66 BOTANY 
grain that is richest in the dissolved substances that 
the plant requires. 
Root Pressure.—It is clear that the force exerted by 
the liquid pressing inwards through the membrane in 
both thistle funnel and egg, must be considerable, 
since it is enough to hold up a long column of water 
above the level of that which surrounds the base. 
This pressure could of course be measured. The liquid 
passing into the root hairs exerts a similar pressure, 
the effect of which may be readily seen in the bleeding 
of vine, peach, and other stems when cut in the spring. 
This bleeding is due to the osmotie pressure inwards 
and upwards from the root hairs. This root pressure 
may be measured by the apparatus shown in Fig. 44, 
The stem of a fuchsia or other suitable plant growing 
strongly in a pot is cut off a few inches above the 
ground and a glass tube, bent in the form of a flat 
S, as shown in the figure, is, by means of a piece of 
rubber tubing, firmly fixed to the stump. Now pour 
mercury into the tube till it stands somewhere near the 
top of the middle limb. The mereury will at 
first stand at the same level in both limbs. If the plant 
be well supplied with water, root pressure will however 
soon force the sap out from the cut end, and this, 
filling the part of the tube above the mereury, presses 
on the surface of the latter, and forces it up the left 
hand limb. In a few days the mereury will reach its 
highest point, upon which, by finding the weight of 
the mercury in the left hand limb and subtracting 
from it the weight of the water and mereury in the 
middle limb, the amount of the root pressure may be 
found. The length of the mereury and water columns 
and the bore of the tube must of course be measured. 
Then, taking the weight of 1 ¢@.c. of water ag 1 gram 
and the weight of mercury, volume for volume, 13-6 
times that of water, the necessary calculation may 
easily be made. 
