Dec, 1902.] Observations on Self-Pruning. 327 



OBSERVATIONS ON SELF=PRUNING AND THE 

 FORMATION OF CLEAVAGE PLANES. 



John H. Schaffne;r, 



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 stich 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 tho.se b}' 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 geophilotis 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 

 Avould make an interesting though difficult subject for research. 



The process of self-prtming 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 

 ntnnber of species were added to my former lists, including 

 members of such widely .separated families as M^-ricaceae, Acer- 

 aceae and Vacciniaceae. A number of trees were found to prtuie 

 abtuidantly 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 dtiring 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 rubrtun L. also self -prunes in the spring 

 and shed twigs from one to ten jxars old were collected on May 

 12th. No evidence of self-prtming was discovered in Acer sac- 

 charum Marsh, or Acer negundo L. Ulmtis americana L. also 

 sheds twigs in the spring, althottgh autumn is the more tisual 

 time for this tree to self-prune. On Ma\' i6th a large tree was 



