202 
ON THE RELATION OF VEGETATION TO SEASONS. 
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 phe- 
nomenon, 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 the inclemency of the 
winter. De Candolle has proved, by a simple experiment, 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 cor- 
responds 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 
activity ; 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, it appears from 
experiments that heat applied to the branches alone, is quite sufficient to determine 
and maintain all the phenomena of growth. Once set in action, the branches of a 
tree go on growing according to the laws which have now been explained. They 
and their leaves, by degrees, gain their full growth : bark and wood separate, and 
cambium is deposited between them ; the leaves decompose the fluid they receive, 
send their fibres down within the substance of the branches, gradually secrete the 
substance peculiar to each peculiar species, and transfer them to the bark ; and, 
finally, becoming clogged at every pore, by the earthly and carbonaceous matters 
that are deposited during the process of digestion and evaporation, cease to act 
efficiently as leaves. 
In this state they are principally protectors of the young buds in their axils. If the 
latter have been formed very early, they are so far advanced in their growth by the 
middle of summer, that they have already arrived at the same state as later formed 
buds will be in at the commencement of another spring. Acted upon by the tem- 
perature of the season, they develope and call into play the same class of pheno- 
mena as took place in the beginning of the spring ; the sap which had become 
languid as the leaves had become impotent, is again stimulated by a rapid movement, 
and is secreted anew in increased quantity. This is indicated by what gardeners 
call the running of the bark, that is to say, the bark and wood of exogens separate 
spontaneously as in the spring, depositing a layer of cambium between them. 
Thus are formed what are called midsummer shoots, which only occur in plants 
which bud very early in the spring. 
In the course of the autumn, the increased and prolonged heat and drought 
complete the destruction of the leaves, which had already begun to languish ; and 
their vital actions are destroyed by the quantity of foreign matter with which their 
