GENERAL INFLUENCE OF THE ENVIRONMENT ON PLANT 



LIFE. 



I. EXTERNAL AND INTERNAL CONDITIONS AND PLANT ACTIVITY. 



The behavior of any plant is said to be controlled by the surround- 

 ing conditions, variations in these being the stimuli, or causes, which 

 produce in the internal, physiological complex of the organism various 

 responses or effects. Such responses are, however, quite as dependent 

 upon the nature of the responding organism as upon the nature of the 

 stimuli. With the same set of environmental conditions different 

 plants behave differently, merely because their internal conditions 

 differ, and with unlike environmental complexes plants of the same 

 form exhibit quite different behaviors. The behavior of plants in 

 general thus depends upon two interacting sets of conditions, the one 

 set being external and the other internal. The latter set of conditions 

 makes up, of course, the nature of the plant and serves to define it 

 physiologically. These internal factors determine the abihty of the 

 organism to respond to exposure to any given constant external 

 condition, or to any given change in any condition, and they also 

 determine the extent of such responses. A plant might be rigorously 

 defined by means of these powers or capabilities to respond to stimuli, 

 and it is some of these powers that are, indeed, unconsciously used by 

 taxonomists in their descriptions; but, although certain groups of the 

 bacteria are now described by conscious reference to their physiolog- 

 ical properties, the physiological nature of the taxonomic description 

 in botany may be said hardly to be generally recognized as yet. 



As a plant develops, its internal conditions pass through a series of 

 more or less profound alterations, and the different developmental 

 phases of the same plant often show greater divergency in response to 

 the same environmental complex than do the corresponding phases of 

 distinct plant-forms. Thus, environmental conditions that are favor- 

 able to seed germination or to vegetative growth may be markedly 

 unfavorable to the production of flowers or fruit. It follows that for 

 the best growth and reproduction of many forms the external conditions 

 must vary from phase to phase as growth proceeds. This is one of 

 a number of considerations that make for great difficulty in the incep- 

 tion of any satisfactory quantitative study of the relation of external 

 conditions to the characters of individual plants and of vegetation in 



general. 



A second consideration that enormously complicates our problem 

 is this, that the response, or effect of the external system upon the 

 organism, is definitely dependent upon the duration of the component 



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