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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. 
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 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 
