WATER CONTENT ASSOCIATION 27 



light association in a certain degree in hylophytes and 

 poophytes, it really represents the fundamental zonation of 

 vegetation with reference to water, in which the first zone, 

 the ocean (hydrophytes), has been ignored. The proper 

 series, then, is ocean, forest, grassland and desert, corres- 

 ponding exactly to hydrophytes, mesophytes and xerophytes, 

 in which mesophytes, largely because of difEerences of 

 water content and humidity, but also on account of light, 

 break up into hylophytes (forest) and poophytes (grassland). 

 The same concept is thus seen to underly both series, the 

 apparent difference in the two arising simply from the fact 

 that the one, ocean, forest, grassland and desert represents 

 the fundamental structure of vegetation, while the other, 

 hydrophytes, hylophytes, poophytes and xerophytes, 

 emphasizes the causative factors. 



Prom the above, it follows that Schimper's so-called cli- 

 matic formations, forest, grassland and desert, are merely a 

 somewhat incomplete expression of water content associ- 

 ation. As to the validity of his division of all formations 

 into climatic and edaphic, there is also room for grave 

 doubt. ' While there can be no question of the existence of 

 great formations, or series of formations, which bear a dis- 

 tinctive impress, yet one may well hesitate to apply the 

 term climatic to them. The conditions which have to do 

 with the water supply of a plant are of so much more import- 

 ance in determining its character than those which regulate 

 the water loss, that all plants, and consequently the form- 

 ations which they constitute, are primarily influenced by 

 soil, i. e., they are edaphic. In the ultimate analysis, how- 

 ever, species and formations owe their peculiar impress to 

 a certain constant relation between the mean of climatic 

 conditions and the mean of edaphic ones. This relation be- 

 tween the soil and the atmosphere must be as constant in 

 the case of an extensive formation as in that of a restricted 

 one; otherwise, the formation will change character. The 

 difference between the two formations, other than a different 



^Clements, F. E. Schimper's Pflanzengeographie Science 9:747 



