INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 99 



conceivable that plant physiology may eventually work out the 

 principles whereby the behavior of plants may be one day explained, 

 predicted, and controlled. This is, of course, the hope of ecology and 

 of the developing science of plant-culture. It is our aim in the present 

 chapter to present merely a tentative outline of the general and more 

 superficial relations that apparently obtain between the plant and its 

 environmental conditions. No attempt is here made to make our 

 consideration logically complete, either from the standpoint of physical 

 causation or from that of the plant kingdom in general; attention is 

 mainly turned upon some of the most obvious physiological considera- 

 tions and upon the behavior of ordinary vascular land-plants. 



II. THEORY OF PHYSIOLOGICAL LIMITS. 



In an etiological study of plant distribution, either natural or 

 artificial, the conception of physiological hmits must hold a very prom- 

 inent place. We understand by this term the extremes of intensity, 

 etc., in a given factor which a certain plant can withstand. Starting 

 from the optimum intensity of heat for any plant, for example, we 

 may reduce the temperature till death ensues, thus attaining the 

 minimum temperature limit for life. If the temperature be increased 

 sufficiently above the optimum, another death-point is reached, the 

 temperature maximum. The plant is thus able to retain life only 

 under temperature conditions that fall within these physiological 

 limits. With some other factors a similar pair of limits can be deter- 

 mined; with many, however, only a single limit exists. For example, 

 a submerged aquatic possesses a definite minimum rate of water- 

 supply, but it is impossible to produce death or injury by increasing 

 this rate even to its physical limit, as by surrounding the entire plant 

 with water. It is obvious that in such a case there exists no maximum 

 limit for life. Cases where there is a well-marked maximum but no 

 minimum are also frequent, most toxic substances furnishing examples 

 of this. Plants live normally in utter absence of these substances; 

 they also live normally in their presence, so long as the amount sup- 

 plied does not surpass a .fixed maximum Hmit. 



Life is able to proceed, then, in any particular plant, only so long 

 as the external conditions do not surpass the physiological limits for 

 the life processes of the form considered. In different plants and in 

 different developmental phases or stages of the same plant these 

 hmits may be very different, so that an environmental complex that 

 inhibits life in one phase or form may allow healthy activity in another. 

 It is mainly in accord with this generahzation that distinct climatic 

 areas are characterized by corresponding types of vegetation, and the 

 principle is therefore probably of primary importance in the study of 

 plant distribution. 



