158 THE PI.ANT WORLD 



Our Teachers' Department. 



Edited by Professor Francis E. IvI^oyd, 



Teachers College, Columbia University, New York City. 



THE SHEDDING OF BRANCHES AND LEAVES. 



The Central American rubber tree, Castilla, presents an unusual 

 example of self-pruning. The young tree has long, gracefully-curved 

 branches, which, as they do not persist, may be called temporary or de- 

 ciduous. These arise in the usual fashion in the axils of the leaves, and 

 are loosened and at length released by the softening of a special layer of 

 tissue at their bases. There is left a shallow, conical scar-pit, which 

 is soon covered by the bark and becomes indistinguishable. All the 

 branches which are produced for three or four years are thus got rid of. 

 At the end of this period permanent branches are developed. Such a 

 branch arises as an adventitious bud found near the side of the base of a 

 temporary branch. " On young trees it is very easy to distinguish tem- 

 porary from permanent branches, from the fact that the latter are directed 

 obliquely upward at an angle of about 45 degrees, while the temporary 

 branch near which it arises is almost or quite horizontal." * It appears 

 also that these permanent branches may exceptionally be shed. 



This behavior of the rubber tree in question is, of course, only a 

 special case of a very general phenomenon. It is generally understood 

 that the fall of the leaf in the autumn is brought about by the formation 

 of a separation-layer of tissue which allows the easy removal of the leaf 

 without exposing the inner tissue as a wound. In compound leaves, each 

 leaflet is provided with its own separation-layer, and the leaf therefore is 

 not cast off as a whole. It is interesting to recall that the Boston Ivy 

 has two kinds of leaves, unifoliate and trifoliate. The former are pro- 

 duced on younger shoots, while the latter are found in seedlings and on 

 the older parts of the plant. Whether, however, there is a single blade, 

 or there are three leaflets, each has its own separation-layer, so that we 

 find here an apparent exception to the general rule that single leaves fall 

 as a whole, petiole and all. If compound leaves were absent, it would fairly 

 be argued that the Boston Ivy has been derived from a form with com- 

 pound leaves, on the evidence of the persistence of the habit of separating 

 the terminal leaflet. 



But the special interest attaching to this plant is the example it affords 

 of a habit analogous to that of Castilla. In the autumn a transverse 

 separation-layer is formed at each node of the terminal portions of the 

 stems which have not, on the approach of cold weather, lost their herba- 



*Cook, O. F. " The Culture of the Central American Rubber Tree." U. S. Dept. Agric, Bur. Plant 

 Ind., Bull. 49. October, 1903. 



