ROOT-PRESSURE 91 
the process, of course, cannot go on, and a disturbance of 
the normal constituents of the soil will lead to modifications 
of the process. If there is too great a preponderance of 
neutral salts such as sodium chloride or potassium nitrate, 
so that the liquid presented to the roots is practically a 
saline solution, the exudation will cease; indeed, under 
such circumstances water may actually be withdrawn from 
the plant. 
Root-pressure is continually at work while the trans- 
mission of water is going on; but it is not easily seen later 
in the year when the development of the leaves has caused 
an active transpiration to proceed. If the stem of the vine 
be cut in July instead of in March, no bleeding follows the 
wound. This is not, however, due to the absence of 
activity in the roots, but to the fact that the copious 
evaporation of transpiration prevents the necessary accu- 
mulation of water in the cavities of the woody elements. 
In the experiment in the early spring the conditions were 
different ; there were no expanded leaves, and the water 
absorbed and sent upwards by the root consequently re- 
mained in the vessels of the stem, escaping at once when 
the latter was cut. In July the vessels have been emptied 
by the transpiration, and there is no accumulation of 
water to overflow. The apparatus described will show, 
however, if the experiment with it is continued for some 
time, that root-pressure is still at work, even though tran- 
Spiration ig vigorous until the stem is severed. 
The force of root-pressure must therefore be regarded 
always as a factor in maintaining the transpiration current. 
It ig continually forcing water into the vessels of the axis, 
and the fact that transpiration prevents an accumulation 
there does not show that the influence of root-pressure is done 
away with as soon as it ceases to be easily demonstrated. 
The root-pressure, though always considerable, is not 
the same at all times of the day and night. It can be 
measured by observing the output of water in the second 
form of the apparatus described above, measurements 
