216 TREES AND TREE-PLANTING. 
both, is not entirely restricted or cut off in its flow, but 
allowed a passage and continued supply till the produc- 
tion of roots, by the fact of its being necessarily kept in 
connection with its parent stem. The seasonable flow of 
sap which is drawn up by the roots of trees for the nour- 
ishment and growth of their stems and branches diffuses 
itself throughout, when, having performed its offices, it 
returns through the inner bark and deposits a thin layer 
of sap-wood or alburnum: so it may be conceived that 
the branches and roots are composed alike of this sub- 
stance. 
In layering, a branch may be chosen at a convenient 
height from,the ground, and, if not of such convenience, 
bent till brought in contact with the ground, as shown 
below, where it is to be inserted, or covered over with 
V 
earth, and kept in position by means of a forked peg. 
Previous to insertion or covering over of the bent branch 
or layer an incision is commonly made in the lower 
