Phe Life of the Plant = 16 
in general the roots, are at length compelled to absorb water. 
Acting in conjunction with this force is the voot-pressure. 
The roots absorb a greater quantity of water than the plant 
requires ; and this, therefore, exercises a pressure which 
drives the water that has been already absorbed higher and 
higher up the stem; and often, as in grasses, Aroidee, &c., 
even causes it to exude in drops at the margins and tips of 
the leaves. Experiments on the intensity of the root-pres- 
sure, performed by cutting off the upper part of the stem, 
and attaching a manometer to the section, show that it is 
sufficiently powerful to balance a column of water more 
than ten metres in height (Fig. 343). 
The organs of transpiration are the leaves, which allow of the escape 
through their stomata of the aqueous vapour that has accumulated in 
the intercellular spaces. The amount of transpiration depends on the 
moisture of the air, on the intensity of light, on the temperature, on - 
concussions to which the plant is subject, and on the age and size 
of the leaves. It has not yet been ascertained how all these forces 
act ; observation has nevertheless shown that the stomata are closed” 
at night and open in the daytime under favourable circumstances, 27.c. 
when the atmosphere is not too moist, and under the influence of light 
and warmth. 
If the loss of water by transpiration from the leaves is greater than 
the quantity supplied by the roots, the conducting parts become 
first of all deficient in it; and when at length the evaporation from the 
more delicate organs can no longer be compensated, they lose their 
stiffness and hang down from their own weight, or in other words 
- wither. This condition is caused at an earlier period in succulent 
herbaceous plants than in trees which possess large reservoirs of water 
in their stems. 
ASSIMILATION AND METASTASIS. 
The life of the plant is associated with a continual con- 
sumption of plastic substances, which are of service to it 
as formative materials for the growth of cells already in 
existence, and for the production of new ones. A number ~ 
of the sources of force in the plant are being continually lost 
through these processes, because many of these formative 
M 
