OF BOTTOM HEAT. 103 



jnitted that, in some cases, the amount is extremely 

 small ; for Von Baer found Ranunculus nivalis and 

 Oxyria reniformis flowering in Nova Zembla, where 

 the soil was not warmed above 34 J ° ; and, in Jakutzsk, 

 Brdmann states that Summer Wheat, Eye, Cabbages, 

 Turnips, Radishes, and Potatoes are cultivated, al- 

 though the ground is not thawed above three feet in 

 depth. 



That elevating the temperature of moist soil causes 

 an unusual degree of vigour in plants unaccustomed 

 in nature to such an elevation, is a fact which requires 

 no proof: it is attested by 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 hotbeds, 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 which 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 (108). 



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

 unless the temperature of the soil be raised permanent- 

 ly to at least 75°, the seeds of tropical trees will not 

 germinate ; or, if they do, they push forth feebly, and 



warmth communicated to the general system from the soil, 

 through the roots ; so that the lowering of the temperature of the 

 air, by radiation during the night, is unable to affect plants injuri- 

 ously, in consequence of the antagonist force exercised by the 

 heated soil. 



