16 ASSOCIATION 



epiphytes, lianes and commensalism. Commensalism cor- 

 responds to the primary principle of association which has 

 given rise to vegetation. Homogeneous commensalism is the 

 term applied to social exclusive plants, in which the patch 

 is composed of a single species,. Such association is 

 extremely rare in nature, and, if the most minute forms be 

 considered, probably never occurs. On the other hand, 

 heterogeneous commensalism, in which individuals of more 

 than one species are present, is everywhere typical of vege- 

 tation. Warming regards sap rophytism merely as a special- 

 ised kind of parasitism, an opinion that may well be 

 defended. Helotism, however, is also a mere modification of 

 parasitism, if it is not indeed parasitism pure and simple. 

 Mutualism is an altogether vague concept, including para- 

 sites, epiphytes, and endophytes of doubtful physiological 

 relation. Pound and Clements (18*8:101) treated lianes, 

 parasites and saprophytes as vegetation forms, relating 

 herbaceous creepers and twiners to the lianes, and dividing 

 the fungi and lichens into nine groups. Whatever the value 

 of these divisions may be from the standpoint of vegetation 

 forms, they represent the same relation between plant and 

 nutritive stratum, and with respect to association should 

 be merged in one group. To Schimper (1898:208) belongs 

 the credit of having been the first to perceive the essential 

 similarity of all such groups from the standpoint of associ- 

 ation. He terms these, plant societies (Genossenschaften), 

 retaining the four groups already established, lianae, 

 epiphyta, saprophyta and parasiticae. It is evident that 

 dependent association comprises extremely divergent forms, 

 from the slightly clinging herb, such as Oalium, to the most 

 intense parasite. The distinction, however, is a clear one, 

 if restricted to that relation between plants, in which one 

 acts as a mechanical support or stratum, or as a nutritive 

 host for the other. 



Ground association (associaiio terrena). The first 

 division of formations into open and closed seems to have 

 been made by Engler and Drude(I893 : 55). Open formations 

 were defined as those having incomplete stability and 



