74 LAYERS. [chap. xv. 
A Verbena layered. 
The only art required in layering is to 
contrive the most effectual means of inter¬ 
rupting the returning sap, so as to produce as 
great an accumulation of it as possible, at the 
joint from which the roots are to be pro¬ 
duced. For this purpose, sometimes, instead 
of cutting the branch half through, a ring 
of bark is taken off, care being taken that 
the knife does not penetrate into the wood; 
