NATURAL TEMPERATURE OF SOIL. 135 



of the air may fall to such a degree that the excitability of a plant 

 woTild be too much and suddenly impaired, if it acquired the coldness of 

 the mediiun surrounding it ; this is prevented, we may suppose, by the 

 warmth communicated to the general system, from the soU, through the 

 roots ; so that the lowering of the temperature of the air, by radiation 

 during the night, is unable to affect plants injuriously, in consequence 

 of the antagonist" force exercised by the heated soil. It is not impro- 

 bable that this fact may be hereafter applied in gardening to the 

 acclimatising of half-hardy plants. "Were an open border heated 

 artificially in the winter, it is possible that plants might endure an 

 amount of cold upon their stems and leaves, which in the absence of 

 such heat would be fatal to them. An experiment upon this subject 

 was tried some years ago, and although it was conducted so negligently 

 and unskUfully, as not to justify any inference being drawn from it, 

 yet the result, such as it was, was full of promise. 



That elevating the temperature of moist soil produces an 

 unusual degree of vigour in plants unaccustomed in nature to 

 such an elevation is a fact which requires no proof; it is 

 attested hy the condition of vegetation round hot springs, and 

 in places artificially heated by subterraneous fires ; and this 

 has probably been the cause of the employment of tan and 

 hot-beds, by which means bottom heat has been generally 

 obtained for rearing delicate species, and especially seeds. 

 But if this stimulus acts in the first instance beneficially 

 in all cases alike, it soon becomes a source of mischief in those 

 species vyhich are natives of climates where such terrestrial 

 heat is unknown, the latter " drawing up," as the saying is, 

 becoming weak and sickly, and speedily presenting a diseased 

 appearance. 



On the other hand, it is equally well known that, unless the 

 temperature of the soil be raised permanently to at least 75°, the 

 seeds of tropical trees will not germinate ; or, if they do, they 

 push forth feebly, and from the first present the sickly appear- 

 ance of plants suffering from cold. Hence arises the impossi- 

 bility of making the seeds of tropical plants germinate when 

 sown in the open air in this country, where the mean tempera- 

 ture of the earth seldom rises to 65°, and that for only short 

 periods of time. It is, therefore, obvious that aU plants require 

 some bottom heat ; but the amount varies with their species, 

 and the only means of determining what the amount should be 



