INFLUENCE OF CLIMATE ON PLANTS. 
205 
of where the plants are to be grown, or is rendered unnecessarily extensive in 
order to fulfil some other design, the object of the planter is more than counteracted, 
and a positive evil is entailed. 
Few can be unapprized of the fact, that from every leaf of the minutest vege- 
table an insensible moisture is constantly being exhaled. Let but this circumstance 
be considered, in connexion with the immeasurable surface which a large tree 
presents, and, again, with the augmented evaporation of a considerable number of 
them, and it will be confessed that the quantity of fluid evolved cannot be trivial. 
Further, instead of this vaporised fluid ascending into the higher regions of the air 
— at least on those occasions when its influence can be injurious — it is wafted 
over the superficies immediately beneath it, and there suspended, till the action 
of cold precipitates it to the earth, or condenses it on the vegetation that may 
intervene. 
For those who have witnessed the additional violence with which frost acts upon 
plants saturated with moisture, (and by the least scientific cultivator this circumstance 
cannot have been unobserved,) it will be needless to add, that the increase of fluid 
just noticed, cannot, when succeeded by frost, prove innocuous. But atmospheric 
and external moisture are even more prejudicial in such a case, if the vessels of the 
plant are likewise moderately filled with liquids, than any internal and inherent 
repletion would be. Vapour spread over the surface of any vegetable substance, 
actually serves as a conductor for the transition of latent heat ; and a plant is much 
more speedily and dangerously deprived of its temperature by this means, than it 
could be if suffering under extreme saturation, provided, at the same tiine, its 
surface were kept comparatively dry. 
Many may think the mode we have taken for demonstrating this point far- 
fetched and abstruse ; some will probably also consider our conclusions hyperbolical. 
If, however, great masses of trees can be proved (and none, we imagine, will deny 
this) to diminish the temperature of an immense district, there can be no question 
that the causes which operate in this instance will, when more circumscribed, 
produce within an equally limited area precisely the same effects. The purport of 
these remarks has not the most remote reference to the prejudicial shade which 
would be afforded by large trees, this latter particular being a distinct and powerful 
objection to their employment as materials for shelter, and one which we have 
heretofore discussed. 
What we have now to adduce may, to some minds, appear completely to nulHfy 
the suggestions already advanced ; though we think it will be easy to enlist it in 
our train of argument against the plantation of trees for the defence of tender exotics. 
Strong currents of air, and even the slightest breezes from certain quarters, are 
known materially to decrease the temperature. Particular localities, and especially 
elevated spots, suffer the greatest reduction of heat from this source ; while, as 
winds are proverbially changeable, and their effects on the temperature appear or 
vanish with their mutations, such sudden transitions are exceedingly likely to 
