84 VEGETABLE PHYSIOLOGY 
not immersed. A similar exudation can be caused to take 
place from the hyphe of fungi and from the tissues of 
mosses. 
We must, however, be cautious not to attribute every 
escape of water from a plant to this cause. When a tree 
trunk is wounded or cut on a warm sunny day in winter, 
there is frequently an exudation of water from the wound. 
This is generally due to purely physical causes, being 
brought about by the expansion of 
5 the air which is contained in the 
vessels of the wood. It can be 
r artificially produced at any time 
in winter by warming a freshly 
cut piece of wood ; and its cause in 
v hk this case can be seen to be physical 
= by the fact that as the wood cools 
the water in contact with the cut 
surface is again absorbed, owing to 
the contraction of the air, which 
was expanded by the warming. 
To meastre the root-pressure 
in a plant the apparatus shown in 
fig. 62 may be used. It consists of 
a T-piece of glass tubing (R), which 
i is fastened by indiarubberi_rings 
Fic. = ror tus (7) to the top of a cut stem, such 
Hstmation oF Boow-Paes- authat of Helianthus, ‘'To-the side 
arm of the tube a manometer (q), 
with a capillary bore, is attached by a tightly fitting cork 
(k), and the T-piece is filled with water from the upper end 
(k'). Mercury is poured into the manometer till it stands at 
a level a little below the cork %, and the aperture k’ is 
then tightly closed. As the root continues to take up water, 
it forces it into the tube R, whence it overflows into the 
proximal arm of the manometer, causing the mercury in 
the two limbs to be at unequal levels. By the displacement 
of the mercury the force of the root- -pressure can be 
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