212 HOAV CHOI'S FEED. 



The application of these principles to the question be- 

 fore us is apparent. The young, active roots tliat are in 

 contact with the soil are eminently hygroscopic, as is de- 

 monstrated by the fact that they supply the plant with 

 large quantities of water when the soil is so dry that it 

 has no visible moisture. They therefore share with the 

 soil the moisture which the latter contains. As water 

 evaporates from the surface of the foliage, its place is 

 supplied by the adjacent portions, and thus motion is es- 

 tablished within the plant which j^ropagates itself to the 

 •roots and through these to the soil. 



Each particle of water that flies ofl" in vapor from the 

 leaf makes room for the entrance of a particle at the root. 

 If the soil and air have a surplus of water, the plant will 

 contain more ; if the soil and air be dry, it will contain 

 less. Within certain narrow limits the supply and waste 

 may vary without detriment to the plant, but Avlien the 

 loss goes on more rapidly than the supply can be kept up, 

 or when the absolute content of water in the soil is re- 

 duced to a certain point, the plant shortly Avilts. Even 

 then its content of water is many times greater than that 

 of the soil. The living tobacco plant cannot contain less 

 than 80°!^ of water, while the soils in Sachs' experiments 

 contained but 12.3" |„ and 1.5° |„ respectively. When fully 

 air-dry, vegetable matter retains IS'l^to 15° j^, of water, 

 while the soil similarly dry rarely contains more thaq 



The plant therefore, especially when living, is much 

 more hygroscopic than the soil. 



If roots are so hygroscopic, why, it may be asked, do 

 they not directly absorb vapor of Avater from the air of 

 the soil ? It cannot be denied that both tlie roots and fo- 

 liage of plants are capable of ti)is kind of absorption, 

 and that it is taking place constantly in case of the roots. 

 The experiments before described prove, liowcver, tlat 

 the higher oi'ders of plants absorb very little ia this W~^y« 



