ASSOCIATION 205 



kinds of associations: parasitism, helotism, mutualism, epiphytism, lianism, 

 and commensalism. Commensalism corresponds to the primary principle 

 of association which has given rise to vegetation. Homogeneous commen- 

 salism 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 vegetation. Warming re- 

 gards saprophytisni merely as a specialized 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 parasites, epiphytes, and endophytes 

 of doubtful physiological relation. Pound and Clements^ treated lianes, par- 

 asites, 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 nu- 

 tritive stratum, and with respect to association should be merged in one 

 group. Schimper- was the first to perceive the essential similarity of all 

 such groups from the standpoint of association. He terms these plant so- 

 cieties (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 Galium, to the most intense parasite. The distinction, how- 

 ever, 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. 



255. Ground association. The first division of formations into open and 

 closed was made by Engler and Drude.^ Open formations were defined as 

 those having incomplete stability and heterogeneous composition, while 

 closed formations have a more definite uniform stamp. What is true of for- 

 mations IS equally true of vegetation, so that association may be regarded 

 as open or closed with reference to the density and thoroughness with which 

 the plants occupy the ground. In open association, the ground is slightly 

 or partially occupied, readily permitting the entrance of new plants without 

 the displacement of those already present. Such an arrangement is char- 

 acteristic of the early stages of a formation, or of a succession of forma- 

 tions. It produces unstable open formations, which arise, usually 



' Phytogeography of Nebraska, 1st ed., 101. 1898. 



' Pflanzengeographie auf physiologischer Grundlage, 208. 1898. 



" Die Vegetation der Erde. Engler Bot. Jahrb., 17 :b55. 1893. 



