92 REACTIONS. 



(17) Reaction by means of saprophytes. — ^These have to do chiefly with the 

 formation of humus or with its modification in such a way as to make its 

 nitrogen again available for plants. This is true even of those fungi which 

 exist in the soil as saprophytes, and become parasitic when the proper host 

 becomes available. A few of these are very destructive in their action, and 

 sometimes effect the complete disappearance of a dominant. The fleshy 

 fimgi which play a large part ia the ground layer of boreal and mountain 

 forests have to do largely with hastening the conversion of plant remains into 

 humus, with its attendant effects upon water-content, nutrients, etc. This is 

 the well-known r61e of a large number of soil bacteria, especially those which 

 free ammonia or elaborate nitrates from nitrogenous substances or fix free 

 nitrogen. In the case of both fleshy fimgi and bacteria, the final effect is to 

 produce conditions in which plants with greater requirements can enter and 

 displace those with less exacting demands. The same general effect is exerted 

 by animals living in the soil, though there is some evidence that protozoa may 

 play an antagonistic rdle. 



AIR REACTIONS. 



The reactions of plant communities upon atmospheric factors are less numer- 

 ous and usually less controlling than those upon soil. The notable exception 

 is the reaction upon light, which plays a decisive part in the later stages of 

 the majority of seres. The effects upon the other air factors are so inter- 

 woven that it seems best to consider the reactions upon humidity, tempera- 

 ture, and wind together. As a consequence, the reactions may be grouped 

 as follows: (1) upon light; (2) upon humidity, temperature, and wind; (3) 

 upon the local climate; (4) upon aerial organisms. 



(18) Reaction upon light. — The primary reaction upon light is seen in the 

 interception of sunhght and the production of shade of varying degrees of 

 intensity. There may also be a secondary effect upon the quality of the light 

 (Zederbauer, 1907; Knuchel, 1914) where it has to pass through a dense 

 canopy of leaves. The preponderance of results up to the present time indi- 

 cates that the Ught beneath the tree-layer passes between the leaves and not 

 through them, and is essentially unchanged as to quality. The reduction of 

 light intensity is usually slight or even lacking in the early stages of succession, 

 though exceptions occur whenever plants are tall and dense, as in consocies of 

 Phragmites, Spartina, or Typha, or when leaves are broad and spreading, 

 Nymphaea, etc. As the population becomes denser, it intercepts more and 

 more light, with the result that a subordinate layer appears. With the 

 entrance of shrubs and trees, the reaction steadily becomes more marked 

 and the demarcation of subordinate layers more striking. In a layered forest 

 the reduction in Hght value is a progressive one from the primary layer down- 

 ward. In many forests of this type the cumulative reaction is so complete 

 that the ground layer can consist only of fungi and mosses, the latter with 

 the lowest of light requirements. As the canopy becomes denser and denser, 

 either by the growth of individuals or by the entrance of trees with closer tops, 

 the layers begin to disappear. This usually takes place in a downward direc- 

 tion, the final stage of a closed forest containing only mosses, fungi, and sapro- 

 phytic phanerogams, with occasional low herbs. Thus, even after the estab- 



