TRANSPIRATION 93 
these are in the alternation of day and night. This alter- 
nation, affecting successive generations of plants through 
an enormous length of time, may have impressed upon the 
protoplasm a peculiar rhythm of greater and less general 
activity, which has become ultimately automatic and inde- 
pendent of the immediate surroundings. Of this the vary- 
ing action of the roots may be a particular expression. 
It is remarkable, however, that very young plants do 
not exhibit this diurnal variation, but they gradually 
acquire the power of doing so as they develop, subject as 
they are under normal conditions to the alternation of light 
and darkness. In many cases, again, the diurnal periodicity 
is not manifest at all. 
The effect of the periodic alternation of Jight and dark- 
ness cannot in any case have been originally appreciated by 
the roots, as they are implanted in the soil and so escape 
its influence. If it was originally due to such variations, 
these must have been impressed upon the general orga- 
nigation of the plant. 
TRANSPIRATION.—The modified evaporation by which 
the protoplasts get rid of water and enable the contents of 
their vacuoles to be continually renewed takes place ulti- 
mately from the surfaces of all the succulent parts of 
plants, and to a less extent from portions of the exterior 
which are covered by a layer of cork. Like the activity 
of the absorbing organs of the root, it is essentially a vital 
process, and is regulated by the protoplasm of the cells 
which take part init. As we have seen, it is usually spoken 
of as transpiration. 
It is easy to demonstrate the fact of its continuous 
existence during daylight by enclosing a plant, or part of 
one, in a dry glass vessel which can be closed so as to admit 
no air. Very soon the surface of the glass becomes covered 
by a fine dew, which is the condensed vapour that has 
escaped from the plant. The same thing may be seen when 
a vigorous plant is covered over by a bell-jar, the water 
condensing copiously upon the sides of the latter. 
