NOCTURNAL RADIATION. 199 



houses, or guarded by a mere coping of thatched hurdles, suffer 

 less in winter than if they were fully exposed to the sky. 



It is also obvious from what has been stated that plants 

 growing upon grass will be exposed to a greater degree of cold 

 in winter than such as grow upon gravel: but it does not 

 therefore follow that hard gravel is, with respect to vegetation, 

 a better coating for the surface of the ground than turf ; it has 

 its disadvantages as well as its advantages, and the former 

 probably outweigh the latter. Its superior heating power is its 

 only advantage ; the objections to it are, its dryness in summer, 

 and its comparative impermeability to rain, so that it causes 

 the force of perspiration to be inversely as the absorbing 

 power of the roots. 



In Grermany, where the winters are very severe, it is customary to 

 cover the roots of plants on grass with a mulching of leaf mould, six 

 inches deep and a foot in diameter ; but this can have, I think, no 

 sensible effect upon roots, because of the inconsiderable area that it 

 occupies. 



It is well known that blackened surfaces absorb heat much 

 more than those of any other colour ; and it has been expected 

 that the effect of blackening garden walls, on which fruit-trees 

 are trained, would be to accelerate the maturation of the fruit ; 

 but, notwithstanding a few cases of apparent advantage, one of 

 which, of the Vine, is mentioned in the Horticultural Transac- 

 tions, vol. iii., p. 330, this has been, in general, found either 

 not to happen at all, or to so small an extent as not to be 

 deserving of notice in practice. It is true, that so long as the 

 wall is but little covered by the branches and leaves of a plant, 

 the absorbent power of the blackened surface is brought into 

 play; but this effect is lost as soon as the wall becomes covered 

 with foliage. In the early spring before the leaves appear, the 

 flowers are brought rather more forward than would otherwise 

 be the case, which is in England a disadvantage. It would 

 seem, however, that in autumn the wood becomes more com- 

 pletely ripened ; but the effect is very slight. 



It is rather to a judicious choice of soil and situation that 

 the gardener must look for the means of softening the rigour 

 of climate. Wet tenacious soUs are found the most difficult to 



