THE ELOBAL WOULD AND GARDEN GUIDE. 
303 
that do not grow wild on this side the Channel ; there is not a 
dozen species common to England and Bengal. Species, in fact, 
are in general limited by similarity of temperature, and cannot exist 
beyond such limits. One of the first considerations for the propa- 
gator, therefore, is what amount of heat is natural to a species 
during its season of growth. With less than that it is hopeless to 
make cuttings grow. It is only when plants strike freely that the 
natural amount of heat is sufficient ; in general they require more. 
The amount of heat found in their natural climate may be enough 
for them to grow in ; but a greater degree of excitement, by means 
of a higher temperature, v\ ill be demanded by them to strike root in, 
when cut up into the fragments called cuttings. 
A. willow cutting, stuck into the open ground, will strike root ; 
but it does so mm h faster, and more vigorously, if placed in a hot- 
bed. A white-thorn cutting in 1 he open ground will not root at all ; 
in a warm propagating house it will do so readily. And, to reverse 
the illustration, cuttings of tropical plants, which naturally enjoy a 
very high temperature, will perish if it is reduced, and will only put 
forth roots when it is raised considerably above their natural 
standard. Thus Mr. Neumann mentions that nutmegs, guiacum, 
mangoes, etc., will not succeed unless in a temperatuse of about 
100° Fahr. That degree of heat, again, would be fatal to green- 
house plants. But it is not the temperature of the atmosphere 
that requires to be maintaiued abov~ that to which plants are 
naturally subject — it is the soil that must be warmed. The first 
object is to obtain roots — those organs once formed, leaves will 
follow. The vital action winch causes the productiou of roots is, in 
the first instance, local. Roots are produced by the development of 
the cellular matter of the underg-ound part. That cellular matter 
requires to be stimulated by unusual warmth, but the necessary 
stimulus cannot be communicated by a heated atmosphere ; it is 
the warmth of the soil in which the cellular 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 anything rather than conducive to the 
success of a cutting. If the 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 con- 
sideration ; lor all other processes are subservient to that funda- 
mental requisite. And the rule is, that it should always be higher, 
by several degrees, than that to which plants are naturally subject. 
Unfortunately, we have very little evidence to show what that is; 
but a rough estimate of it may be formed by regarding it to equal the 
mean temperature of the summer. Hence the great value of good 
meteorological observations to gardeners. Suppose, for example, 
that it is required to strike a cutting of s.>me plant from Algiers, and 
October. 
