INFLUENCE OF CLIMATE ON VEGETATION. 
15 
known, there can be no difficulty in apportioning to it the heat necessary and proper for its 
growth, both by day and night, nature being followed in the mode by which that appointment 
is regulated. 
Impunity has been spoken of. Of course, this expression is intended to take into 
account the injury that may be wrought on the constitution of a plant, both by excitement, 
and paralysis — by excess of heat and cold. 
The acquirement of a knowledge of the highest and lowest temperatures that each species 
of plant will bear without injury, is matter for direct experiment ; and, in fact, can only be 
surely acquired by such an experimental trial with each individual species. It is certain 
that many plants will bear an amount of cold which, but for the knowledge having been 
acquired by experience, would not be credited. Probably this is the case with most plants, 
if not with all. Attention has, however, been a good deal averted from this view of the 
matter, by the practice of referring all exotic plants to an insufficient number of groups. 
Virtually, there are in gardens but three recognised groups of exotic plants, namely, stove, 
greenhouse, and hardy ; and the cultivator’s ideas of climate, for the most part, adapt 
themselves to this division. The exceptions to this, in which intermediate groups are 
formed, are comparatively few and rare. This requires to be exactly reversed, if perfection 
in the art of culture is to be obtained. Universally and not partially, it must be admitted, 
and practically acted on, that a greater variety of climate than is at present generally 
afforded, is required, to cultivate successfully the great variety of exotic plants, which have 
been introduced to our gardens from every part of the world. 
These considerations obscurely indicate some of the difficulties which the cultivator has 
to contend with, and why it is that he finds a greater number of buildings, though of small 
size, more favourable to his cultivation, than the same extent of accommodation in fewer 
structures. Practically, it would seem desirable, where a miscellaneous set of exotic plants 
is grown, to provide structures, in which an average of every tenth or fifteenth degree of 
temperature, between 40° and 80° or 90° Fahr., may be kept up ; and even some of these 
average temperatures, would require to be connected both with a dry and moist atmosphere. 
It seems to me, that the old titles of “ stove ” and “ greenhouse,” as applied to the plants 
of temperate and tropical climates, ought to be given up, as only calculated to mislead ; and 
that exotic plants should be classified anew into a greater number of groups, correspondent 
with certain gradations of climate, adapted to meet the requirements of cultivation. This 
opens up the great question of Botanical Geography, a subject of the highest importance to 
cultivators generally. 
A remark just made, might, without explanation, seem to favour the idea (already far 
too prevalent) that it is essential, in the management of artificial climates, to keep the 
temperature accurately to a certain point of the thermometer. This has no support in 
nature. A certain range of the thermometer should not be exceeded either above or below ; 
but to maintain a particular degree of heat, under all circumstances, either by day or by 
night, is a practice founded on error, or a total misconception of a natural climate. 
In considering the influence of climate on vegetation, three things must be kept in view, 
namely, the properties of light, heat, and moisture. No one of these may be lost sight of ; 
and the successful imitation of a natural climate will depend on the adaptation of these, each 
to the other. Now, one of these, the property of light, is altogether beyond our control ; 
therefore the applied heat and moisture must be adapted to the existing degree of light. 
An increase of heat excites growth, but growth in the absence of sufficient light is imperfect. 
Thus we reach the conclusion, that the highest permitted degree of heat should be coincident 
with the brightest light ; and that in the absence of light, the lowest degree of heat 
sufficient for the plants, should be applied. Between these extremes, the modifications of 
applied heat should correspond with the modifications of existing light. The lowest degree 
of heat proper for growing plants may be a fixed point, or nearly so ; but what the highest 
should be, is dependent partly on external conditions, both as to temperature and light. 
