OF THE RELATION OF VEGETATION TO SEASONS. 
263 
flow of sap from certain trees, even in the midst of that dreary- 
season. 
“ But whatever power of attracting sap by its roots a plant may 
possess during winter, it is obvious that it has little means of parting 
with any part of it again by evaporation at that period of the year ; so 
that during the winter the whole of the tissue must gradually acquire 
a state of turgidity, which will go on increasing till the leaves and 
new branches are developed to carry off the sap, or decompose and 
assimilate it. 
“ This turgid state is eminently favourable to rapid growth when 
vegetation once resumes its activity; for it acts as a force from behind, 
which continually presses upon the new-born tissue, and causes it to 
expand. It is well known that after very long winters, or when a plant 
has been prevented by artificial means from shooting at its usual season, 
its branches and leaves are developed with extraordinary vigour—a 
circumstance which has been ascribed to accumulated irritability , but 
which is, in fact, owing to the turgid state of the tissue. 
It is when the temperature of the air is raised sufficiently high, 
that the^vital energy of a plant is excited, and buds are developed with 
their leaves. Light has certainly nothing to do with this phenomenon, 
although it afterwards colours and consolidates the young parts; for if 
a plant be exposed to an elevated temperature, in total darkness, its 
growth takes place as if in the light. The common experiment of 
introducing into a hothouse the branch of a vine growing in the open 
air, is another familiar illustration of this fact: the temperature of the 
hothouse excites the buds into action, they immediately attract fluid 
from beneath them, and thus the whole system is put in motion, 
although the vine-plant may be exposed beyond the house to all the 
inclemency of the winter. De Candolle has proved by a simple experi¬ 
ment, that in such a case as this, the fluid consumed by the young 
leaves is really attracted out of the earth, and not absorbed from the 
atmosphere of the hothouse. If you select a tree with two principal 
branches, and two principal roots to correspond with them, and adapt 
to each root in the earth a bottle of water, you will find that the bottle 
which corresponds with the branch in the hothouse will be quickly 
emptied, while that which is connected with the branch in the open air 
remains nearly full. It may be supposed that in a natural state of 
things, a corresponding effect is produced upon the roots by the warmth 
of the surface of the soil, and that they also are stimulated into acti¬ 
vity ; but it is doubtful whether this amounts to much, if, indeed, it is 
of any importance whatever; for provided only the earth is not frozen, 
