WATER RELATIONS OF PLANTS 28o 



cubic centimeters per day. The pressure developed in this 

 process is also small. In herbaceous plants, it usually constitutes 

 but a fraction of an atmosphere. Only in some trees and in the 

 grapevine has a pressure above 1 to 2 atmospheres been measured. 

 It must be remembered, however, that such experiments are 

 usually conducted under conditions where the root system is 

 separated from the aerial parts of the plant and, therefore, may 

 soon weaken. As the water in the manometer accumulates but 

 slowly, the recorded pressure is considerably lower than that 

 which actually exists. The action of the root cells in producing 

 bleeding and guttation is dependent upon the protoplasts being in 

 an active condition, and in particular upon their osmotic proper- 

 ties. A simple experiment may serve as an illustration. A small 

 pot with strongly guttating wheat seedlings is placed under a bell 

 jar filled with vapors of ether or chloroform — or watering the soil 

 with a solution of some poisonous substance may serve the same 

 purpose. After a time, the root cells will be killed, and guttation 

 will cease. The same results may be obtained by killing the roots 

 with heat or by depriving them of oxygen. This result is easily 

 understood, since it has been seen that all osmotic properties of 

 the cell are closely dependent upon the normal structure and 

 function of the protoplasmic membrane. With the death of the 

 cell, this structure undergoes profound changes. The cell loses 

 its semipermeability, along with its turgidity and suction tension. 

 The dependence of root pressure on the life activity of the plant, 

 however, has been found to be very complex. Not only a normal 

 unaltered condition of the physicochemical properties of the pro- 

 toplasmic membrane is required, but likewise an uninterrupted 

 supply of nutrient substances. If this supply is checked, as when 

 the aerial organs are cut, bleeding rapidly decreases or ceases. 



To understand the cause of this phenomenon, the mechanism 

 of the root pressure must be examined. It has been seen that 

 the water enters the root under the force of suction tension and 

 having passed through a series of parenchyma cells is driven 

 with considerable force into the vessels. From the osmotic 

 systems of the cell, however, no conclusion can be dra^vn as to 

 the reasons for the possible expulsion of water from the cell. 

 Although Dutrochet, who was the first to investigate osmotic 

 phenomena in plants, compared the rise of the sap in the vessels 

 of roots and stems to the rise of the Kquid in the tube of an 

 osmometer, the similarity is not very great. The tube of the 



