PROP A GATION BY LA YERS. 233 



they will form a callosity there, and root more rapidly when planted. 

 The roots of plants which contain latent buds may be stimulated to 

 develop them by the exposure of portions of them to the light, or by 

 bending, or twisting, or cutting notches in them, in the same manner 

 as in stems. Piercing the stems or roots by a longitudinal cut through 

 a joint, and keeping the wound open with a wedge or splinter, or 

 driving pegs or nails through them, will facilitate both the formation 

 of roots and the development of buds ; and various other modes of 

 exciting buds, and causing the protrusion of roots, will occur to the 

 gardener who understands what has been already said on the subject. 

 It is only necessary to bear in mind that when the ascending sap is to 

 be interrupted by cutting, the knife must penetrate into the alburnum, 

 and that when roots only are the object in view, it is only necessary 

 to penetrate the bark. 



Propagation by Layers. 



The theory of layering is founded on the following facts : The sap 

 absorbed from the soil by the roots rises to the buds and leaves chiefly 

 through the alburnum ; for though it has been proved, by the trans- 

 mission of coloured fluids from the roots upwards, that a communica- 

 tion is maintained throughout the whole stem, yet the greatest flow of 

 sap, whether ascending or descending, takes place through the youngest 

 layers, whether of wood in ascending, or inner bark in descending. 

 A decortication may therefore be made with little or no interruption 

 resulting to the ascent of the sap. The elaborated fluid, in returning 

 from the leaves, descends by the inner bark, depositing in its progress 

 an organized layer of alburnum, a portion of this extending to the 

 extremities of the roots, where it protrudes in the form of spongioles. 

 From these facts it will appear evident that although ringing does not 

 interrupt the upward flow of sap, because the incision does not reach 

 the vessels in which it proceeds, yet that the descent is prevented 

 by the chasm formed by the operation ; on the brink of this chasm it 

 accumulates, and under favourable circumstances a callosity is formed, 

 or mass of cellular substance protruded, which by degrees assumes a 

 granulated form, and these granulations ultimately elongate into spon- 

 gioles ; or the teguments above the incision, being rendered soft by 

 the earth or other suitable moist covering, are ruptured, and afford 

 egress to the nascent roots. From this the principle of the operation 

 of ringing, applying ligatures, twisting, tonguing, or splitting the parts 

 about to be laid, will be easily understood. 



The operation of layering, like that of forming cuttings, is chiefly 

 applicable to plants having leaf-bearing stems ; and the advantage 

 which a layer has over a cutting is that it is nourished, while roots 

 are being formed, by the parent plant ; whereas the cutting has no 

 other resource than the nutritive matter laid up in it, or that produced 

 by the functions of the leaves. Hence, layering is one of the most 

 certain modes of propagation by division, though it is in general slower 

 than any other mode. In whatever way layering is performed it 

 consists in the interruption of the descending sap at a joint of a stem, 



