'913.] OF THE UNITED STATES. 259 



determine whether or not it can thrive tliere, is actually to make the 

 experimental test. 



Frequently a simple inspection of the plant dealt with, or the 

 approximate measurement of certain of its characteristics, may 

 suffice for an indication of its ability to withstand the water-with- 

 holding or water-extracting power of the environment. Thus, it has 

 long been appreciated that the ability of a plant to thrive under arid 

 conditions is often indicated by its observable physical structure. 

 The power of an organism to withhold moisture from an arid environ- 

 ment seems to be closely, and usually directly, connected with struc- 

 tural characters which can be recognized at sight, and, on the basis 

 of this principle, ecologists have classified plants into xerophytes, 

 mesophytes and the like. Of course this classification must be sub- 

 jected to a much more definitely quantitative treatment than the one 

 now generally employed, that of mere observation and personal judg- 

 ment, before ecology can begin to partake of the characters of an 

 exact science. For such a resurvey of the moisture-retaining powers 

 of plants we have now at least two practicable and fairly quantitative 

 methods^^ besides the directly experimental one of trying various 

 plant forms under various climatic complexes. This is not the 

 place to enter into a consideration of these methods, but it should 

 be emphasized that it now appears to be possible, within a single 

 period of twenty- four hours, to determine with considerable accu- 

 racy the position of almost any plant individual in what might be 

 termed an absolute scale of xerophytism, as far as the water-with- 

 holding power of its aerial parts is concerned. 



For the study of the effects of temperature conditions within the 

 plant, no means is yet available excepting that of direct experiment. 

 In one way the problem here met with is simplified by the well-known 

 fact that plant temperatures practically always follow very closely 

 the temperatures of the surroundings. In attempts to determine the 

 relations between temperature and the various plant processes, it is 

 therefore only necessary to know the effective temperature condi- 



" Livingston, B. E., " The Resistance Offered by Leaves to Transpira- 

 tional Water Loss," Plant World, i6 : 1-35, 1913. Also references there 

 given. 



PROC. AMER. PHIL. SOC, LII, 209 Q- PRINTED JUNE I 7, I913. 



