28 CONCEPT AND HISTORY. 



of all other investigators/the values ranging from 1 per cent to 16 per cent, 

 or in the heaviest clays to 30 per cent. But a striking departure from all 

 previous results occurs with respect to echard for different species. While 

 Heinrich, Gain, Clements, and Hedgcock found differences between species 

 in the same soil represented by a ratio of 1 to 1.5 or 1 to 2, or even more in the 

 case of hydrophytes, the greatest ratio found by Briggs and Shantz was 1 to 

 1.1. The thoroughness of their work seems to leave little question of the 

 soundness of the conclusion "that the differences exhibited by crop plants in 

 their ability to reduce the mositure content of the soil before wilting occurs 

 are so slight as to be without practical significance in the selection of crops for 

 semi-arid regions." The issue must still be regarded as open with reference 

 to material differences in the echard of native species, and this can only be 

 settled by further research. Recent studies by Dosdall (1919) have shown 

 that Eguisetum differs greatly from Helianthus and Phaseolus in its ability to 

 draw water from the soil, as was likewise demonstrated by growing them side 

 by side in the same spots. In seeking to harmonize the discordant results of 

 qualified investigators, it has become more and more probable that types of 

 echard must be recognized. 



Water requirement. — In summing up the results of their own researches, as 

 well as those obtained by many earlier observers, Briggs and Shantz (1913 : 

 1:46; 2:88) reach the following conclusions: 



Experiments upon the effect of water-content on the water requirement 

 show that the latter increases as a rule when the water-content approaches 

 either extreme. 



A reduction in water requirement generally accompanies an increase in the 

 nutrient-content, while a higher water requirement may result from a defi- 

 ciency in the amount of a particular nutrient. 



The type of soil affects the water requirement only though the water or 

 the solutes it contains. 



The water requirement increases with the dryness of the air, and is pro- 

 foundly affected by climatic conditions. 



The water requirement varies greatly for different species and varieties. In 

 Colorado, it was found to be approximately 1,000 for alfalfa, 700 for sweet 

 clover, and 300 for millet and sorghum. The grains ranged from 369 for corn 

 to 507 for wheat and 724 for rye. 



The greatest value of water requirement work for indicator studies is in 

 connection with the phytometric analysis of climates and habitats. So far as 

 the water relation is concerned, the values obtained by means of phytometers 

 can be expressed in terms of water-loss per unit area or rate of growth, or in 

 the water requirement in terms of dry weight or seed production. For crop 

 plants, the latter are the most important, but for native species all four values 

 must be taken into account, in addition to photosynthetic efficiency. 



CONCEPT. 



General. — Every plant is an indicator. This is an inevitable conclusion 

 from the fact that each plant is the product of the conditions under which it 

 grows, and is thereby a measure of these conditions. As a consequence, any 

 response made by a plant furnishes a clue to the factors at work upon it. 

 While this general principle seems to be of universal significance, its applica- 

 tion is far from simple. This is because the most direct responses are physio- 



