482 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



less perpetual in that the depths are all the time enriched by a ' rain ' 

 of dead bodies containing the all-important nutrients. 



It is chiefly in late summer and autumn, when there has been an 

 extensive depletion of mineral nutrients but some replenishment 

 and, meanwhile, a copious increase in organic substances, that 

 water-blooms of Cyanophyceae occur. Then again in autumn there 

 may be another Diatom ' maximum '. To the extent that each new 

 population appears only after the requisite conditions have been 

 provided by its predecessor, there is here a kind of successional 

 sequence, although actually such phytoplanktonic stages are probably 

 all proseral in being non-essential to, or at all events not forming 

 part of, the autogenic main sere. 



In connection with the spatial distribution of plant communities 

 which is the mainstay of our subject, we should recall that each 

 physiological activity, such as photosynthesis and reproduction, is 

 greatly affected by various conditions of the environment. Usually 

 with each such ' function ' there is for every pertinent environmental 

 factor a minimum below which, and a maximum above which, there 

 is no activity ; somewhere between lies an optimum at which the 

 function involved is carried on best. These ' cardinal points ', how- 

 ever, may vary with other environmental conditions, even as they do 

 of course with different organisms. With such rare exceptions as 

 perspiration, which generally increases with increasing temperature 

 until death from overheating occurs, each physiological function 

 responds in this manner. So does the organism respond as a whole 

 to change in an external factor — hence the importance of physio- 

 logical considerations in plant geography. But because several fac- 

 tors normally change at one time, and indeed go on changing all the 

 time, the effects are exceedingly complex and usually difficult to 

 analyse. Moreover the demands and reactions of various types, 

 species, and even lower entities or different stages of plants are 

 themselves extremely various. 



In water, as we have already seen, some of the environmental 

 factors which are most variable on land are damped down, but 

 others retain a strong hold, as it were, on plant activity and distri- 

 bution. Furthermore, the planktonic population depends not merely 

 (and obviously) on the systematic groups present and able to grow 

 and maintain life in the face of often unfavourable conditions, but 

 also on the rate of reproduction and depletion of the component 

 forms. Such depletion may be more rapid than in most other 

 types of plant communities, and numbers may fluctuate greatly 



