320 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



floating vegetation. Yet, in major bodies of water, far more exten- 

 sively occupied plant habitats are usually provided by the surface 

 waters where sufficient light penetrates for photosynthesis. Here 

 develop various planktonic communities of free-floating or swimming 

 organisms, the vast majority of which are microscopic. The habitat, 

 and consequently the community, may vary greatly with climatic 

 factors and the presence of solutes and suspensions in the water, the 

 whole being often subject to marked seasonal fluctuations including 

 exhaustion of nutrients when the population is around its ' peak '. 

 In the deeper layers of water and on deep ocean or lake floors where 

 light does not penetrate, there are still habitats — especially for 

 saprophytic plants living on the ' rain ' of sinking bodies. Indeed 

 it is here that Bacteria are often especially numerous. 



MiCROHABITATS 



We have seen that the environmental and internal factors of living 

 organisms have intricate and highly complex interrelationships, be- 

 longing as they do to a plethora of variables and potentialities that 

 may be set in motion by all manner of ' master ' forces. Yet it is 

 only the ' thin shell ' of environment directly impinging on, or 

 immediately adjacent to, the organism that is of primary causal 

 significance to it. So we get what in eff^ect are * partial habitats ' 

 (microhabitats), for example in areas of drastic relief or uneven 

 ground, or indifferent situations in a forest or other gross and complex 

 community. Consequently it is rare for the measurements recorded 

 by meteorological instruments to be actually those of the conditions 

 of the microhabitat affecting the growing plant or, more precisely, 

 the growing part of the plant. 



Microclimates are really the ultimate multiple expression of the 

 local climatic effect which is so commonly and variously engendered 

 by physiographic change, and microhabitats are their environmental 

 result, though they may often be based on edaphic or other vari- 

 ations. Very commonly one factor will compensate for another so 

 far as some plants are concerned, but not in a manner satisfactory 

 to the requirements of other plants — so leading to a jumbling of 

 local communities — and this eff^ect may be extended to microhabitats. 

 Moreover, an alteration in one factor may initiate whole series of 

 adjustments in others, often having far-reaching consequences. 

 Especially are such effects apt to be complex when concerned with 

 groups of factors that are closely related to one another — such as 



