INSTRUMENTAL METHODS. 439 



surface downward; (3) measurement of humidity and evaporation in the chief 

 reaction level of the air, and especially at the soil-surface, where the effect 

 upon the seedling is critical; (4) determination of the degree of acidity of soil- 

 water at different depths; (5) determination of the degree of alkalinity at 

 different depths. 



Methods of determining the holard are so numerous and so simple as to 

 need Uttle comment. From the standpoint of succession, however, it is imper- 

 ative to determine the holard at levels marked by the root-layers, and espe- 

 cially in the soil-layer occupied by the roots of dominants. But, while it is an 

 easy matter to measure the reaction in terms of increased water-content, the 

 successional significance of this increase can be determined only be ascertaining 

 the amount of it available, i. e., the chresard. Our knowledge of this available 

 water and of the water requirements of plants has greatly increased since the 

 chresard was emphasized as the critical factor in vegetation (Clements, 

 1905 : 30; 1907 : 9). In spite of the excellence of the work done under control 

 conditions (Briggs and Shantz, 1912; Crump, 1913 : 96; 1913^ : 125), it seems 

 certain that the r61e of the chresard in succession can only be determined under 

 field conditions, owing to its variation at different soil-levels and perhaps with 

 the conditions for transpiration. The measurement of evaporation has been 

 so standardized by the porous-cup method in the work of Livingston (1910 : 

 111) and others, and by the open water method of Briggs and Belz (1910 : 17) 

 that there is httle left to be desired. Readings of hmnidity, temperature, 

 and wind have become unnecessary, except as they are required for the analy- 

 sis of evaporation or for other piuposes. In the case of evaporation, however, 

 while this gives a measure of reaction, it may not have a causal connection with 

 succession. This is apparently the case in the successive consocies of scrub 

 and forest, where the evaporation decreases toward the climax, but the reaction 

 in control of the sequence is that upon Hght. Moreover, evaporation measures 

 fail to reckon with the compensating effect of water-content, and it seems 

 inevitable that measures of transpiration be largely substituted for those of 

 evaporation in the study of serai reactions. Considerable success has already 

 been attained in selecting species and standardizing individuals for this 

 purpose, and the method gives promise of universal application. Until we 

 have a clearer notion of the actual effect of an acid holard, the present methods 

 of determining the degree of acidity by means of Htmus or phenolphthalein 

 are fairly satisfactory. It seems increasingly certain, however, that the acid 

 is merely a by-product of decomposition under a lack of oxygen, and that the 

 absence of oxygen is the real factor. Experiments now under way seem to 

 prove this, and hence to indicate that measurement of the primary reaction in 

 acid soils must be directed toward the effect upon the oxygen content, i. e., 

 upon aeration. The determination of the alkaUnity of the soil solution has 

 been so thoroughly worked out by Briggs (1899) by means of electric resistance 

 apparatus that it seems to leave nothing more to be desired. 



Measurement of light reactions. — Since the pioneer work of Wiesner (1895) 

 in measm-ing light intensity, a number of methods have been devised to 

 measure light values (Clements, 1905 : 48; 1907 : 72; Zon and Graves, 1911). 

 Most of these have had to do with light intensity, but the spectro-photo- 

 meters of Zederbauer (1907) and Knuchel (1914) have been devised for the 

 purpose of determining the quality of forest light. Most instruments for 



