant (and 
animal) form, but it is often slurred over, 
especially when the structure is obviously of 
use in a particular connection. To describe 
it as an adaptation to a particular condition 
of the environment is merely to state an 
impression. Such descriptive phrases furnish 
no explanation of origin, nor do they illum- 
inate in any material degree the hidden 
relations of cause and effect. 
There are other flowering epiphytes which 
absorb water not by their roots at all, but 
