On Epiphytes. 



PLANTS are much more directly dependent on the nature of their 

 physical surroundings than are animals, and display a cor- 

 respondingly greater plasticity of form under varying external 

 conditions. Hence among plants examples of structural harmony 

 with the environment are both striking and numerous ; but this 

 malleability in the nature of a plant renders it more difficult to decide 

 when we are dealing with a true instance of adaptation. A leaf 

 exposed to direct sunlight transpires at a greater rate than it would 

 in a damp shaded habitat ; and we find that leaves which develop in 

 the sunlight have a structure different to that of leaves of the same 

 species formed in the shade. The leaves developed in the light are 

 structurally so constituted that they transpire less than those formed 

 in the shade, provided that both sets of leaves be exposed to the same 

 conditions ; but these peculiarities of structure are not transmitted 

 to the offspring of the respective plants. Here we are not dealing 

 with a case of adaptation — in the narrow sense — but with an instance 

 of the direct effect of the environment on the individual plant. Now 

 this is obviously very different to the case of desert plants, in which 

 the leaves have structural characteristics (succulence, hairs, thick 

 cuticle, sunken stomata, etc.), which are handed down to the offspring 

 grown under any conditions. Here we can see that the leaves are 

 constituted so as to avoid excessive loss of water — they are suited to 

 the dry habitat. But can we at once pronounce this a case of 

 adaptation to the peculiar mode of life on the desert ? Decidedly not. 

 Either the ancestor of the desert plant possessed another form, and 

 only changed it gradually as it became a desert plant ; or the 

 ancestor possessed these structural peculiarities even before it became 

 a desert plant, and, in fact, only assumed its present mode of life in 

 virtue of these characteristics. In the latter case, correctly speaking, 

 the desert plant is not adapted, though it is suited, to its peculiar 

 mode of life. The environment has not impressed itself on the nature 

 of the plant. That such cases are possible is shown by the fact that 

 plants occupying widely different positions in nature often betray 

 remarkable structural similarities. For example, many Epiphytes, 

 desert-plants, littoral forms, neighbours of mineral springs, alpine 

 plants, and temperate evergreens possess leaves which agree in being 



