No. 510] PROBLEMS IN PLANT ECOLOGY 



371 



that the underground portions of ordinary plants are well 

 worthy of more attention than has heretofore been ac- 

 corded them. 



We turn now to the environmental factors. When it 

 is sought to determine the causes influencing the behavior 

 of plants growing under natural conditions, two very 

 different methods may be resorted to, the observational 

 and the experimental. By the experimental method we 

 try to determine, on the one hand, the kind and amount 

 of vegetation and, on the other, the magnitude of the vari- 

 ous physical conditions which make up the environment. 

 From these two series of observations are selected paired 

 concomitant factors or groups of factors, and when 

 the same concomitants appear in a number of such pairs 

 the conclusion is drawn that the relation between the 

 paired factors is a causal one. By the experimental 

 method we seek to control the conditions to a greater or 

 less degree, either synthetizing an artificial environment 

 (experimental physiology), or growing the same plant 

 forms under various natural environmental complexes. 

 This sort of work may be termed experimental ecology. 

 In so far as the environment is artificially synthetized, it 

 is comparatively easy of measurement, but where natural 

 conditions are allowed to obtain, the experimental and 

 the observational methods both require the measurement 

 of uncontrolled factors, and thus present great difficulties. 



The complexity of natural conditions makes it neces- 

 sary often to break them up into component parts and to 

 measure the parts separately. For convenience in hand- 

 ling, these factors have been classified into climatic and 

 edaphic, but I fail to see that such a classification has any 

 relation to the activities of the plant. I shall, therefore, 

 speak here merely of environmental factors, classifying 

 them, for ease in discussion, into those which are active 

 above the soil surface and those which are active below 

 it. Each group must, of course, be further analyzed 

 according to the purposes of the investigation. But it 

 must be remembered that the data from separate com- 



