82 VEGETABLE PHYSIOLOGY 



similar part. Against this theory we have the fact that, if 

 the transpiration current is made to contain substances 

 that are poisonous to the living cells, and the latter are 

 consequently killed, the current still goes on. Considerable 

 lengths of a stem have been killed by heating it to the 

 temperature of boiling water, and the dead part has proved 

 to be no obstacle to the transport. Nor do differences of 

 gaseous pressure within and without the plant, or at 

 different portions of the axis, explain the matter more 

 satisfactorily. 



BOOT-PRESSURE. We have seen how the absorption of 

 water osmotically from the soil by the root -hairs leads to a 

 great turgescence of the tissue of the cortex of the root, not 

 only in the regions of absorption but along the whole length 

 of the younger portions, which turgescence exerts consider- 

 able pressure on the sides of the vessels and tracheids of the 

 xylem of the stele. By this means water, containing various 

 salts and other constituents in extremely small quantity, 

 is forced into the nbro-vascular tissue. The process is 'not 

 a purely physical one of nitration under pressure, but is 

 regulated to some extent by the protoplasm of the cells which 

 abut upon the xylem. When these are distended to their 

 greatest capacity, their protoplasm appears to be stimulated, 

 perhaps by the very distension, and in consequence to 

 allow water to transude through its substance. This mode 

 of response to stimulation is not infrequent in vegetable 

 tissues ; indeed it appears to correspond to the response of 

 a muscle to stimulation by the process of contraction. We 

 must not push this comparison too far, for the protoplasm 

 of the vegetable cell seems to respond not by contracting 

 but by modifying its permeability, so that the hydrostatic 

 pressure existing in the cell is able to force the water 

 through the living substance with greater facility than it 

 could before the stimulus was appreciated. By thus modify- 

 ing the turgor of the cell, the protoplasm relieves itself of 

 the over-extension, and we get an intermittent pumping 

 action set up, which has a certain rhythm. By it large 



