RELATION TO LIFE FORMS 69 



by the reaction of trees upon light. The shadow of a single tree shifts 

 with the sun, and, in consequence, the reduced light intensity is 

 permanent only over a small area about the base. Thus, while a com- 

 munity of trees casts less shade than the same number of isolated 

 individuals, the effect is constant and continuous, and hence becomes 

 controlling. The significance of the community is likewise clearly 

 demonstrated in the production of duff and leaf mold. The litter is 

 again only the sum of all the fallen leaves and twigs of the individuals, 

 but its accumulation is dependent upon the practical cessation of wind 

 action. The reaction of plants upon wind-borne sand and silt-laden 

 waters further exemplifies the unique importance of the community. 



The animal members of the terrestrial community are less effective 

 than plants in producing reactions, as a general rule. In spite of this, 

 waste products and hard parts often accumulate in great amounts, 

 while burrowing animals in particular frequently exert a pronounced 

 effect in disturbing soil or bottom, and sessile and sedentary ones in 

 protecting or perforating the substratum. Moreover, animal reactions 

 may be more or less direct consequences of food coactions, as in the 

 tunnels and mounds of pocket gophers, moles, etc. 



Relation to Life Forms. Some reactions are the direct consequence 

 of a normal functioning of the organism. AVith respect to plants this 

 is illustrated by the decrease of water content through absorption, the 

 increase of humidity as a result of transpiration, and the weathering 

 of rock by the excretion of carbon dioxide. The amount of oxygen, 

 carbon dioxide, or solutes in the medium is directly affected by both 

 plants and animals, and animals produce many deleterious excreta. 

 Furthermore, such a functional complex as growth may lead to the 

 direct modification of physical factors, but as a rule this is much more 

 strikingly related to life form. 



Reactions in both plants and animals may be directly connected 

 with form. Growth form in plants, however, is primarily an outcome 

 of reaction as brought about by competition. The plant matrix of the 

 community owes its predominant ability to modify land habitats to 

 the vegetation forms represented by the dominants especially. This 

 is best revealed by the contrast between forest and prairie ; the former 

 exerts a controlling action upon aerial factors, the latter a much 

 slighter effect. On the other hand, the reaction of forest on soil is 

 primarily surficial, while that of grassland is usually profound, owing 

 to the fibrous root systems, which ramify much more completely 

 through the soil. The successful reaction of pioneers in dunes of sand 

 or topsoil, in gravel slides and badlands, is chiefly a matter of the 

 form assumed by the shoot and root, in consequence of which the 



