410 Dar bishire. — Observations on Mamillaria elongata, 
grounds, that plants develop in such a way as to offer armed resistance 
to animals, although they may as a matter of fact carry organs which may 
actually inflict injury and pain on animals. 
Most green land-plants are during the vegetative phase of their 
development of sedentary habits. They are tied to the spot to which 
they have once affixed themselves. It is necessary therefore that under 
these conditions plants should possess the property of adapting themselves 
to a certain extent to external conditions. I am not referring here so 
much to ontogenetic as rather to phylogenetic adaptation. The former 
differs from the latter only in degree. 
A race of plants incapable of adapting itself to altered climatic and 
edaphic conditions must succumb. 
Only those external conditions influence the plant which in some way 
adversely or favourably affect the carrying out of the vital functions of the 
plant. Of these there are the two already referred to, through which the 
plant is most powerfully influenced by the external world, namely, 
transpiration and photosynthesis. 
The green plant must carry out these two life-processes itself and 
under all conditions. It must provide its cytoplasm with organic food, 
representing matter and energy. It is dependent on its immediate neigh- 
bourhood for the supply of both. The main idea which therefore I consider 
underlies the adaptation of plants is that of building up organic food — that 
is, of carrying out at all costs the processes of transpiration and photo- 
synthesis. The plant has inherited in an increasing degree this property 
of adapting itself in such a way as to carry out these functions most 
effectively. Thus we get the various plant-forms— -which in each case 
represent the balance between the tendency of the plant to develop its 
organs of transpiration and photosynthesis, and the influence of the 
external conditions on the carrying out of these functions. 
In every plant-form a struggle is going on between transpiration and 
photosynthesis, until a compromise is arrived at. In a very dry place the 
function of transpiration, influenced by the adverse external conditions, 
is withdrawing the plant away from exposure to light and air. Photo- 
synthesis is drawing the organs to the light. 
To put it briefly I might say that the main principle which underlies 
adaptation in plants has a physiological basis. It is on this basis mainly, 
if not entirely, that many of the remarkable, as yet little understood plant- 
forms will be explained. Schimper has thus rested his book on the 
geographical distribution of plants on a physiological basis. It will then 
be found, I think, that organs, which are considered as offering protection 
against attacks by animals, and therefore would not of course represent 
a physiological adaptation, are really very often, if not entirely, structures 
with a physiological meaning. This would not however prevent, in 
