PRESIDENT 3 ADDRESS. 
599 
destroyed. This conservative action is not confined to leaves, but 
takes place also in stems. In the case of other leaves which 
blacken at the first frost, there appears to be some physiological 
condition obtaining in the plant, by which this conservative action 
is not brought about, and probably the cells are ruptured from the 
first, and the destruction of the leaf takes place at once. 
In herbaceous plants and trees of moderate height, water is 
conveyed to the leaves under the force of root pressure, and its 
path of transit is through the fibro-vascular bundles in the former, 
and the younger portions of the wood in the latter, but by what 
force it is carried to the tops of high trees, is not at present known. 
A great many experiments have been made, but as yet the problem 
remains unsolved. It was held by Sachs that water travelled by 
imbibition through the walls of the wood cells. Now no doubt the 
force displayed by a body absorbing water is tremendous, for dry peas, 
as they expand under the iniluence of moisture, have been shown 
by Hales to be capable of raising a weight of 160 lbs. Against 
this theory is the fact that a very little force would stop the tlow, 
and, moreover, when transpiration was active, the necessary quick 
supply could not be maintained. Another theory was, that the 
water is conveyed through the hollow tracheal tubes ; but the 
difficulty involved here, is the presence of air which would divide 
up the columns. Still, as it is easily shown that in herbaceous 
plants the tracheids are always full of water, it may be proved 
perhaps in time that these are the channels. There is no doubt 
that a strong sucking action goes on in the leaves, and this would 
contribute a good deal to sustaining the rising columns. 
Besides the physiological effects, water contributes powerful 
mechanical aid to a plant. In the normal state the cells, as we 
have seen, are in a state of extreme turgidity, and the osmotic 
force is very great in leaves and succulent stems. This force 
maintains their stability until the woody tissue has developed 
sufficiently to act as the natural support. It is by the temporary 
loss of water that the leaves and leaflets of the sensitive plant, 
Mimosa pio/ica, droop on the application of a stimulus. When 
a leaf is touched, a physiological response is conveyed to some 
