272 



PROPAGATION BY LAYERS. 



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 inter- 

 rupted 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. 



§ IV. Propagation by Layers. 



019. Hie 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 transmission 

 of coloured fluids from the roots upwards, that a communication is main- 

 tained 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 organised layer of alburnum, 

 a portion of this extending to the extremities of the roots, where it pro- 

 trudes in the form of spongioles. From those 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 aff^ord egress to 

 the nascent roots. From this the principle of the operations of ringing, 

 applying ligatures, twisting, tonguing, or splitting the parts about to be laid, 

 will be easily understood. 



620. The operation of layering^ like that of fonning 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, 

 hy 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 whichever way 

 layering is performed it consists in the interruption of the descending sap at 

 a joint of a stem, or shoot, and placing it under circumstances favourable for 

 the production of roots. The interruption is most successful when it takes 

 place immediately under a bud or joint, when the shoot is more or less 

 matured, and when it penetrates into the alburnum ; though, if the albur- 

 num is penetrated too far, the ascent of the sap will be interrupted, and the 

 supply to the buds or leaves will be insufficient to develop tliem, or 

 keep them from flagging. The descending sap may be interrupted either 

 wholly by cutting off" a ring of bark, or partially by a cut or notch, 

 by driving a peg or nail through it, by a slit kept open, by twisting 

 the stem at a joint, by strangling it there with a wire, by bending it so 

 as to form an angle, by pressure by laying a stone on it, or by attracting 

 it by heat and moisture. The latter mode of causing a branch to 

 protrude roots may often be observed in nature, in the case of the lowest 



