438 THE INVESTIGATION OF SUCCESSION. 



decisions as to ecesis and competition are reached. The measurement of 

 reaction at other levels is not without value, but it is rarely of primary impor- 

 tance. Since the critical period for each species is usually the seedling stage, 

 it indicates that the depth of the reaction-level above and below the surface 

 is only a few inches, or at most a foot or so. This greatly narrows the field 

 of measurement, and makes the appUcation of the results much easier. The 

 most critical area of all is where the reaction-level of one community meets that 

 of another, i. e., the ecotone. This is strikingly illustrated by the reciprocal 

 behavior of the seedlings of both communities at the ecotone between grass- 

 land and forest. It is in such areas that reactions can be best determined and 

 their influence measured. 



Measurement of reactions. — ^The methods of instrumental investigation 

 (Clements, 1905 : 20; 1907 : 7, 73) are now so numerous and detailed that no 

 adequate account of them can be attempted here. It must suffice to point 

 out the general method of attack and to emphasize the necessity of such study 

 for the understanding of succession. At the outset, it must be recognized 

 that general measurements, such as are made in the usual meteorological 

 observations, are of little or no value. This is true to some extent also of the 

 data obtained by ecograph batteries in various habitats. These bear some 

 relation to the conditions in actual control of ecesis and competition, but the 

 direct attack must be made upon these conditions themselves, since they 

 characterize the reaction-level. Thus, while the experienced investigator 

 may find it possible to interpret and apply general factor data, one can expect 

 to obtain httle fight upon succession unless the instrumental study is con- 

 centrated upon the factors in primary control. This means that water and 

 light reactions must be given the first place, though it is certain that water 

 reactions in particular must ultimately be carried further back into the plexus 

 of intricate cause-and-effect relations found in the soil. Moreover, in dealing 

 with water and light, it must be borne in mind that the reaction may affect the 

 quality as well as the quantity, and that hiunidity as well as water-content 

 must be considered. FinaUy, it should be recognized that, while instruments 

 furnish the readiest means of measurement, the use of standard plants for 

 determining the effect of each reaction brings us much nearer to the explana- 

 tion sought. 



Measurement of water reactions. — The reaction of a community upon 

 water may affect the amount of holard and echard, or the degree of humidity. 

 It may change the nature of the water-content by modifying the nutrient 

 content, by making it acid or decreasing its acidity, or by decreasing its alka- 

 linity. So far as is known the alkalinity of the soil can not be increased by the 

 accumidation of plant remains in it, except by artificial means. The develop- 

 ment of instruments and instrumental methods for the study of water-content 

 and hmnidity has gone so far that even a brief mention of them is impossible 

 in the scope of the present discussion. The great majority of them have not 

 been developed with reference to succession, and hence the number which 

 require mention is small. Those of the first importance for the accurate field 

 study of reaction are: (1) determination of the holard in the reaction-level 

 of the soil, and especially at the germination level; (2) determination of chre- 

 sard and echard at the depth of various roots in the reaction-level, since the 

 addition of humus, and often the abstraction of water also, decreases from the 



