146 PROPAGATION OF PLANTS. 



soil mspring is generally warmer than the atmosphere, 

 and tliefTower end of a cutting, from which point it is 

 always desirable to have the roots produced, receives 

 more heat than that portion which is exposed to the air. 

 In latitudes where the ground freezes to a considerable 

 depth, every one who has ever taken the trouble to exam- 

 ine the soil at the time of its thawing out in spring, must 

 have noticed that it thaws from below upward, far more 

 rapidly than from the surface, downward. Heat descends 

 slowly, but cold rapidly, and just as soon as the weather 

 becomes so warm that the surface does not freeze, the 

 heat from below will rise to the surface. The hot-bed 

 used by gardeners in forcing vegetables in cold climates 

 is made on the same principle, the object being to secure 

 warmth for the roots, while the leaves and upper portions 

 of the plant are kept cool. Thus plants are forced with 

 what is termed "bottom heat." 



In warm climates it is just as important to give the 

 cutting plenty of time to form roots, or the advance 

 process^callejl a callus., as in cold ones ; for if roots are 

 not formed when the leaves expand, the cutting is very 

 likely to die. The callus which always precedes the, for- 

 mation of rpots on all kinds of cuttings, whether from 

 ripe or green wood, leaves or roots, is composed of cellu- 

 lar matter formed principally from the assimilated sap 

 of the plant, and is of a similar nature to the nutrients 

 stored up in seeds for nourishing the embryo and young 

 plantlet ; and while the callus does not itself become a 

 root, it is the immediate source from which the young 

 rootlet on a cutting obtains nutriment. By a close ex- 

 amination of the callus on a cutting at the time the young 

 rootlets are pushing out, it can readily be seen that the 

 roots are distinct formations, and not in any manner the 

 result of an unfolding or prolongation of the irregular 

 masses of exuded matter which is termed the callus. 



Cuttings that are jemovedjrom^ a Cellar or other plajee, 



