3i8 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



such matters of size, depth, and shelter also introduce factors of 

 their own, including light and temperature variations and the question 

 of whether rooted or otherwise attached plants can grow up sufficiently 

 to perform all their vital functions. This also depends upon the 

 possibility of rooting or other attachment, which is usually dependent 

 upon a suitable ' bed ' that of course varies for different types of 

 plants. 



As regards the content of dissolved substances, this can range 

 from ' ocean ' or even more extreme salinity down to varying degrees 



Fig. 88. — Margin of tropical oligotrophic lake, with steep rocky sides and rapidly 

 deepening water, supporting few larger plants. The hill-top vegetation is a semi- 

 arid savanna with prominent Acacias. Lake Tanganyika, E. Africa. (Phot. 



R. Ross.) 



in ' fresh ' water. Often in bodies of fresh water the acidity and 

 especially the nutritive salt content are of key significance for the 

 development of planktonic communities — at least, within particular 

 temperature ranges. In this connection it is often useful to dis- 

 tinguish three types of such bodies, of which the first may give rise 

 to the others : (i) oligotrophic, of waters poor in dissolved minerals, 

 typically with Desmids abundant but supporting at most a narrow 

 zone of rooted higher plants because of a hard rocky bottom and 

 rapidly deepening water (Fig. 88) ; (2) dystrophic, with waters also 

 poor in nutrients but rich in humus and acidic in reaction, often 

 coloured, containing Desmids and Bog-mosses ; and (3) eutrophic, 



