96 REACTIONS. 



families and colonies, and has an interesting and probably important relation 

 to the presence and behavior of pollinating insecte. 



Correlation of reactions. — The eflScient reactions in the great majority of 

 seres are those that have to do with the increase or decrease of water-content 

 and the decrease of light intensity. These are the controlling reactions in 

 all primary seres, though a portion of the development may be dominated by 

 the presence of acids or toxins, as in peat-bogs, or by the nutrient relations, 

 as in heath. Up to the appearance of the first shrub stage the water-content 

 reactions are directive. With the entrance of trees and shrubs it becomes 

 wholly or largely subordinate to the light reactions. In the development of 

 grass or herbaceous climax formations reaction upon light plays little or no 

 part. On the contrary, many secondary seres, especially those originating 

 in burns or clearings, may be controlled almost entirely by the decreasing 

 light value. In short, the chief reaction upon the habitat is necessarily upon 

 the soil and its factors, until a community develops sufficient height and 

 dominance to control air conditions. 



The accumulation of plant remains or humus is the most complex of all 

 reactions, as it is the most universal, since it is the direct and inevitable out- 

 come of the presence of plants. In initial water or wet areas it decreases 

 water-content and increases nutrient-content and aeration, unless decomposi- 

 tion produces an excess of acids or other deleterious substances. Its effects 

 in dry areas are largely opposite. It is the great factor in increasing the water- 

 content, but at the same time it also increases the available nutrients. With 

 the appearance of woody communities its influence is masked by the light 

 reactions, but continues to be felt in some degree. It becomes obvious again 

 in woodlands where conditions cause the development of acids, as in beech 

 peat, and may lead to a critical decrease in water-content. 



Quantitative study of reactions. — Our exact knowledge of the amount and 

 effect of community reactions is very slight. The investigation of habitat 

 and community by means of instruments is still exceptional. The few quan- 

 titative studies so far made have been directed for the most part to other 

 problems, and have rarely dealt with the measurement of reactions. The 

 earliest attempts to measure the reactions of the stages in succession were 

 made in the woodland and prairie formations of Nebraska from 1898 to 1906, 

 and in the moimtain and plains formations of Colorado from 1901 to 1910. 

 As already indicated, the first account of the quantitative study of the major 

 reactions of a succession was published in 1910, in connection with the life- 

 history of the secondary sere in burned areas. This was followed by a similar 

 account of the reactions in the grassland stages of the Great Plains (Shantz, 

 1911). The earlier results in Nebraska and Colorado have as yet been pub- 

 lished only in part (Thomber, 1901; Hedgcock, 1902; Clements, 1904; E. S. 

 Clements, 1905; Shantz, 1906). 



In addition to the pioneer work of Wiesner (1895, 1904, 1907) upon the 

 reaction on Kght, a number of measurements have been made dining the last 

 decade of habitat factors. While these were not directed at reactions as such, 

 they are often of much value in this connection. Such are the studies of 

 Livingstone (1906) on the relation of desert plants to holard and evaporation, 

 Zederbauer (1907) on the composition of forest light. Yapp (1909) on evapora- 

 tion and temperature in swamps, Dickey (1909), Brown (1910), and Sherif 



