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THE GREEN-HOUSE. 195 



The improvement wbicli man lias made on the 

 natural mode of layering, is to bury the shoot deeper 

 in the soil, by which means the stimuli of moisture, 

 the very considerable one of pressure, and the ne- 

 cessary condition of the exclusion of air, are increased. 

 Another improvement is that of wounding that part 

 of the shoot which is buried, by which the tendency 

 to put out roots is greatly increased, partly by the 

 quantity of nourishment derived from the stock or 

 parent being diminished, and partly from the return- 

 ing sap being impeded and stagnated in the layer, at 

 the lower end of which it first produces a callus, and 

 then roots. 



There are different ways of wounding the shoot 

 to be layed : a very common mode is giving it such 

 a twist as shall fracture longitudinally both the bark 

 and wood; another mode is by thrusting a knife 

 through the shoot, withdrawing it, and then inserting 

 a bit of wood, slate, stone, or a nail, in its stead : 

 some twist a wire tightly round the part from which 

 the roots are to protrude ; others pierce it with an awl 

 in two or three places ; the most common mode is to 

 make an incision half way through the shoot, and 

 return it longitudinally upwards or towards its extre- 

 mity, so as to separate about half an inch of half of the 

 shoot ; this half inch is technically called the tongue, 

 and the process tonguing. About one-third of this 

 separated part or scalp (being what is generally 

 fractured by the operation) should be cut off in a 

 transverse direction, and directly across a bud or 



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