ECOLOGY 51 



of the Heather and the Cotton Grass, but of the Sphagnum 

 Moss as well. Such groups of quite dissimilar plants 

 growing together form the communities, or " forma- 

 tions," as they are sometimes technically called, and, 

 in a way, they correspond to a city among men where 

 there is room for a certain number of tanners and 

 bakers and printers and postmen, but where, if the com- 

 munity is to succeed, the types must not all be adapted 

 to the same trade nor exactly the same environment. 

 The interaction of the individuals on each other is as 

 important a part of the environment as are the merely 

 physical conditions. Indeed, among plants as well as 

 among animals, they largely determine the physical 

 conditions. For example, the ground immediately 

 under a tall, spreading tree is often quite dry even in 

 the heaviest rain ; it is then futile to measure the rain- - 

 fall for the district and to assume that in that district 

 all plants that require that rainfall would be happy in 

 it. So in any community, because the plants are 

 growing together, it does not at all follow that they 

 require the same conditions for life ; but that they fit 

 into each others needs, and together help to adapt to 

 their requirements the natural physical environment. 



In speaking of a plant community or formation, 

 however, one does not only consider the plants that 

 form it, for to some extent we have, subconsciously in 

 our minds, the thought of the physical nature of the 

 locality in which the plants are growing. For instance, 

 " a marsh " almost postulates the conception of a flat, 

 low-lying, water-logged piece of ground, while " a 

 heath " conveys the idea not only of a mixture of Heather 

 and dry grasses but of a stretch of comparatively high 

 land of a dry and often sandy nature. 



If we take such communities as units and imagine a 



