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, 



(&) The reproductive parts of plants (Part II.)- 



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 III.). 



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 ceU 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, Mght ; synthesis, a putting 

 together), or carbon-assimilation. The term assimilation 

 has rather a wide meaning in plant physiology ; but it 

 rilways implies making or building up. In its wide sense 

 it embraces not only aU 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 



