66 ELEMENTS OF AGRICULTURE 



the more rapidly the water passes through the membrane. 

 The solutions in the root-hairs are more dense than the 

 soil solutions, hence more water passes into the root than 

 passes out into the soil. If extremely strong fertilizer is 

 used, the soil solution may be so concentrated as to. cause 

 more water to leave the root than enters it. In this case, 

 the plant will wilt and may be killed. An excess of any 

 plant food in solution may thus kill plants. The alkali 

 soils of arid regions often contain so much soluble material 

 as to prevent the growth of plants. 



Some of the cell sap does pass from the roots to the soil. 

 This cell-sap is slightly acid, so it helps to make more 

 of the material in the soil soluble. The acidity of a root 

 may be easily shown by pressing the root of a sprouting 

 seed against blue litmus paper. 



If a plant were a dead thing, the solution in the cells 

 would eventually become of the same density as the soil 

 solution, so that the moisture would pass out of the roots 

 as rapidly as it passed into them. But the plant cells are 

 alive. The leaves are constantly using such of the materials 

 in the cell-sap as are needed for the manufacture of plant 

 tissues. They remove the surplus water by transpiration. 



The transpiration keeps the cell-sap of the leaves and 

 upper parts of the plant densest, so that the balance 

 of osmotic movement is always upward. 



The plant foods are not taken up as elements, but in 

 compounds. Nitrogen constitutes four-fifths of the at- 

 mosphere, but it is in the form of an element. No plant 

 can take up nitrogen except when it is combined with 

 other elements. It is taken up in soil solutions in the 

 form of nitrates. No solid particles can be taken up by 



