252 



DISEASES OF TREES 



The contention that such pruning is too costly is justified only 

 when it can be proved that the difference in value between a log 

 free from knots and one where they are abundant is not equal to 

 the cost of pruning plus interest on the outlay. 



When we come to consider the removal of green branches 

 that is to say, branches or twigs that are living and provided with 

 leaves we find that, except in a few exceptional cases which 

 will be presently discussed, a loss of growth attends the oper- 

 ation. This is the case no matter whether the separation 



from the stem be effected 

 by the hand of man or 

 by such natural agents 

 as storms, accumulations 

 of snow, &c. If one re- 

 duces the number of the 

 organs of assimilation 

 (the foliage leaves), the 

 products of assimilation 

 generally suffer to a like 

 extent. As I have proved 

 conclusively, 1 it is only 

 in the case of trees that 

 are growing in a per- 

 fectly open situation, 

 whose stems are branch- 

 ed to the ground, and 

 which have a very large 

 mass of foliage, that 



limited pruning may be performed without diminishing the 

 amount of growth. In the case of such trees there is a greater 

 extent of foliage than is necessary to effect the metabolic 

 processes in the plant-food that is taken in by the roots. Of 

 course the amount of growth depends essentially upon the 

 quantity of such food. Under such circumstances a reduction 

 in the extent of the foliage merely results in more active 

 assimilation in the leaves that remain. 



In the great majority of cases the practical operation of 

 pruning is followed by more or less considerable reduction in 

 1 Das Holz der Rothbuche. Berlin, Springer, 1888. 



FIG. 145. The dead and rotten stump of an oak- 

 branch over which an occluding callus has 

 formed. Two thirds natural size. 



