88 VEGETABLE PHYSIOLOGY 
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 periodi- 
city is not manifested at all. 
The effect of the periodic alternation of light 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- 
nisation 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 in it. 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. 
A more elaborate method of demonstrating transpiration 
