lo] ENVIRONMENTAL FACTORS 285 



conditions, for example in forests, the form tends to be more elongated 

 and narrow. In temperate forests, too, the seasonal aspects are 

 apt to be important — in particular the prevernal {i.e. before spring) 

 one of herbs which flower before the shading tree-leaves expand. 

 Thereafter diff"erent levels or layers in the forest usually have 

 different light-climates. 



The measurement of light tends to be unsatisfactory, for no instru- 

 ment indicates precisely the quality as well as intensity, much less 

 the total quantity ; usually only the intensity at a particular point 



Fig. 7y. — A'loss Campion (Silene acaitlis agg. ) riov\ering onK on the south-tacing 



sides and tops of its domed tussocks near 80*^ N. in Spitsbergen. The surrounding 



terrain is a mixed ' half-barren '. 



in time and space is measured, and that is done only as far as the 

 measuring device employed is sensitive to the component wave- 

 lengths encountered. The particular rays of the spectrum which 

 are most effective in the diverse plant-functions affected by light, 

 also tend to be different. Nevertheless photoelectric cells and 

 actinometers of sensitized paper are useful, especially for purposes 

 of comparison, and both are widely employed by ecologists in the 

 field. 



(2) Temperature. This factor is vitally important as it conditions 

 the speed of the chemical actions and activities comprising life. 

 The great world vegetational zones, like altitudinal ones, depend 

 primarily on temperature, and we find it convenient to distinguish 

 between megatherms (plants favouring warm habitats), microtherms 

 (plants favouring cold habitats), and the intermediate mesotherms. 



