FACTOR INDICATORS. 83 



munities must be regarded as primarily related to water, and hence treated as 

 indicators of it. While these doubtless have relations to temperature which 

 are susceptible of measurement, they are subordinate, and in our present 

 incomplete knowledge can not be regarded as indications of it. 



Indicators of solutes. —The term solute is used here to indicate any sub- 

 stance dissolved in the holard. It may be solid or gaseous, or even liquid. 

 The best-known solutes are the mineral salts found in the soil, of which some 

 are nutrients, others more or less inactive, and some actually deleterious to 

 the plant. Of the gases dissolved in the holard, oxygen and carbon dioxid 

 are the most important, but oxygen is the only one which bears a clear rela- 

 tion to indicator plants. In addition, there are the debatable toxic exudates 

 and soil toxins, the existence of which is in doubt or the relation to the plant 

 uncertain. Livingston, who has devoted much attention to this subject 

 (1918 : 93), states: 



"Evidence that agricultural plants do actually excrete toxic substances into 

 the soil is not very strong in any of this work, however. As to the manner in 

 which these poison substances arise in the soil, no definite statements can yet 

 be made, but they are surely not excreted as such from plant roots. There 

 is physiological evidence, however, that such substances are given off by living 

 roots when the latter are practically deprived of oxygen." 



In so far as indicator plants are concerned, the effects ascribed to toxins 

 are much better explained on the basis of an inadequate supply of oxygen. 



The ordinary nutrient salts of the soil rarely leave a distinctive impress 

 upon plants, owing to lack of concentration. When the concentration reaches 

 a point where absorption is interfered with, the plant makes a definite physio- 

 logical and structural response to the saline or alkaline conditions. The 

 relation to lime and magnesia is less clear and the indicator impress less 

 marked. In the case of deficient aeration, the response is clear, but its expres- 

 sion is often limited to physiological and histological features. Since all 

 solutes act through water or in conjunction with it, their effects are often 

 obscured by the responses to it. This is particularly true of saline indicators, 

 which are merely xerophytes of a more or less peculiar type. 



Saline indicators. — The term saline is preferred as the general term for all 

 soil conditions in which soil salts occur in excess or a deleterious alkaline salt 

 is present. In the West it is practically synonymous with the word alkali, 

 and the two are employed interchangeably. Saline indicators are typical of 

 sea-shores the world over, but their most striking development is found in the 

 arid basins of the interior of continents, such as the Great Basin of North 

 America. Practically all the work with them has been done in such regions, 

 where the limits set by alkali to agricultural development are of the greatest 

 importance. The outstanding studies in this field are those of Hilgard (1906) 

 and Kearney (1914), and their respective associates. The work of Hilgard 

 touched a large portion of the West, but dealt especially with California; that 

 of Kearney and his associates was confined to the Tooele Valley in Utah, but 

 it is applicable to the major part of the Great Basin. Both dealt specifically 

 with the tolerance of the important dominants, but the work in Utah was 

 much more intensive, treating the plant communities in detail, and measuring 

 the water-content and salt-content at different depths and in a wide variety 



