86 
INFLUENCE OF CLIMATE ON PLANTS. 
of caloric, appears to be their principal office, as plants seldom absorb nutriment in 
this manner, unless they fail in obtaining an adequate supply from the soil. 
Natural evaporation is undoubtedly due to solar lieat ; and although it is 
augmented when the plant is subjected to the rays of hght, this latter agent can 
only be regarded as an auxiliary in its promotion, since it proceeds unrestrainedly 
when a due degree of heat is present, even though the plant be enveloped in total 
darkness. The effects of this evaporation are somewhat remarkable. In tropical 
countries, it is periodically so profuse, as to suspend the functions of vegetation, and 
occasion a season of repose. The water which is vaporized during this period,, 
descends again upon plants after a certain time, and causes them to resume their 
growth with renewed vigour. 
The vegetation of temperate climes experiences a still greater degree of benefit 
from this source. Saturated with the fluids absorbed during winter, the genial 
warmth of the sun in the spring calls these into admirable avail, in the develop- 
ment of stems and foliage ; by which, also, it simultaneously provides an imme- 
diate exit for their redundant inhalations. As the young branches, too, approach 
maturity, the higher temperature which prevails at this season increases the 
exhalations from their surface to such an extent, as to dry and concrete their 
newly-formed substance and cuticle ; thus enabling them to combat successfully 
the rigours of the succeeding winter. 
Frost is found to be particularly destructive to plants in the spring, after they 
have commenced growing. This is owing, in a great degree, to the protracted 
interruption of the process of transpiration which has preceded that period, and 
its aggravated suspension when absorption and circulation have been increased. 
Hence, the newly-formed leaves and shoots, being the media through which this 
accumulated moisture is exhaled, are so completely charged with it, as to be highly 
susceptible of injury from cold. The tender, nascent state of the new develop- 
ments, and the total absence of cuticle, or its amalgamation with the cellular 
tissue,— forming, in fact, a similar aggregation of cells, filled with fluid matters, — 
increase the liability to derangement from such circumstances. 
To the same source may be traced the frequent detriment which the young- 
shoots of plants receive, on a severe winter succeeding an unusually cold summer 
and autumn. Being unable to rid themselves of the refuse portion of the moisture 
they abstract from the soil, without a sufficient stimulus, their parts remain succu- 
lent, flexible, and incapable of repelling frost. It would appear from these facts 
that water or any other fluids radiate heat much more readily than dry and solid 
substances ; and such is really the case : therefore it may be affirmed as a funda- 
mental principle in cultivation, that the less moisture a plant is allowed to absorb 
during the autumnal months, the better will it be prepared to retain its needful 
temperature through the winter. 
Extreme cold is, as stated before, much more injurious to plants in the early 
stages of their periodical growth ; and, as analogy teaches, while experience 
