32 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



complicatedly lobed) halves by a constriction around the centre. 

 Also included are a wide range of freshwater and marine slimes and 

 scums, the sometimes bulky Sea-lettuces, and, according to most 

 students, the peculiar and fir-like Stoneworts. 



Different kinds of Green Algae are to be found in a vast array of 

 habitats including damp soil, the surfaces of rocks, and the bark of 

 trees. Here and on such objects as posts and palings they may 

 form a green investment most typically on the pole-facing side, 

 which tends to be less dried by the sun. But mainly they are 

 aquatic, being especially abundant in freshwater lakes and streams, 

 although numerous types occur in the sea. Many Green Algae are 

 among the most widespread of plants, the group as a whole being 

 virtually ubiquitous. Their nutrition is mainly by photosynthesis 

 — that fundamental series of reactions on which they and practically 

 all other forms of life depend, directly or indirectly. In this vital 

 process, chlorophyll absorbs radiant energy from light and catalyzes 

 the building up of simple materials into complicated carbohydrates 

 in which the energy is locked — there to remain stored until it is 

 liberated, for example by burning or the slower process of respira- 

 tion. Such carbohydrates made by green plants constitute the main 

 basic food materials of the world. Although some simulate higher 

 plants, Algae do not need (or have) roots or other special absorbing 

 organs, but take in the raw materials they require (chiefly water 

 with certain salts and gases in solution) more or less all over their 

 surfaces. 



The Green Algae reproduce by various methods, several of which 

 may be practised by the selfsame species. The chief types are 

 asexual reproduction by cell division, fragmentation of the thallus, 

 or the liberation of spores — which may swim actively by means of 

 flagella. Or there may be sexual reproduction following conjuga- 

 tion or, more often, the fusion of special bodies (gametes) which are 

 frequently differentiated into large female and small male ones. In 

 some cases both of the gametes and in others only the male ones 

 are motile. In spite of their great diversity and virtual ubiquity, 

 the Green Algae are of rather little importance except as food 

 for aquatic animals ; but they are of great interest in indicating 

 some of the lines along which evolution to higher plants may have 

 taken place. They are widely important constituents of aquatic 

 vegetation and are often dominant in freshwater pools, though as 

 constituents of human or domestic animals' food they are at best 

 minor. 



