UPON PLANT-HOUSES. 



that influence of heat or light, as may be proved by hundreds of 

 instances. Thus the leaf-bud turns visibly away from the light 

 at the moment of bursting ; the young leaves ai'e not always 

 developed towards the south, but follow the general course of 

 warmth in the atmosphere, expanding themselves most actively 

 between south and west, in order to turn green. Nor are plants 

 insensible to the radiation of heat emanating from neighbouring 

 plants, which in some degree affect their development. The 

 degree of warmth which plants receive from the solar light differs 

 according to their solidity and thickness, and should become 

 perceptible towards evening and during night. The radiation of 

 heat from powerful Cactuses and the like plants deserves to be 

 especially attended to. 



All these combined influences become greater and more 

 complicated if we reflect, that the vegetable structure consists of 

 a threefold system of growth : the ascending and the descending, 

 which are perpetually in a state of antagonism ; that while the 

 atmosphere with its light and heat constitute only one-half of 

 the agencies which affect these green, deaf and dumb, but still 

 excitable beings, the earth — that dark source of warmth — and 

 its moisture, call forth other processes in them. Not to insist 

 on these, physical and chemical powers operate differently 

 according to tlie degrees of longitude and latitude, elevation of 

 the sea, exposure, season, time of day, character of the 

 soil, &c. ; and all the phases in the life of a given plant occur 

 within certain periods in succession, according to its native place. 

 All which tends to prove that the vegetable \Yorld must possess 

 the power of adaptation to a considerable degree, in order to 

 accommodate itself to and thrive under the deficient appliances 

 which the art of gardening can substitute for the realities of 

 nature. 



In a state of nature, as well as in a hot-house, plants derive 

 warmth from two distinct sources — from the sun as the illu- 

 minating, and from the earth and whatever else can radiate heat, 

 as the dark, source. We are unable to give plants from hot 

 countries the same advantages which they derive from those 

 sources in their native places, because the heat of our less 

 vertical sun is not so powerful as in those regions ; and the 

 plants consequently I'eceive a smaller amount of stimulus from 

 light and heat from above. We err if we imagine that the heat 

 from a stove or from heated tubes has the same effect, physiolo- 

 gically, as the former combined. We might approach a state of 



