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 all 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 



