92 COMMUNAL LIFE OF ORGANISMS sect, ii 



different growth-forms are combined to form a single aggregate which 

 has a definite and constant guise. 



The analysis of a plant-community usually reveals one or more of 

 the kinds of symbiosis as illustrated by parasites, saprophytes, epiphytes, 

 and the like. There is scarce a forest or a bushland where examples of 

 these forms of symbiosis are lacking ; if, for instance, we investigate 

 the tropical rain-forest we are certain to find in it all conceivable kinds 

 of symbiosis. But the majority of individuals of a plant-community 

 are linked by bonds other than those mentioned bonds that are best 

 described as commensal. The term commensalism is due to Van Beneden 

 who wrote ' Le commensal est simplement un compagnon de table ' : 

 but we employ it in a somewhat different sense to denote the relationship 

 subsisting between species which share with one another the supply of 

 food-material contained in soil and air, and thus feed at the same table. 



More detailed analysis of the plant-community reveals very consider- 

 able distinctions among commensals. Some relationships are considered 

 in the succeeding paragraphs. 



LIKE COMMENSALS 



When a plant-community consists solely of individuals belonging to 

 one species for example, solely of beech, ling, or Aira flexuosa then we 

 have the purest example of like commensals. These all make the same 

 demands as regards nutriment, soil, light, and other like conditions ; as 

 each species requires a certain amount of space and as there is scarcely 

 ever sufficient nutriment for all the offspring, a struggle for food arises 

 among the plants so soon as the space is occupied by the definite numbers 

 of individuals which, according to the species, can develop thereon. 

 The individuals lodged in unfavourable places and the weaklings are 

 vanquished and exterminated. This competitive struggle takes place 

 in all plant-communities, with perhaps the sole exceptions of sub-glacial 

 communities and in deserts. In these open communities the soil is 

 very often or always so open and so irregularly clothed, that there is 

 space for many more individuals than are actually present ; the cause 

 for this is obviously to be sought in the climatically unfavourable condi- 

 tions of life, which either prevent plants from producing seed and other 

 propagative bodies in sufficient numbers to clothe the ground, or prevent 

 the development of seedlings. On such soil one can scarcely speak 

 of a competitive struggle for existence ; in this case a struggle takes 

 place between the plant and inanimate nature, but to little or no extent 

 between plant and plant. 



That a congregation of individuals belonging to one species into one 

 community may be profitable to the species, is evident ; it may obviously 

 in several ways aid in maintaining the existence of the species, for instance, 

 by facilitating abundant and certain fertihzation (especially in anemo- 

 philous plants) and maturation of seeds ; in addition, the social mode 

 of existence may confer other less-known advantages. But on the other 

 hand it brings with it greater danger of serious damage and devastation 

 wrought by parasites. 



The bonds that hold hke individuals to a Hke habitat are, as already 

 indicated, identical demands as regards existence, and these demands 



