253 



yield lumber that is much freer from knots than that cut from 

 trees growing in the open, where the lower branches have per- 

 sisted, but natural pruning appears to be . of little advantage in 

 _ the economy of the plant. 



Many trees, however, prune themselves. This phenomenon, 

 though described by Foerst * in 1893, and more fully by 

 Schaffner and Tyler, in the " Ohio Naturalist" for 1901, appears 

 to be not widely known. This self-pruning is distinguished from 

 natural pruning in that the tree itself is throughout the active 

 agent in the process. The branch dies as the result of the 

 pruning, rather than being pruned after it is dead. In some 



s ther 



t the 



layer, like that formed at the base of petioles in leaf-fall, and 

 thus the branch is severed from the tree. 



A striking demonstration of self-pruning may now be had 

 along the walks leading up to the museum building. The 

 sapling poplars (Populus deltoides), that alternate with the tulip- 

 trees on either side of the walk, are vigorously pruning them- 



general attention. Most of the pruned branches are two years 

 old, though some are older and some younger. They bear 



well-for: 



buds, 



leaves still remain on the branch. 



Self-pruning, in some families, is accomplished in other ^ 

 than the one mentioned above. In the willow, for exar 

 instead of the abscission-layer, a brittle zone is formed at the 

 of the branch. Out of about twenty-five genera where 

 pruning occurs, Catalpa, Ailanthus, horse-chestnut, elm, lilac 

 mulberry, may be mentioned as illustrative examples in 



The significance of self-pruning seems to be quite analo ; 

 to that of artificial pruning, viz., to get rid of superfluous . 

 weak branches. It does not seem to be a means of veget; 

 propagation, for in most cases the branches cut off either do 

 take root, or do not fall into conditions suitable for that. £ 

 trees, as, for example, the red and the 



267. 



i»93- 



