418 BIOLOGY: GENERAL AND MEDICAL 



massive development of what is called callus which 

 gradually extends over the denuded surface until it is 

 once more entirely covered. As the callus grows, the 

 cells become suberized and a cork-forming phellogen 

 arises in the periphery. In the stems of gymnosperms 

 and dicotyledons, the seat of injury is gradually sur- 

 rounded and covered by a layer of tissue arising from 

 the exposed cambium layer. While the callus is gradu- 

 ally spreading over the wounded surface, an outer pro- 

 tective covering of cork is formed, at the same time that 

 a new cambium is forming within the callus, through 

 differentiation of the inner layer of cells continuous with 



FIG. 145. Budding leaf of Bryophyllum. (From Bergen and Davis' "Prin- 

 ciples of Botany." Ginn & Co., publishers.)' 



the cambium of the stem. When the margins of the 

 growing callus meet and close over the wound, the 

 edges of the new cambium also unite and form a complete 

 cambial layer continuous with the stem and covering 

 the entire seat of injury with a new and complete cam- 

 bium. The new wood formed by this new cambium 

 never becomes continuous or coalescent with the old 

 wood, and marks that cut deeply enough into the stem 

 to penetrate the wood are merely covered by new wood 

 and may be found within the stem. The ends of sev- 

 ered branches may similarly become so completely 

 covered as to be concealed from view. The callus wood 

 differs in certain particulars from the normal wood, con- 

 sisting at first of isodiametrical cells which are, how- 

 ever, followed by the formation of more elongated 

 cell forms. 



2. The Restoration of Lost Parts. It is at this point 



