290 AMOUNT OP HEAT NECESSARY. 



nearly alike, that distant countries are clothed with vegeta,tion 

 of entirely different kinds, and that the distinction in the 

 vegetation is in proportion to the distance of the countries 

 from each other. There are not, perhaps, a dozen species in 

 Normandy that do not grow wild on this side the Channel ; 

 there are not a dozen species common to England and Bengal. 

 Species, in fact, are in general limited by similarity of tempe- 

 rature, and cannot exist beyond such limits. One of the first 

 considerations for the propagator, 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, will 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 

 faster and more vigorously if placed in a hotbed. A WhitethOTn_ 

 cutting in the open ground wiU not root at all; in a warm 

 propagating house, it wiU d o so readi ly ; and, to reverse the 

 illustration, cuttings of tropical plants, which naturally enjoy a 

 very high temperature, wiU 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, 

 Gruaiacum, Mangoes, &c., will not succeed unless in a tempe- 

 rature of about 100° Fahr. That degree of heat would be fatal 

 to greenhouse plants. 



But it is not the temperature of the atmosphere that requires 

 to be maintained above that to which plants are naturally 

 subject : it is the soil that must be warmed. The ^ratobjectj^, 

 to obtain roots; those organs once formed, leaves will follow. 

 The vital action which causes the production of roots is, in the 

 first instance, local ; roots are produced by the development of 

 the cellular matter of the underground part; that cellular 

 matter requires to be stimulated by unusual warmth ; but the 

 necessary stimulus cannot be coiniiiunicated by a heated 

 atmosphere : it is the warmth of the soil in which the cellular 



