64 THE FOREST TREE CULTUKIST. 
crease strength, as it will enable those leaves that remain 
‘to enlarge and act with more vigor than if all had remain- 
ed. Some varieties will require but little pruning, while 
others imperatively demand it at least once a year, or very 
little progress will be observed. 
Pruning should not be practiced to such an extent that 
the tree will be eventually weakened or checked in growth. 
Neither should too many of the lower branches be taken 
off at one time, but expose the stem gradually to the sun. 
A tree when grown in the open field will usually produce 
branches sufficient to shade the whole of its stem. This 
appears to be not only natural but beneficial, for when the 
stem is fully exposed to the sun the bark becomes dry and 
hot, and the flow of sap is retarded in its circulation. It 
is only while young and the bark thin that any particular 
injury will be perceived. The stems of trees do not re- 
quire the direct rays of the sun, but the leaves can not 
exist long without them. : 
When trees are grown in nurseries, the stems are par- 
tially shaded, consequently the lower branches are not re- 
quired for shade, but only to assist growth until a sufficient 
number of others have been produced, so that their services 
can be dispensed with without injury to the tree. 
Trees when standing alone should have at least two 
thirds of their height occupied with branches. But when 
grown in thickets, and for the purpose of producing tim- 
ber, this rule may be reversed, and the branches occupy 
only one third, varying the rule according to the natural 
habit of the tree. If it is one of those which produce a 
conical head, like the Balsam, Tulip, and Larch, then allow 
