Dec., 1902.] 
Observations on Self-Pruning . 
3 2 7 
OBSERVATIONS ON SELF-PRUNING AND THE 
FORMATION OF CLEAVAGE PLANES. 
John H. Schaffner. 
Plants form separation layers and brittle zones for a variety of 
purposes, and the process is one of great importance in the life 
of many species, especially in those of a woody nature. The 
most familiar example is the annual phenomenon of the shedding 
of leaves and leaflets in our deciduous trees and shrubs. By this 
means the plant gets rid of useless members or of such as would 
be injured by long periods of dry or cold weather. 
When fruit or seed is developed there is again the necessity 
for some means by which these bodies may be separated from the 
parent. In many cases, both in perennials and annuals, the 
parts are pruned off by the formation of cleavage planes. 
The most interesting examples of the development of separa- 
tion layers and brittle zones are those by which ordinary buds, 
twigs and branches are cut off or self-pruned. This may be an 
adaptation for getting rid of leaves which do not themselves have 
the deciduous habit ; for vegetative propagation ; for the cutting 
off of herbaceous stems near the ground, as in certain perennial 
tumble-weeds and other geophilous plants ; or it may be a pro- 
cess whose primary object is simply to rid the plant of surplus 
branches, thus preventing decay and allowing for the production 
of more new twigs and leaves the following season. 
Whether separation layers are ever produced in roots or under- 
ground stems is not known to the writer. But it seems that this 
would make an interesting though difficult subject for research. 
The process of self-pruning has been studied by the writer for 
several years, and during the past summer further observations 
were made to supplement previous work on this subject. A 
number of species were added to my former lists, including 
members of such widely separated families as Myricaceae, Acer- 
aceae and Vaceiniaceae. A number of trees were found to prune 
abundantly in the spring. The soft maple ( Acer saccharinum L. ) 
was self-pruning abundantly from the first of May to the first of 
July. From some trees hundreds and even thousands of twigs 
from one to ten years old were shed during this period. The 
formation of the joint and cleavage plane is much the same as in 
the poplars, and most of the twigs had leaf buds with partly 
developed leaves. Acer rubrum L- also self-prunes in the spring 
and shed twigs from one to ten years old were collected on May 
1 2th. No evidence of self-pruning was discovered in Acer sac- 
eharuin Marsh, or Acer negundo L. Ulmus amerieana L. also 
sheds twigs in the spring, although autumn is the more usual 
time for this tree to self-prune. On May 16th a large tree was 
