Jan., 1901] Schaffner-Tyler — Self-Pruning of Trees. 
31 
Among the plants which come in the first class, the most 
striking perhaps, is Populus alba, in which very large branches are 
frequently cut off in such a perfect manner that one might think the 
pruning had been done with a sharp knife. Green twigs from one to 
fifteen years old were found to be shed and no doubt still older ones 
are cut off. The twigs have well-developed winter buds and this is 
also the case in other species of Populus and in certain species of 
Quercus and Primus. In Quercus alba shed twigs were found from 
one to four years old while in Q. acuminata there were some seven 
years of age. In Prunus serotina twigs from one to six years old 
were cut off. In Ulmus Americana not only are joints formed at 
the base of the twigs, but the twigs also break apart at the nodes, 
caused by annual growth. 
The following species were found belonging to the first class: 
Populus alba L. 
“ deltoides Marsh. 
“ grandidentata Mx. 
“ tremuloides Mx. 
“ dilatata Ait. 
Quercus alba L. 
“ robur L. 
“ macrocarpa Mx. 
“ acuminata (Mx.) Sarg. 
Ulmus americana, L. 
Prunus serotina, Ehrh. 
Tamarix gallica L. 
Tsuga canadensis (L.) Carr. 
Ali the plants found with brittle zones belonged to the willows. 
It is interesting to note that the branches shed may be one to 
several years of age and that certain branches do not develop a 
brittle zone. Salix nigra and S. amygdaloides seem to show the 
character most perfectly and it is remarkable to see how readily the 
branches drop off. 
The following species show the adaptation: 
Salix nigra Marsh. 
“ amygdaloides anders. 
’ “ fragilis L. 
. “ alba vitellina (L.) Koch. 
“ babylonica L. 
Among those which come in the third class, the most remark- 
able plant observed was Ampelopsis cordata. About the time that 
the leaves are §hed nearly all the slender green branches literally 
fall to pieces and drop off. Most of the fruit is on these branches 
