BOTTOM HEAT. 291 



matter lies buried that must be secured. Unusual warmth of 

 the air would have the effect of stimulating the buds, and would 

 cause a premature appearance of leaves, which would be any- 

 thing rather than conducive to the success of a cutting. If soil 

 were to be kept at 33° and the air at 84°, leaves would form, 

 but no roots would be emitted under ground, however skilful 

 the operator ; and then, unless roots were thrown out above 

 ground, the cuttings would speedily exhaust themselves. On 

 the other hand, if the soil were kept at 84°, and the air at 33°, 

 leaves would certainly be formed as soon as the roots had 

 struck out, although in a pinched and shivering condition. 



A proper degree of bottom-heat, then, is the first point for 

 consideration, for all other processes are subservient to that 

 fundamental requisite ; and the rule is, that it should always be 

 higher by several degrees than that to which plants are 

 naturally subject. Suppose, for example, that it is required to 

 strike a cutting of some plant from Algiers, and that the mean 

 temperature of the summer there were 70°, the safe course for 

 the gardener to take would be, to plunge his cutting in soil 

 warmed up to 75°. 



The action of light and moisture upon cuttings is hardly 

 inferior to that of heat. The moment li^t strikes a green 

 plant it excites perspiration. Let us imagine that a cutting 

 weighed 20 at daybreak ; the uninterrupted action of light upon 

 it during the day would perhaps reduce its weight to 5, unless 

 it were supplied with water to replace that which the sunlight 

 drives off: the effect of this would, of course, be to kill it. 

 But such a result does not often happen to rooted plants, 

 because they are able to suck fluid out of the earth as fast as 

 the sun drives it off from tteir leaves, and the circulation of 

 the plant is active enough to prevent ajiy part from being 

 exhausted of fluid. If it is not sufficiently active, then we have 

 leaves withered at the end, or branches struck with dryness 

 But a cuttmg, having no roots, is unable to contend against the 

 suns influence, and therefore it must be shaded; for, as we 

 cannot make it feed,, we must prevent its wanting food. Thus 

 m tropical countries we learn from Mr. Neumann that cuttings 

 are struck m sheds shaded by straw and watered occasionally; 



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