62 STUDIES IN THE VEGETATION OF THE STATE 



plant to withdraw water from the soil. The vigor of the 

 plant is lessened by light, heat, temperature and moisture 

 effects which vary too widely from the optimum necessary 

 for the plant, and such physical factors are indirect causes 

 in increasing or decreasing the amount of non-available 

 water in the soil. The limit of physiological water varies 

 directly with the square root of the per cent of soluble salts 

 in the soil. 



'2. Water in Plants. 



The per cent of water in the tissues of the plant is usually 

 the greatest in seedlings, and decreases gradually as the 

 plant grows older. The individual plants containing the 

 highest per cent of water are not necessarily the healthiest, 

 since the most vigorous growing plants contain a lower per 

 cent than less thrifty ones. Shade plants and plants grown 

 under humid conditions have a relatively high per cent of 

 water, due probably to a poor development of the fibrovascu- 

 lar system. Plants grown in different soils indicate the fol- 

 lowing series beginning with the soil bearing plants of 

 the highest water content, viz., sand, loam, saline, clay, 

 loess, humus. The most watery plants were more often 

 attacked by fungous plants than those less watery. 



Mesophytes apparently lose water when first wilting in 

 almost the same ratio by weight as the soil. This may 

 indicate that the plant gets little or no water from the soil 

 after it begins to wilt excessively, and that the root system 

 is already beginning to die at the extremities. Hydro- 

 phytes probably lose weight more rapidly than the soil. 

 Xerophytes lose weight very slowly when subjected to 

 drought, and some may retain most of the water in their 

 tissues even after the substratum is air dry. The tenacity 

 with which the water in soils resist the absorbent action of 

 the root hairs of the plant varies in diiferent soils. The 

 soils of experimentation arranged in order of their resist- 

 ance from the highest to the lowest are : saline, humus, 

 loam, loess, clay, sand, The chief probable factors of vari- 



