ABSORPTION OF WATER 



139 



Fig. 55. — Guttation of a nastur- 

 tium leaf (after Noll). 



The pressure developed in this process is also small. In her- 

 baceous plants it usually constitutes but a fraction of an atmos- 

 phere. Only in some trees and in the grapevine, has a pressure 

 above 1 to 2 atmospheres been observed. It must be remem- 

 bered, however, that such experi- 

 ments 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 manom- 

 eter accumulates but slowly, the 

 recorded pressure is considerably 

 lower than actually exists. Indirect 

 determinations of the root pressure, 

 recently carried out by Sabinin have 

 given greater values. In some her- 

 baceous plants, such as corn, 

 wheat, and hnpatiens, the bleed- 

 ing pressure has been found to 

 approach 1.5 to 2 atmospheres. 



44. The Osmotic Mechanism of Root Pressure. — The pressure 

 of the root cells, as revealed by the phenomena of bleeding and 

 guttation, is closely connected with their life activity, but in par- 

 ticular with the unalterable condition of their osmotic properties. 

 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. Watering of 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 oyxgen. 



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 

 pressure. 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 proper- 

 ties of the protoplasmic membrane is required, but likewise an 



