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 saprophytism 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. 
