LIMIT OP PHYSIOLOGICAL WATER 53 



systems which may supply them with the nutriment necessary 

 for rapid growth. In hard soil a plant is hindered in the 

 development ol its roots even from the beginning. As a re- 

 sult it does not grow so large as a plant in looser soil because 

 it does not have the means to absorb water nor can it draw 

 from such a large and varied soil area. As a result, in 

 periods of drought, it is first to die. 



The limit of physiological water for various plants in the 

 field was determined for both hard and loose soils. The 

 average results give a slightly higher limit for plants in 

 hard soils. The results for the herbaceous plants when 

 averaged give 5.9 per cwnt as a limit in hard soils, and 5.2 

 for loose soils. Ten species of grasses have an average limit 

 of 6.8 per cent in hard, 6.4 per cent in loose soil. These 

 results may be slightly inaccurate because the soils compared 

 were from different localities and probably differed slightly in 

 composition and porosity. The plants were taken from sim- 

 ilar conditions of heat, light, and physiography. 



The soils used in the comparative plots were of a loose 

 texture, care being taken in filling the riots not to pack the 

 soil, which was allowed to settle and become denser by grav- 

 ity. The time of experimentation was short, being less than 

 two months, and none of the soil became hard. This enabled 

 each of the plants to root throughout the pot, and added 

 much to the accuracy of the results. 



7. The Recovery of Plants from Drought Effects. 



"When a plant has suffered from loss of water by wilting 

 badly, it may be revived by watering it. One box of potted 

 plants in the plant house, grown under nearly normal condi- 

 tions, was allowed to dry until the plants were wilted badly, 

 in fact were dying, for excessive wilting is but the condi- 

 tion preliminary to death. Water was then sprinkled upon 

 the soil in sufiBcient quantity to make it moist enough either 

 to fully or to partially revive the plants. Again the plants 

 were allowed to dry out, this time to the death point. Deter 

 minations of non-available water in these pots showed that 



