NATURAL PRUNING. 



59 



51 height above the ground. (See figs. 52-5G and PI. 

 XXX.) The trunks of trees grown in the open, where 

 even the lower limbs have abundance of light, are 

 branched either quite to the ground or to within a short 

 distance of it. But in the 

 forest not only are the lower 

 side branches continually 

 dying for want of light, but 

 the tree rids itself of them 

 after they are dead and so 

 frees its trunk from them en- 

 tirely. When a branch dies 

 the annual layer of new wood 

 is no longer deposited upon 

 it. Consequently the dead 

 branch, where it is inserted 

 in the tree, makes a little 

 hole in the first coat of living 

 tissue formed over the live 

 wood after its death. The 

 edges of this hole make a 

 sort of collar about tlie base 



of the dead branch, and as 



Fig. 57. — Poiutedcrowua of saplings 

 of Longleaf Piue growing rapidly 

 in height. Southern Florida. 



a new layer is added each 

 year they press it more and 

 more tightly. So strong does this compression of the 

 living wood become that at last what remains of the 

 dead tissue has so little strength that the branch is 

 broken off by an ice storm or by the wind, or even falls 

 of its own weight. Then in a short time, if all goes 

 well, the hole closes, and after a while little or no ex- 

 terior trace of it remains. Knots, such as those which 

 are found in boards, are the marks left in the trunk 

 by branches which have disappeared. 



