78 KINDS OF INDICATORS. 



terms for indicators, it must be clearly understood that the reference is to the 

 nature and size of the area concerned, and not to the position of the factor 

 in the soil or the air. In the sense employed here, climatic and edaphic indi- 

 cators are synonymous with climax and serai ones, respectively, though the 

 emphasis in the former case is upon the factors rather than the process of 

 development. 



Water indicators. — A detailed account of our present knowledge of the 

 indicators of each factor is impossible within the limits of the present treat- 

 ment. It must suffice to point out here the general relations of each factor to 

 its plant and community indicators and to consider the most important and 

 best understood of the latter in the chapters which have to do with climaxes 

 and with practice indicators. The broader correlations of water and its 

 indicators have already been touched upon in Chapter II, and the following 

 brief statement is intended primarily to emphasize some of the basic points 

 involved and to suggest probable lines of advance in future work. 



Water use will undoubtedly become the primary basis for interpreting the 

 water-relations of plants, when the use of phytometric methods becomes 

 general. Expressed in terms of transpiration per unit area and per gram of 

 dry matter produced, this will furnish the first exact basis for the classifica- 

 tion of plants on the basis of water. The application of such methods to 

 native species will be a slow matter, however, especially under field conditions. 

 Consequently, the indicator value of native plants for water must still rest 

 largely upon determinations of water-content, humidity, evaporation, and the 

 transpiration of standard plants, supplemented to some degree by studies 

 of the form, structure, and growth of the plants themselves. Thus it becomes 

 particularly important to refine the concept of water-content, since this 

 exerts the basic control in water relations, and to render its expressions more 

 definite and comparable (plate 13). 



The general value of the echard for the various kinds of soils is now so well 

 known that determinations of the holard are helpful in refining the values 

 gained from sequences. This is particularly true when a single uniform soil 

 is concerned, though even here account must be taken of differences at the 

 various levels. The importance of the echard at the critical period has 

 obscured the fact that it is the chresard which represents the amount of water 

 available for the work of the plant, and that a very large number, if not the 

 majority of species, probably never reach the echard during their lifetime. 

 The water-response of such plants, and hence their indicator value, is con- 

 cerned with the chresard. In the case of xerophytes and xeroid plants, includ- 

 ing the crop plants of arid regions, the echard may be reached more than once 

 during the growing season, or the plant may remain at that point for a con- 

 siderable portion of the year. When the latter occurs, the plant bears a dis- 

 tinctive xerophytic impress, the intensity of which is apparently correlated 

 with the length of the period of deficiency. The difficulty of making echard 

 determinations in the field is such that in practice it is much more satisfactory 

 to obtain this indirectly by means of the moisture-equivalent method of 

 Briggs and Shantz (1912 : 56), and to express the seasonal chresard graphi- 

 cally, as has been done by Weaver (1917). 



The lack of agreement between the results of the earlier investigators and 

 those of Briggs and Shantz may be due in part to the more exact physical 

 methods of the latter. So far as native plants are concerned, however, there 



