Cliniatal Influences. 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. CLIMATE, ITS INFLUENCE ON VEGETATION. 



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 seenis 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 is a distinction with 



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