676 
MOLECULAR FORCES IN THE PLANT. 
certain time of nearly the same size, and the stem and the channels of the 
current (woody bmidles) which traverse it consequently retain their diameter 
unchanged. 
The movements of water caused by growth as well as those induced by 
evaporation have this in common, that their direction is towards the places where 
they are required. If growth or transpiration begins at a certain time at a 
definite spot, the nearest portions of the tissue give up their water first of all, 
then the more distant ones, the organs at the greatest distance, generally the 
roots, absorbing water from without. The movement therefore propagates itself 
continually further and further from the point to which 
it tends, and finally over the whole plant to the root. 
The kind of motion may therefore — without consider- 
ing for the moment its actual causes — be described 
as a process of suction. This is especially evident 
in leafy stems and branches which, having been cut 
off and placed with their cut surface in water, suck 
up as much water through their woody bundles as 
is required for transpiration and for the unfolding of 
fresh leaves, unassisted in this case by any pressure 
from below. 
Another kind of motion of water in the plant, 
depending not on suction but on pressure from below, 
is caused by the roots, and is altogether independent 
of the use of the water for the purpose of growth or 
of transpiration. If the woody stem of a land-plant 
is cut through above the root, the root being attached 
to the ground in the ordinary manner, and if the 
ground is damp and sufficiently warm, water exudes 
from the transverse section of the stem either at once 
or after some time, the current continuing for days, 
and the quantity of water which flows out amounting 
sometimes to many times the volume of the root. 
This current of water, which rises through the wood 
and especially in the vessels, can only be induced by 
a pressure existing in the lower parts of the root. If 
a manometer of a proper form is fixed in the section 
(Fig. 467), it shows that even in smaller plants with but Httle wood (as Tobacco, 
Maize, the Stinging Netde, &c.) the water which exudes stands at a pressure 
which holds in equilibrium a column of mercury several centimetres in height ; 
while in some woody plants, as for instance the Vine, this pressure may amount 
to 76 cm. (or one atmospheric pressure). 
In many plants of small height this root-pressure is observable from the fact 
that water exudes at particular points of the leaves in the form of drops, pro- 
vided that the internal supply of water is nowhere diminished by powerful transpira- 
tion, and the pressure thus removed. Thus drops of water appear abundantly 
and repeatedly on the margins and apices of the leaves of many Grasses (especially 
Fig. 467. — Apparatus for observing the 
force with which water escapes under root- 
pressure from the transverse section of a 
stem r. The glass tube R is first of all 
firmly fastened to the stem, and the tube 
then fixed into it by the cork Ar. R is com- 
pletely filled with water, the upper cork k 
then fixed in it, and mercury poured into 
the tube r so as to stand from the first 
higher at than at ^, the level q' rising 
above ? according to the intensity of the 
root-pressure. Tiie apparatus is much 
more convenient to handle than that hither- 
to in use. 
