V. REACTIONS. 



Concept and nature. — By the term reaction is understood the effect which a 

 plant or a community exerts upon its habitat (Clements, 1904:124; 1905: 256; 

 1907 : 282). In coimection with succession, the term is restricted to this special 

 sense alone. It is entirely distinct from the response of the plant or group, i. e., 

 its adjustment and adaptation to the habitat. In short, the habitat causes the 

 plant to function and grow, and the plant then reacts upon the habitat, chang- 

 ing one or more of its factors in decisive or appreciable degree. The two proc- 

 esses are mutually complementary and often interact ia most complex fashion. 

 As a rule, there is a primary reaction with several or many secondary ones, 

 direct or indirect, but frequently two or more factors are affected directly and 

 critically. Direct reactions of importance are confined almost wholly to 

 physical factors, with the exception of parasitism, which can hardly be regarded 

 as a rieaction proper. With almost no exceptions, reactions upon biological 

 factors have barely been touched by investigators as yet. Any exact under- 

 standing of them must await the quantitative study of the community as 

 a biological unit. 



The reaction of a community is usually more than the sum of the reactions 

 of the component species and individuals. It is the individual plant which 

 produces the reaction, though the latter usually becomes recognizable through 

 the combined action of the group. In most cases the action of the group 

 accumulates or emphasizes an effect which would otherwise be insignificant 

 or temporary. A community of trees casts less shade than the same number 

 of isolated individuals, but the shade is constant and continuous, and hence 

 controlling. The significance of the community reaction is especially well 

 shown in the case of leaf -mold and duff. The leaf -litter is again only the total 

 of the fallen leaves of all the individuals, but its formation is completely 

 dependent upon the commxmity. The reaction of plants upon wind-borne 

 sand and silt-laden waters illustrates the same fact. 



Some reactions are the direct consequence of a functional response on the 

 part of a plant. This is exemplified by the decrease of water-content by 

 absorption, the increase of humidity as a consequence of transpiration, and 

 the weathering of rock by the excretion of carbon dioxid. Others are the 

 immediate outcome of the form or habit of the plant body. The difference 

 between woody plants and grasses in the reaction upon Ught and humidity is 

 one of the critical facts in succession. Almost any obstruction may cause 

 the deposition of dime-sand or of water-borne detritus. The actual formation 

 of a dime depends, however, upon the aerial and soil forms so typical of sand- 

 binders. The accvunulation of leaf-mold, filling with plant remains, and the 

 production of humus are all due to the death and decay of plants and plant 

 parts. Marl, travertine, calcareous tufa, and sinter are partly or wholly the 

 result of little-understood processes of the plant. The successful reaction of 

 pioneers in gravel-shdes and in bad lands is almost wholly a matter of mat, 

 rosette, or bunch forms and of extensive or deep-seated roots. In a primary 

 area the reaction is exerted by each pioneer alone, and is then augmented by 

 the family or colony. It extends as the communities increase in size, and 



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