PEOBLEMS OF PBOPAGATION. 



453 



from the point of section and induces root -formation beyond 

 the area of the callus, one may find a means by which to 

 overcome the influences that retard root-formation. The effect of 

 intermittently repeated wound- stimulus is wonderful in the massive 

 development of callus upon stems in positions where no special function 

 beyond that of healing is required. Its effect upon callus where there 

 is demand for new absorptive organs imperatively required for the 

 hfe of a cutting is no less remarkable. The wound-stimulus induces 

 root -formation. By paring off the surface of the callus-tumour it is 

 possible to stimulate almost immediate root-development. If a first 

 paring is ineffectual a second or a third may bring about the desired 

 growth. Proteaceae, which are regarded as difficult subjects, readily 

 respond to the stimulus of callus paring (fig. 161). The callus itself 

 here forms the roots in the first instance. If one pares the callus of a 

 nodal cutting in Clematis, roots are formed, but always from the inter- 

 node above the callus. 



In contrast with plants which readily and rapidly form an abundant 

 callus — and that is characteristic of the soft-wooded — there are the 

 dicotylous plants commonly called "hard-wooded," which are attuned 

 to live in conditions where water is physically and physiologically 

 scanty, and which, if they have enough to satisfy their requirements, 

 live only by the exercise of the strictest economy of the resources 

 available to them — Erinacea pungens and species of Dillwynia and 

 other Australian Leguminosae, are examples. 



In them, then, there is none of the lavishness observable in the 

 soft-wooded, although the processes of striking are the same. The 

 march of events is relatively slow. Wound-stimulus is not transmitted 

 rapidly. The initiation of callus is late, is often not abundant, and 

 formation o'f new absorbing roots is delayed. It is these elements that 

 govern the difficulties encirchng the propagation of such plants. Their 

 drought attunement makes special demands for nice adjustment of 

 . aeration of the nidus. Their slow development prolongs the period 

 during which these have to be satisfied. The double danger of wilt- 

 ing of the cutting from insufficient water and of suffocation from want 

 of air becomes therefore very real. These considerations may suggest 

 particular conditions that require attention in propagating such plants, 

 and not the least important of these is the necessity of using as sniall 

 cuttings as possible. Size of cutting is a matter of relatively less 

 moment in the soft-wooded. 



Another problem confronts the gardener when he attempts to propa- 

 gate by cuttings plants which are full of resin as are so many of the 

 Coniferae. 



For long it was said, and even now it is said, as my text specifically 

 shows, that propagation by cuttings of some conifers is not possible. 

 That such propagation from the point of view of commerce is im- 

 practicable in the case of many coniferous genera we may admit. As 

 a fact of science all can be propagated. 



An obstacle in the way of commercial propagation by cuttings of 



