Il] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 32I 



light, heat, and moisture relations, which vary simultaneously with 

 every change in the intensity of insolation. 



In such circumstances it is difficult or sometimes impossible to 

 segregate individual factors experimentally. Instead of physical 

 apparatus we may use ' phytometers ', which are standard plants or 

 clumps of vegetation that have the advantage of integrating all effective 

 factors of the environment and expressing the result in their own 

 responses. They react only to changes that matter to the plant or 

 plants, whose protoplasm has the power of making adjustments. 

 But much as species are usually composed of more or less numerous 

 biotypes, so may general habitats or even single examples of them 

 be made up of hiotopes, w^hich are the ultimate expression of environ- 

 mental variation, being defined as the smallest natural area of space 

 that is characterized by a particular environment. The biotope (or 

 ' ecological niche ' of some authors) is thus the primary' topographic ' 

 unit used in habitat classification. The community of forms in- 

 habiting it is termed a biocoenosis, and the biotopes having similar 

 characters are united into larger divisions called biochores. Many of 

 these are apt to be represented in any small area, so whole series of 

 phytometers, covering at least the different microclimates, may be 

 necessary to determine the impress of even a single example of a 

 general habitat ; moreover they should be accompanied by batteries 

 of instruments adequate for the establishment of quantitative re- 

 lationships between stimulus and response. 



In addition, matters can change rapidly with time. Thus even 

 under a canopy of vegetation, such items as the movement of leaves 

 by the wind, the changing angle of the sun, and various effects of 

 weather and season, cause variations in the movement of shadows and 

 sun-flecks across the ground. This in turn causes drastic changes 

 in the amount of light-energy received at a given point — on w^hich, 

 as we have already seen, much else may depend. Altogether it is 

 not surprising that experimental ecology has become an exacting 

 (though scarcely exact) science. A saving grace is the fact that, 

 apart from the grosser direct effects of animals and drastic physical 

 agents, the actual effects of environmental factors upon plants are 

 resolvable into a few physical and chemical processes. Examples 

 are those which underlie the influence of light on photosynthesis and 

 growth, the effect of temperature on chemical changes in the plant 

 body, the evaporating power of the air on the water in the plant, 

 and the effect of the soil solution on the absorbing organs and other 

 parts of the plant. 



