84 



KINDS OF INDICATORS. 



of conditions. The indicator values of this classic study were completed by 

 Shantz (Clements, 1916 : 233), who brought out the successional relations of 

 the various communities (plate 15, a). 



Plants indicate alkali by their presence or absence. The positive indicators 

 are the halophytes, which bear a distinctive xerophytic impress, caused pri- 

 marily by the decreased chresard in the presence of an excess of salts. When 

 the relation is chiefly one of concentration, the condition is known as "white 

 alkali." This is due to the presence of sodium chloride, sodium sulphate, 

 calcium sulphate, or other salts which possess no directly injurious action. 

 Sodium carbonate produces "black alkali," which is directly deleterious to 

 the plant, probably through corrosion of the tissues. The latter renders the 

 soil useless agriculturally, while the former does not, except when present to 

 an excessive degree. Since the three sodium salts often occur together, the 

 plants of alkali soils serve chiefly as indicators of the total concentration, and 

 the significance of the "black alkali" can be determined only by chemical 

 analysis or crop test. Hilgard (1906 : 535) regards the following species as 

 indicators of irreclaimable land when they occur as dominants, unless the 

 land is underdrained to remove the excess of salts: Sporobolus airoides, Dis- 

 tichlis spicata, Spirostachys occidentalis, Salicorniaspp., Dondia torreyana, D. 

 suffrutescens, Sarcobatus vermiculatus, Frankenia grandifolia campestris, and 

 Cressa truxillensis. In the Tooele Valley the crop-producing power of saline 

 lands have been summarized by Kearney et al. (1914 : 414) in the following: 



Community indicators of crop production in saline lands. 



Lime indicators. — The original plan of giving a concise but complete 

 account of the various views as to the effect of lime in native vegetation has 

 necessarily been abandoned by reason of the limitations of space. Conse- 

 quently, it must suffice to point out that the former views of the calciphily or 

 calciphoby of various species are untenable, and that the effects usually 

 ascribed to lime are either due to a complex of factors or to its indirect action. 

 Schimper (1903 : 94) has presented the best summary of the arguments which 

 support the assumption that lime is a factor of primary importance, but even 

 his account reveals the many weaknesses of the theory. The latter are clearly 

 brought out in the following statement : 



"External conditions, however, change with the area. In one area, the 

 silica-form, in another the lime-form, is better adapted to local conditions, 

 whilst in a third area both forms may be able to maintain themselves in the 

 struggle for existence. Accordingly, one and the same species is calciphobous in 

 the first area, calciphilous in the second, and indifferent in the third." (p. 104.) 



