304 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Besides various essential or beneficial substances including growth- 

 stimulating ones, harmful toxins may be produced in the soil by 

 living organisms or parts of organisms — with corresponding effects 

 on plant distribution. And then there are various mycorrhizal 

 associations, Algae, and other units in the microcosm, all of which 

 may have their importance to the vegetation and to plant geography, 

 though quite how is often not well understood. Nor should we 

 forget the importance of soil temperature, which can act in so many 

 and various ways — as, for example, through the soil's living content. 



BlOTIC 



The biotic factors in the wide sense are those due to living 

 organisms, whether animal or plant — ranging, as it were, from Man 

 and the great herbivores and trees down to the lowly but often 

 vitally important soil microorganisms with which we have just dealt. 

 It is useful to visualize the total components of an immediate 

 environment or recognizable habitat as forming a self-contained 

 ecosystem, composed on one hand of the inorganic and dead parts 

 and, on the other, of the various organisms which live together in 

 it as a sociological unit and comprise the biota. A large, primary 

 biotic community in which the climax vegetation [see next chapter) 

 is more or less uniform is termed a biome. Our interest is primarily 

 in the living components, the inert ones being from our point of 

 view mere factors conditioning the existence, structure, and develop- 

 ment of the biome — chiefly through their effect on one or more of 

 its component organisms. These are the individual species or other 

 taxa, divisible approximately into animals and plants. The latter 

 form the plant community, which may be loosely defined as an 

 entire population of plants growing together and maintaining as a 

 whole a corporate individuality that is not the same as the sum 

 total of the separate manifestations and effects of its components. 



The component plants of a community have many immediate 

 internal (' autogenic ') effects upon one another and upon their own 

 habitat, as for example in competition and the deposition of humus, 

 and by producing various changes in the soil ; they even have 

 ' allogenic ' effects in sheltering other plants and in dispersing them- 

 selves outside the immediate community. It is convenient, however, 

 to treat these plant-engendered repercussions under succession [see 

 next chapter) and to regard separately as introducing the (collective) 

 biotic factor chiefly those animals which have a marked effect upon 



