30 PLANT LIFE 



must obviate the inevitable difficulties of 

 existence in another way altogether. Some, 

 like fungi and many parasites, have adopted 

 the latter alternative; but as regards the 

 vast majority of the green plants, we shall 

 find that a recognition of these two dominating 

 factors, water and light, will serve us in good 

 stead, as furnishing an important clue to 

 much of the complexity of structure to be 

 observed in so many of the more advanced 

 types of plants. Such complexity is intimately 

 related with a corresponding differentiation 

 and specialisation of function, and indeed 

 it is largely to this circumstance that many of 

 the more striking examples of " adaptation 

 to the environment " are to be attributed. 



The best way of arriving at a clear concep- 

 tion as to how the higher plants, with their com- 

 plicated structure and high degree of differ- 

 entiation, have come into existence in past 

 times, is to study the more primitive types 

 which illustrate various degrees of advance 

 on the simple unicellular stage. The class 

 of Algae, which includes the simpler water 

 and land plants, will furnish us with excellent 

 material wherewith to reconstruct, in outline 

 at least, the course of vegetative evolution. 

 This must not, however, be taken to imply 

 that the simple types in question are to be 

 regarded as new forms which are now on an 

 upward grade of evolutionary development. 

 They are to be understood, rather, as per- 

 manent representations of some of the phases 



