96 AGRONOMY 



exactly the same conditions and absorbing the same soil water. 

 Clover when growing with barley may take up five or six times 

 as much lime as the barley does, while the latter takes up 

 eighteen times as much silica as the clover. Similar differences 

 in the absorption of food materials are found in other plants. 

 It is as if each plant exercised a conscious selection. Such a 

 condition, however, is to be explained on purely physical 

 grounds by what is known as selective absorption. When minute 

 quantities of any mineral are dissolved in the soil water, they 

 will pass into the plant by osmosis, but in every instance each 

 substance in the water acts with reference to similar substances 

 in the plant as if it were the only element concerned. It fol- 

 lows, therefore, that if the plant happens to be using a certain 

 substance, the depletion of the supply in the cells will induce 

 more of it to filter in ; but if the plant has no use for it, the 

 density of the solution for this particular substance on both 

 sides of the cell wall soon becomes equal and the osmotic 

 action with reference to it ceases. Plants cannot exclude 

 poisons and other harmful or useless substances when suffi- 

 ciently diluted by the soil water. 



Use of water to the plant. In addition to carrying the dis- 

 solved minerals into the plant, water forms a very essential 

 part of the plant food, maintains the turgor of the cells and 

 thus keeps the plant in shape, is the medium in which all the 

 vital processes of the plant go on, aids in the transfer of food 

 within the plant, and, finally, by its evaporation, serves to cool 

 the plant and keep the cell sap denser than the soil water. 

 The amount of water transpired by growing plants is remark- 

 able. It is estimated that for every pound of dry matter pro- 

 duced by ordinary crops, from 250 to 400 pounds of water is 

 transpired, while mustard is said to require 900 pounds of 

 water for each pound of dry matter. A healthy apple tree has 

 been estimated to transpire 35,000 pounds of water during 

 the growing season. A moist spot may be drained through 



