CHAPTER VII. 



THE SOIL AND WATER. 



The plant food is dissolved in water and enters the plant 

 through its roots. Water also serves as the medium by which 

 matter is transferred, and it supplies the hydrogen and a part of 

 the oxygen used in the synthesis of organic matter. 



A large amount of water is required by plants, not only because 

 plants contain considerable water, but also because the passage of 

 water through plants is one of the most important means of plant 

 nutrition. The evaporation of water from the surface of the 

 leaves produces an upward current of water which carries into 

 the plant needed mineral material. The greater the evaporation, 

 the greater is the transference of plant food from the soil to the 

 plant, other things being equal. 



Transpiration. The loss of water through the leaf of the 

 plant is termed transpiration. The amount of loss by transpira- 

 tion is easily determined with plants in pots. 1 We weigh the pot 

 of soil containing the plants at the beginning of the experiment 

 and at the end of certain periods weigh it again; the loss of 

 weight plus any water added, is water evaporated by plant and 

 soil. The water evaporated from a pot of similar soil but with 

 no plants, under the same conditions, is taken to show the 

 evaporation from the soil alone, although the shade of the plant 

 make's a difference. The loss of water by soil and plants, less the 

 loss from soil alone, gives the loss by the plant alone. Correction 

 must be made for the increase in weight of the plant, if the 

 experiment is conducted for some time. In another form of 

 apparatus, the soil is covered with a galvanized iron cover, and 

 the openings through which the plant extends is rendered water- 

 tight by means of modelling clay. Plants may be grown in the 

 free air by this apparatus, without danger of irregular results due 

 to varying amounts of rain falling in different pots of the same 

 series. 



If the plant is contained in pots or other vessels set in the 

 1 Montgomery, Proc. Am. Soc. Agron., 1911, p. 257. 



