AIR REACTIONS 91 



and deposition, but these are more local and to a large extent condi- 

 tioned by vegetation. In view of these facts and especially the devel- 

 opmental correlation between climax and habitat (Clements, 1905:292; 

 1916:357) , soils may properly be distinguished on the basis of climaxes, 

 as has already been done to some extent in the recognition of forest 

 and prairie, humid and arid categories. In addition, they may be sub- 

 divided with respect to climax and sere, the role of topography being 

 especially important in connection with the latter. 



AIR REACTIONS 



From the very nature of the medium, the reactions of plants upon 

 the air are usually less definite and controlling than upon the soil. 

 Naturally, the chief reason for this lies in the fact that effects are not 

 readily accumulated in a gaseous medium. However, a notable excep- 

 tion exists in respect to light, in which the time element produces 

 results not unlike those of accumulation. The absence of air reac- 

 tions by animals is noteworthy, since the functions that produce their 

 striking reactions in water are almost without effect on land. 



Reaction upon Light. The leaves of plants react upon light by 

 virtue of reflection and absorption, while the chief role of branches and 

 trunks is interception. The obvious consequence is to reduce the inten- 

 sity, especially of sunlight, and to produce varying degrees of shade. 

 Since the absorption of leaves is selective, it has frequently been 

 assumed that the quality of light is changed under forest canopies 

 especially. It is now known that this takes place in a considerable 

 degree only in dense forests and thickets, owing to the fact that in the 

 great majority of cases the light beneath the crowns is derived from 

 rays reflected by the leaves or passing between and not through them. 

 The same principle holds true for the successive forest layers, the 

 quantity being affected to the extreme point where the ground layer 

 can consist only of mosses and fungi. 



In the initial stage of the hydrosere, the reduction of light intensity 

 by the water itself may be greatly augmented by the reaction of the 

 plant community. This is most notable for floating leaves, such as 

 those of pond lily and pondweed, but it may sometimes be as great 

 for submerged species, for duckweeds, and, under optimal conditions, 

 for microscopic algae of the plankton. The reaction of the dominants 

 of reed swamps is often much more decisive than it appears, and this 

 is true likewise of grassland, in spite of the more or less complete 

 absence of a canopy. In ponds, the minute animals of the plankton 

 exercise an effect when abundant, and this may likewise be true of 



