KOOT-PKESSUKE 87 



of the root. The medullary rays of the stele in tall tree 

 trunks have been held to behave similarly. 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. 



ROOT-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 trachei'ds 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 

 stimulation of a gland cell of the animal body. When one 

 of the nerves supplying the parotid salivary gland is stimu- 

 lated by an electric current the gland pours out its secre- 

 tion in a very similar way, by modifying the permeability 

 of the cell protoplasm, 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 modifying the 

 turgor of the cell, the protoplasm relieves itself of the 



