Chinatal [nflucnces. 563 
as this process not only changes the colour of a plant, but also 
greatly modifies its other secretions. 
The sources of nitrogen and many other constituents of the 
ternary and quaternary proximate principles of plants, and by 
what agencies these combinations are effected, are subjects still 
engaging the attention of scientific men. 
§ 2. Crimare, vs Inriumyce on Vucerarion. 
Vegetable organisms, more than animal, and especially than 
man, are subject to the influences of relatively slight deviations of 
both heat and cold in diverse degrees, according to the climatal 
conditions of their natural habitats. Taking those plants in 
general cultivation for our guide, an access of heat beyond 
what may be termed normal seems to be less injurious than 
an increase of cold. But even heat above the temperature 
natural to a plant will eventually kill it, though its action 
is slow in comparison with that of cold. Frost is very 
decided and rapid in its action, its effects being visible 
almost immediately after a fall in the temperature. This 
peculiar susceptibility is only understood by its results, though 
doubtless an explanation must be sought in the differences of 
organisation of species restricted to different ranges of tempera- 
ture. It is now almost universally conceded that by no process 
of acclimatization can we succeed in making a plant frost- 
proof, even to the extent of a single degree. Under other- 
wise favourable conditions, it is true, a plant will withstand a 
somewhat lower air-temperature than it is subject to in a 
natural state. To illustrate the certain and unchangeable 
effects of temperature on plants, we need only mention 
such familiar examples as the Potato and the Dahlia. We 
mention these because they have been under cultivation 
with us for a long series of years, without producing any 
visible or appreciable alteration in their constitution, in so 
far as it concerns their power of enduring cold. From the 
same cause, acting inversely, it is difficult, and in some cases 
impossible, to grow mountain and arctic plants success- 
fully; that is to say, to keep them alive and in health for a 
number of years. In this case, perhaps, the actual increase of 
temperature is less injurious than the lengthened growing 
period to which plants from colder regions are subject under cul- 
tivation in this country; but, after all, this isa distinction with 
002 
