RESISTANCE BY LEAVES TO WATER LOSS 



35 



the various forms naturally occur, and, on the other, the appear- 

 ance of the plant itself (including of course its internal as well as 

 its peripheral structures and characters). That the character of 

 its place of occurrence cannot satisfactorily be used in the defini- 

 tion of a natural object — whether this be an element, a mineral 

 or an organism — ^is a principle which gains in prominence with the 

 advance of every branch of science, and it appears that the growth 

 of plant ecology may soon lead to the dynamic or physiological 

 study of the organism, as offering the only really useful criteria 

 for ecological classification. In so far as the attempted classifica- 

 tion is to be based on the water relation (and all far-reaching 

 attempts toward such classification have been so based, e.g., 

 those of Warming, Schimper and others), it appears that there are 

 just three features of the plant that need to be involved in an 

 adequate ecological description. These three features are (1) the 

 power of the plant to absorb water from its surroundings, (2) its 

 power to distribute absorbed water within its body and (3) its 

 power to prevent the loss of water to its surroundings; all other 

 features are negligible in a classification based upon the water 

 relation. 1^ We are still very far from the possession of any well- 

 tried means for studying the first two features just mentioned, 

 but the method of relative transpiration and that of the index 

 of transpiring power do seem to furnish a means at least tempora- 

 rily adequate for the study of our third feature, and this is the 

 main point with which this paper has had to deal. A statement 

 of the limits of variation of the index of transpiring power of a 

 given plant should be much more satisfactory than its mere refer- 

 ence to an ill-defined category such as that of xerophytes, etc. 



It may be suggested finally that the conception of transpiring 

 power (or its reciprocal, resistance to water loss by transpiration), 

 together with the somewhat widely applicable method of hygro- 

 metric tests, should prove of great practical value to students of 

 agriculture. We should now be able to distinguish, physiologi- 

 cally and quantitatively, in so far as their aerial water relation is 

 concerned, between different varieties of cultivated plants. 



For an outline of the main features of the water relations of plants, see: Liv- 

 ingston, B. E., A schematic representation of the water relations of plants. Plant 

 World, 15: 214-18, 1912. 



