254 DISEASES OF TREES 



situated on the basal portion of the severed branch that is 

 embraced by the stem which produce the shoots. 



When a spruce is pruned, numerous shoots spring apparently 

 from the cortex of the main stem. These are chiefly due to 

 the vigorous development of small weak dwarf shoots, which 

 originated at the base of the branches in their first year, and 

 which have become occluded during the increasing thickness 

 of the stem. I have not been able to prove that true adventitious 

 buds are formed in the case of this tree. 



If in pruning green branches one leaves the stump of a 

 branch (snag) without any foliage, the same state of things 

 occurs as when branches are suppressed naturally. The 

 snag dies, except for an inch or two at the base, and the 

 formation of callus is either rendered impossible or is so much 

 interfered with and delayed that the dead stump has time to 

 become completely rotten. If the bark is removed from the 

 snag, the conditions are rendered more favourable for the forma- 

 tion of callus, and a covering will more easily grow over the 

 snag from the base than is possible when the dead and dry 

 cortex remains in situ on the stump. In Fig. 146 I have 

 represented the progress of the formation of callus on a thick 

 snag, where for clearness the bark has been mostly removed. 

 The bark of the dead snag presses firmly on the wood, and the 

 formation of the new growth (a, b\ which already covers more 

 than half the stump, has been rendered possible only by its 

 pushing in like a wedge and separating the dead cortex from the 

 dead wood, so that the thin and primarily non-vascular edge of 

 the living tissues has been enabled to grow into the space that 

 has thus been formed. The familiar curled growths on the 

 stumps of branches are formed when the new tissues advance 

 unequally, as is most frequently the case when they are growing 

 over an irregularly fractured surface (Fig. 146, x x, in the upper 

 part). 



As a dead snag interferes with occlusion, the general rule in 

 pruning is to cut as close as possible and to make the cut 

 parallel to the stem. If this is attended to, a callus is formed in 

 the way already described, its formation proceeding most rapidly 

 from the lateral edges of the wound. For obvious reasons the 

 bark is there most easily raised, much more easily, in fact, than 



