148 NECESSITY OF BOTTOM HEAT. 



the soil. He found that, in a cantered orchard, the roots of 

 the trees had entered the earth to the depth of three feet; and 

 he also ascertained that, during the summer months, the 

 average heat of the soil, at six inches below the surface, 

 was 61°; at nine inches, 57°; at 18 inches, 50°; and at three 

 feet, 44°. He took measures to confine the roots to the 

 soil near the surface, and the consequence was, the disap- 

 pearance of canker, and ripeniag of the fruit. (Memoirs of 

 Caledonian Hort. Soc, vi., part 3 ; and Gardener's Magazine, 

 vii. 56.) The same fact has been observed in many other 

 instances. 



If, on the other hand, we take cases of growth in the artificial 

 climate of hot-houses, we find that Bignonia venusta, and many- 

 other tropical plants, will not flower, unless in a high bottom 

 heat ; and that Palm-trees, planted in the soil of conservatories 

 which it is impracticable to heat sufficiently, soon become 

 unhealthy. 



The reason why it is necessary to plants in a growing state, 

 that the mean temperature of the earth should be higher than 

 that of the air, is sufficiently obvious. Warmth acts as a 

 stimulus to the vital forces, and its operation is in proportion 

 to its amount, within certain limits. If, then, the branches and 

 leaves of a plant are stimulated by warmth to a greater degree 

 than the roots, they will consume the sap of the stem faster 

 than the roots can renew it ; and, therefore, nature takes care 

 to provide against this by giving to the roots a medium perma- 

 nently more stimulating, that is, warmer, than to the branches 

 and leaves. 



We regard warmth not merely as a stimulus of vegetation; it is 

 extremely necessary for tlie solution of various substances witk wMcli 

 tlie water comes in contact. It also seta free certain gases which the 

 leaves take up, and through these sources of nourishment promotes the 

 growth of plants. — Oerman Hd. 



Such being the fact, it is obvious that one of the first of a 

 gardener's cares should be, to secure the means of insuring a 

 proper -temperature to the soil in which he grows his plants, 

 and that this is Requisite for hardy as well as tender species. 



