20 THE VEGETATION OF THE UNITED STATES. 



should the unicentric foliage of a Yucca be distinguished from the 

 similar leaf arrangement of Echeveria, in which the leaves are separated 

 by intemodes? Should the sessile foliage of Agave be regarded as per- 

 forming its functions in precisely the same manner as the similar leaf- 

 rosette of Aloe, which is raised well above the ground on a stout stem? 

 It is only to future investigations that we can look for knowledge that 

 will enable us to draw a line between the structural features that are of 

 physiological or ecological importance and those that are due to what 

 we might designate as evolutionary inertia. It is still impossible for us 

 to distinguish between structures that are vestigial, in the sense that 

 they no longer perform an office that they were able to perform in the 

 early history of their race, and structures or structural features that 

 arose fortuitously and never served a vital function, at the same time 

 that they were not of such a nature as to be eliminated by selection. 



It does not require an examination of the physiological significance 

 of the criteria used in any of the classifications of growth-forms to 

 discover the fact that the water-relations of plants have done far more 

 to influence their external form than have any other set of relations 

 to environmental conditions. Anyone familiar with the cultivation 

 of plants could predict with great certainty the relative water require- 

 ments of Parosela spinosa, a hoary, small-leaved tree of the Colorado 

 Desert, and such a tree as the red maple. It is not an invariable rule, 

 however, that the water requirements are obvious, as witness the close 

 similarity of Baccharis scoparia of the Jamaican mountains and Bac- 

 charis emoryi of the Colorado Desert, or the general similarity of the 

 grasses of dunes and swamps. The water requirements of a plant may, 

 however, be much more commonly read from its outward form than 

 may its temperature requirements. There is nothing, for example, in 

 the appearance of Pinus divaricata and Pinus carihoea to indicate that 

 the former grows in the cold taiga of Canada and the latter in the West 

 Indian Islands and Florida. 



Schimper recognized the importance of giving equal weight to the 

 water and temperature requirements of plants in grouping them for 

 ecological purposes. He accordingly divided each of the general classes 

 of plants which are recognized on a basis of their water-relations — 

 xerophytes, mesophytes, and hydrophytes — into three classes based 

 on temperature requirements — microtherms, mesotherms, and mega- 

 therms. In this manner nine categories were secured, in which it was 

 possible to place plants only after securing some knowledge of their 

 habitat requirements. 



The only logical basis on which we can proceed to a classification of 

 the vegetation of the world is one in which we take account of the 

 nature of the vegetation itself, and give no weight whatever to any of 

 the natural conditions or circumstances by which vegetation is affected. 

 It is for this reason that importance attaches to the study of growth- 



