INFLUENCE OF CLIMATE ON PLANTS. 85 
Plants, like animals, are supposed specifically to possess a distinct and definite 
constitution, requiring either particular kinds of food, or that their nutriment shall 
be attempered, prepared, and transmitted in and by particular modes and media. 
This is certainly characteristic of some species ; but, applied generally, it appears 
far from accurate. The majority of plants exhibit the same radical organization, 
have correspondent functions, and are regulated in the exercise of those functions by 
the same agents. Of these agents, heat exercises the most extensive and manifest 
jurisdiction. Indeed, so influential is its operation, that vegetation must either 
adapt itself to its various modifications, or become extinct. 
To afford a clear view of the offices and effects of heat on the functions of 
plants, it will be necessary to trace its agency through the different stages of their 
growth. The circulation of fluids, the accretion and secretion of new matter, and 
the process of evaporation, are all mainly referable to the action of solar heat. 
From the time of the first expansion of their seed-lobes, to the period when their 
fruit and seeds attain their utmost perfection, none of the above particulars can be 
accomplished unless the temperature is sufficiently high. 
Heat is material, indeed essential, to the germination of all descriptions of seeds ; 
and when we consider that moisture and air (the other necessary concomitants 
towards inducing this process) are generated, or reduced to a proper rarity of 
consistence, almost entirely by this agent, we shall perceive that heat is principally 
instrumental in producing this first and most important of vegetable metamor- 
phoses. No sooner does the vital lymph begin to flow, and the seed-leaves appear 
above the surface of the ground, than another evidence of the influence of heat is 
elicited. A system of transpiration commences from their surface, which increases 
as the plant progresses and the leaves are formed ; this being the means whereby 
the superfluous moisture imbibed from the soil is evolved, and the plant preserved 
from turgidity, with its inevitable consequence — disease. 
In the incipient vegetation of the seed, as well as in the subsequent enlargement 
of the plant, accretions are continually accruing to its substance, in the form of 
new strata or elaborations of matter, similar in character and disposure to those 
which constituted its original organization. These additions, whether longitudinal 
or horizontal, internal or exterior, are the result of the propulsion of fluids from the 
soil by heat. The secreted deposites by which they are afterwards surrounded and 
consolidated, are occasioned principally by the action of light upon the surface 
beneath which the circulating fluids are spread ; but partly, also, by the external 
agency of heat, which operates upon those fluids through the medium of the pores, 
and abstracts their more aqueous constituents by exhalation. 
Botanists almost unanimously admit the existence of pores in the cuticular 
membranes of plants, although some deny their visibility. These pores are orifices 
of various sizes, extremely diverse in number in different plants, provided for the 
effluence of redundant moisture, and perhaps, also, for the inhalation of genial gases 
or the dissipation of impalpable excrement. The transpiration of fluid, of gas, or 
