THE WATER OF SOILS. 2OI 



the life of vegetation in rainless summers, or in time of 

 drought. It cannot, however, maintain normal growth, save 

 in the case of some desert plants. 



2. High moisture-absorption prevents the rapid and undue 

 heating o( the surface soil to the danger point, and thus often 

 saves crops that are lost in soils of low hygroscopic power. 



CAPILLARY WATER. 



The liquid water held in the pores of the soil, in the 

 form of surface films representing the curved surface seen 

 in capillary tubes, and therefore tending to cause the 

 water to move upwards, as well as in all other directions, 

 until uniformity of tension is established, is of vastly higher im- 

 portance to plant growth than hygroscopic moisture. It not 

 only serves normally as the vehicle of all plant food absorbed 

 during the growth of the usual crops, but also, as a rule, to sus- 

 tain the enormous evaporation by which the plant maintains 

 during the heat of the day, a temperature sufficiently low to 

 permit of the proper operation of the processes of assimilation 

 and building of cell tissue. 



Comparatively few plants have roots adapted to healthy ac- 

 tion while submerged in water, excluding them from free ac- 

 cess of the oxygen of the air; and when such roots are formed 

 by plants not naturally growing in water or swampy ground, 

 they differ so far from earth roots in their structure that when 

 transferred to soil they usually die, normal earth-roots being 

 gradually formed instead. Conversely, there is for all land 

 plants a definite time-limit beyond which their roots cannot 

 live, or at least remain healthy, in submersion. Thus grain 

 fields will with difficulty recover from a week's total submer- 

 sion ; while young rice fields will resist considerably longer. 

 When in the resting (winter) condition vineyards will bear 

 submergence for thirty-five and even forty days, deciduous 

 orchards about three weeks ; but when in the growing condi- 

 tion, injury is suffered much more quickly. 



It follows that \vhenever the soil-pores remain completely 

 filled with water for a length of time, there is danger to the 

 welfare of nearly all plants commonly cultivated in the tem- 

 perate zones. It is therefore important to know how much 



