4 BRITISH PLANTS 
In dealing with environment we have three things to 
consider : 
1. The items or factors of the environment which affect 
vegetation (Part I.). 
2. The effect of these factors upon 
(a) The vegetative, 
(b) The reproductive parts of plants (Part IT.). 
3. The results of competition among plants, for the © 
satisfaction of their needs, as also between plants and 
animals in the general struggle for existence. The results 
are expressed in the flora just as we find it anywhere 
to-day (Part IIT.). 
As the first part of the book is devoted to those factors 
of the environment which influence vegetation, we will 
survey them briefly first, leaving details to future 
chapters. ; 
A. SOLAR ENERGY. 
The sun is the ultimate source of all terrestrial energy ; 
its rays illumine as well as warm. We have therefore 
to consider the solar energy which is poured upon the 
earth, under what appears to us its two manifestations— 
light and heat. To the plant, however, these two mani- 
festations are one—it is simply energy. 
1. Light. 
The world of green plants owes its existence to light. 
In the presence of light, the green cell combines the 
carbonic acid gas which it receives from the air with the 
elements of water derived from the soil to form sub- 
stances like starch and sugar. The process is known as 
photosynthesis (Gr. photos, light; synthesis,-a putting 
together), or carbon-assimilation. The term assimilation 
bas rather a wide meaning in plant physiology ; but it 
always implies making or building up. In its wide sense 
it embraces not only all those processes which, as a result 
of vital activity, lead to the construction of food, but 
also those less known and even more wonderful processes 
by which, step by step, food itself is transformed into 
