ROOT-PRESSURE 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 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 fibro-vascular tissue. The process is not 
a purely physical one of filtration 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 trangude 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 
