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 siiecies of Populus and in certain species of 

 Quercus and Prunus. 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. 



All 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 shed nearly all the slender green branches literally 

 fall to pieces and drop off. Most of the fruit is on these branches 



