REACTIONS ON LAND 71 



Kinds of Reaction. From their very nature, land reactions are 

 most satisfactorily classified in accordance with the effect upon the 

 habitat (cf . Clements, 1928) . However, in extending the scope of the 

 process, the most convenient division is into land and water, with 

 subdivisions into soil and air, fresh water and salt water, bottom, etc. 

 Furthermore, though it is also convenient to refer to them as plant or 

 animal, this distinction often has no corresponding difference in proc- 

 ess or effect. As to the processes concerned, though these differ greatly 

 in detail, all are characterized by the addition or subtraction of 

 material or energy, or by some feature of disturbance. Finally, the 

 life form of the reactor may sometimes be taken into account to some 

 advantage, though this may carry analysis and classification further 

 than present needs warrant. 



Until recently, the treatment of terrestrial reactions has been 

 mostly qualitative in nature, but the increased emphasis upon soil 

 conservation as a national measure has already placed the reaction of 

 the plant cover upon soil in the forefront of investigation. In addi- 

 tion to such general studies as those of Darwin (1881) and Passargc 

 (1904), a promising start in the direct attack upon the measurement 

 of reaction in relation to competition and to succession has already 

 been made (Sampson and Weyl, 1918; Lowdermilk, 1926, 1930, 1931, 

 1934; Formosov, 1928; Forsling, 1931; Greene and Reynard, 1932; 

 Weaver and Harmon, 1935; Weaver and Noll, 1935; Kramer and 

 Weaver, 1936; Kraebel, 1936). The most comprehensive attack upon 

 this problem is that which is now being made at the several erosion 

 stations of the Soil Conservation Service. Meanwhile, the increasing 

 attention given to physical factors in the ocean is providing the neces- 

 sary background for evaluating the reactions of the plankton in 

 particular. 



REACTIONS ON LAND 



The reactions exhibited by terrestrial communities are logically 

 divisible into the effects exerted upon the soil complex and those that 

 modify aerial factors. In a comprehensive classification, these are 

 further divided on the basis of organism or agent, life-form or life- 

 habit process, effect, and degree. For the present purpose, however, 

 it will suffice to pass reactions in review on the basis of the process 

 chiefly concerned and to discuss the respective parts taken by plants 

 and animals. 



