50 FUNDAMENTALS OF FRUIT PRODUCTION 



that investigations indicate are intimately associated with the question 

 of winter injury. 



Bouyoucos" points out that the relative amounts of these forms of water 

 vary greatly in different soils. He says: "In some soils only one or two forms 

 predominate, while in others all three are about equally represented. In the 

 sands and fine sandy loams, it is the free water that predominates, which amounts, 

 in some cases, to about 95 per cent, of the total water present; the other 5 per 

 cent, consists as a rule, of combined water; capillary adsorbed water is apparently 

 not present in these classes of soil. In the loams and silt loams, it is the free 

 and combined water which predominates. Here, again the capillary-adsorbed 

 water is present in small amounts. In some of the heavy loams all three forms 

 are about equally distributed. In clay loams and humus loams and clay, it is 

 the combined water which predominates followed by capillary-adsorbed and 

 free. Although the amount of free water tends to decrease and the amount of 

 the capillary-adsorbed and combined water tends to increase correspondingly as 

 the soils ascend from the simple and non-colloidal to the complex and colloidal 

 classes. There are many exceptions to this rule."" 



Of equal importance are the differences in the amounts of free or 

 easily frozen water in plant cells, as determined by McCool and Millar, ^^ 

 using the dilatometer. The differences found suggest corresponding 

 differences in the amounts of adsorbed water in plant cells. 



The different water-absorbing capacity of plant cells is attributed by Spoehr^^s 

 to their pentosan content. This is confirmed by the work of Hooker.'^^ It seems 

 probable, therefore, that the chemical composition of plant tissue, as of soils, 

 has a most important bearing on the condition in which its water is held. This 

 in turn has a direct relation to the susceptibility of plant tissue to environmental 

 changes, a subject that is discussed in greater detail in the section on Tempera- 

 ture Relations. 



McCool and Millar ^^ measured the amounts of easily frozen water in plants 

 grown in soils of high, medium and low water content. They found that the 

 plants grown in soils of high water content contained more easily frozen water. 

 Rosa"'' has shown that with lower moisture content of the soil there is an increase 

 in the pentosan content of the plants grown in it. This amphfies the discovery 

 that pentosans are produced under xerophytic conditions and that the water- 

 retaining capacity of the cells is thereby increased. ^'^^ The greater amount of 

 adsorbed water that such plants contain would mean the presence of smaller 

 amounts of free or easily frozen water, such as McCool and Millar found. These 

 investigators have also shown a correlation between the depression of the freezing 

 point of plant sap and the amount of easily frozen water it contains; the lower 

 the freezing point the less easily frozen water is present. ^^ This suggests that 

 differences in the concentration of cell sap may be due in part to the relative 

 amounts of free and adsorbed water. It is obvious that with a given amount of 

 soluble material, the concentration of the sap will depend on the amount of free 

 water available for its solution. The less the proportion of free water and the 

 greater the proportion of adsorbed water the higher the concentration of the 

 solution. 



