144 MASSACHUSETTS HORTICULTURAL SOCIETY. 



are able to make their way to the more delicate aud succulent cells 

 beneath, a point to which I shall refer later. 



As the cambium constantly increases in circumference and the 

 new wood and inner bark increase correspondinglj- in bulk, it is 

 plain that the epidermis, unless endowed with the power of 

 increasing in circumference, must soon be n.ptured, thus exposing 

 the more delicate cells beneath. The epidermis does not possess 

 this poAver except to a very limited extent, but to avoid the 

 danger which must follow an exposure of the sub-epidermal cells 

 to the air after the rupture of the epidermis, which must inevitably 

 take place earl}^ in the life of a plant, nature makes provision for 

 the transformation of the sub-epidermal cells into a zone of cork 

 cells, which act as a protective sheath after the epidermis proper is 

 ruptured. The way in which cork cells are formed is seen on a 

 small scale when a potato tuber is cut in halves. The wounded 

 cells shrivel and die, but the more or less spherical cells beneath 

 become divided into a series of thinner, flatter cells by the forma- 

 tion of uew cell walls parallel to the cut surface, and the walls 

 themselves become tough and resistent. The epidermis of the 

 stems and branches in reality remains intact but a short time, 

 usually only one 3'ear, and then is ruptured and soon disappears ; 

 but, meanwhile, the sub-epidermal cells, having been changed into 

 a series of cork cells like those mentioned in the cut potato but on 

 a larger scale, form a new protective covering which replaces the 

 epidermis. Furthermore, the new cork layer itself is only to a 

 moderate extent capable of extension, and as the inner parts of 

 the stem continue to increase it is in turn ruptured, and the breaks 

 are closed bj' the formation of a second laj^er of cork cells 

 beneath. This process is repeated indefinitely, so that in stems 

 several years old, we have what is in popular language called the 

 bark, composed of several different layers of cork cells more and 

 more split up and cracked externally. 



If we now recognize the structure of the normal stem or trunk 

 in its essential points, we can next consider the primary effects of 

 wounds. In the first place, whenever a trunk or branch is 

 wounded, no matter whether l)y the action of wind or snow, by 

 the bites of animals, by pruning, or l)y wilful violence of man, 

 nature itself attempts to heal the wound if possible. If the 

 wound is not too great, it heals l)y natural processes, but many 

 wounds are so large or so severe that even in the course of several 



