Fig. 1—Twig of Populus alba, showing large basal joint and scars where smaller twigs have been detached.

Fig. 2—View of basal joint of same twig as Fig. 1.

Fig. 3—Twig of Salix nigra, showing position of the brittle zone (a).

Fig. 4—Green twig of Ampelopsis cordata, showing joints (a) at the leaf nodes.

The self-pruning of twigs from woody stems, so far as our observations go at present, is accomplished in three general ways: 1st, by the formation of a single joint close to the parent branch; 2nd, by the formation of a brittle zone near the base of the limbs which are to be shed; 3rd, by a series of transverse joints corresponding to the leaf nodes.

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 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:

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.