SOIL ORGANISMS. 91 



Without attempting to pass upon the matter in general, it may be said that 

 the most persistent search for a decade has failed to reveal any evidence of their 

 r61e in the innumerable examples of succession in the Rocky Moimtains. On 

 the contrary, the detailed study of the ecesis of occupants and invaders in 

 the famiUes and colonies of pioneer stages indicates better development in 

 such areas, as would be expected from their reactions. 



The existence of bog toxins resulting from partial decomposition or from 

 the complex organic interactions of bogs is much more probable (Livingston, 

 1905; Transeau, 1905; Dachnowski, 1912). It is difficult to regard their pres- 

 ence as proved, however, and a long period of quantitative and experimental 

 study of succession is needed to reveal their importance as a reaction. At 

 the present it seems clear that acids, poor aeration, and bog toxins would all 

 have the same effect upon successional movement. The chief task before us 

 is to assign to each one its proper place. 



SOIL ORGANISMS. 



The relation of plants to the organisms in the soil is so complex that it is 

 impossible to recognize all of the effects, or to distinguish the causes of many of 

 them. For the present purpose it will suffice perhaps to draw a distinction 

 between the organisms directly connected with the plant and those not in 

 organic relation to it. The former may be included in the general term of 

 parasites, though many are symbiotic, of course, while the latter are sapro- 

 phytes. Animals as well as plants are found in both groups. The parasites 

 may be regarded as a direct reaction of the plants, while the saprophytes are an 

 indirect reaction, or, better, a consequence of the accumulation of plant 

 remains. 



(16) Reaction by means of parasites. — ^The relation between host-plant and 

 parasite is so intimate that it seems hardly to constitute a reaction. Yet 

 it has a direct bearing upon the fate of the community and its part in succes- 

 sion. The latter is determined largely by the degree of parasitism. If it is 

 intense and destructive, the individual wiU be destroyed or handicapped in 

 its competition or dominance. As a consequence, it may disappear wholly 

 from the cormnunity, though this is relatively rare. The most usual effect is a 

 decrease in niunber or dominance by which the species assumes a less impor- 

 tant r61e. In the majority of cases no direct influence is discoverable, the 

 effect being merged in the general outcome of competition. 



When the relation is more or less symbiotic, its general effect is first to in- 

 crease the dominance of the host-plant, but finally to favor species with higher 

 nitrogen demands. Warren (1909) has pointed out that this is the effect of 

 the nodule-bearing legumes in the prairie formation. The legumes are able to 

 grow in the poorer soils by virtue of their symbiotic partnership and consequent 

 nitrogen production. They thus make possible the greater development of 

 grasses, before which they disappear, sometimes completely. The presence 

 of mycorrhiza alone makes possible the successful ecesis of an increasing 

 number of plants, especially trees and shrubs, and hence controls their appear- 

 ance in succession. Their disappearance may be due to the competition result- 

 ing from the invasion of plants with greater nitrogen demands, but it is also 

 influenced by other reactions. 



