WATER CONTENT ASSOCIATION 23 



{aletophytes), were formed by dividing the mesophytes, 

 largely upon the basis of light association. Schimper (1898: 

 3) merged halophytes in xerophytes as only one of several 

 subdivisions of the latter, and termed mesophytes tropophytes, 

 on account of the alternation of wet and dry conditions dur- 

 ing their period of growth. In postulating the dictum, "E s 

 muss zwischen physikalischer und physiolog- 

 ischer Trockenheit, bezw. Peuchtigkeit unter- 

 scheidenwerden; letztere allein kommt ftlr das Pflanzen- 

 leben, also auch fttr die Pflanzengeographie, in Betracht," 

 Schimper placed the study of vegetation upon a new basis. 

 Pound and Clements (1900:169) followed the division into the 

 three fundamental groups, hydrophytes, mesophytes and 

 xerophytes, retaining hylophytes, poophytes and aleto- 

 phytes as subdivisions of mesophytes. 



Warming's great contribution to ecology lies in his full 

 recognition of the inherent value of water content associ- 

 ation as its basis, and in making a logical and systematic 

 treatise out of the scattered results of many workers. The 

 exceeding timeliness of his work made it practically certain 

 that his views would influence the general character of 

 ecological research for a long time to come. The weak 

 points in his system were the treatment of humic acid (bog) 

 plants under hydrophytes, and the separation of halophytes 

 from xerophytes as a coordinate group. This by no means 

 indicates that Warming failed to see the contradiction in- 

 volved in the occurrence of xerophytes in apparently hydro- 

 phytic situations, or to appreciate the many points of simil- 

 arity between xerophytes and halophytes. On the contrary, 

 he has summed up (176) in what was for that time a very 

 lucid manner, the status of the relation of swamp xerophytes 

 to their habitat. It remained, however, for Schimper to 

 perceive the essential similarity of this relation, as deter- 

 mined by Johansen for bog plants, with that found in crymo- 

 phytes by Kihimann (J890) and Goebel (I890j and in halo- 

 phytes by himself (I890,J89t), and to give expression to the 

 fundamental law of habitats, namely, that it is the physio- 



