PHYSIOLOGY 49 



say, of heat or light that is endirable ; that there is 

 also a maximum quantity of light or heat beyond 

 which the life suffers or dies ; and that somewhere in 

 between them is the best and most suitable quantity, 

 which is called the optimum. This scale of maximum, 

 optimum, and minimum quantities of light, heat, or 

 whatever it is, differs for nearly every plant, and for 

 the different organs in some cases. So that the most 

 favourable, the optimum of heat for example, for one 

 species may be too near the maximum of another to 

 let it thrive at all where the first is most flourishing. 

 The study of these " limiting factors," as they are 

 called, is now one of the great branches of physiological 

 work. 



Each plant's relation to light, heat, air supply, water, 

 and a number of the other physical factors in its environ- 

 ment can be expressed in series of mathematical curves 

 or diagrams. 



