INFLUENCE OF CLIMATE ON PLANTS. 
107 
by an artificial stimulus, which being alike difficult to maintain, and almost desti- 
tute of that sine qua non to the healthy growth of all plants, light, causes a weak, 
partial, deformed, and immature development, and where the plant is of a delicate 
nature, sometimes terminates in its total destruction. 
We are aware tliat much difficulty attends the management of orchidaceous 
plants in this respect ; and that a great diversity of treatment must be practised. 
The natural habits of these and all other plants should not only be studied, but to 
them the treatment must be conformed, as far as is consistent and practicable. 
Summer repose, and winter excitation, however, are too grossly absurd, and too 
fatally prejudicial, to be long followed in this country. Orchidaceous, stove, green- 
house, and all other tribes of plants, must have rest when the climate in icMch they 
are cultivated renders it necessary ; and, according to the principles herein inculcated, 
in Great Britain that period is the winter season. 
INFLUENCE OF CLIMATE ON PLANTS.* 
SOLAR HEAT. 
Pores, besides fulfilling the office of respirators, are likewise the chief vehicles of 
tlie radiation of heat. It has been frequently stated that radiation and evaporation 
are reciprocally dependent or concurrent ; the exhalation of fluids either causing 
or resulting from a simultaneous effusion of heat. In proof of the former of these 
positions, it is said that frost is never hurtful to plants, till evaporation has 
succeeded ; and that the abstractions of temperature which accompany such 
evaporation are mainly productive of the injury. But a more mistaken hypothesis 
could not possibly be imagined. 
We have already described the action of frost upon the structure of vegetation. 
The increased escape of fluids wiiicli follows a renewed application of heat, so far 
from being the operating cause of the damage sustained, is merely one of its effects, 
and that by which it is made manifest. Indeed, it is ridiculous to suppose that 
radiation can be a consequence of evaporation ; because external heat, the agent 
wdiich induces the latter, would evidently repress the former. 
Nor can it be acknowledged that evaporation accompanies radiation, except in a 
very trifling degree. For, although it is remarked that dew is formed on the under 
surface of leaves, it seems to have been forgotten that the vapour of which dew is 
composed by condensation, has so slight a specific gravity, that it is diffused 
through, and held in suspension by, the atmosphere ; and does not therefore neces- 
sarily descend perpendicularly, but may be deposited upon a cooler substance, in 
almost any position. Before we can admit that these processes are simultaneously 
effected, we must assume that they are produced by the same or coincident agents ; 
a theory wholly at variance with existing facts. 
* Continued from page 88-. 
