388 BIOLOGY: GENERAL AND MEDICAL 



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- 



Fio. 141. Budding leaf of Bryophyllum. (From Bergen and Davis' "Prin- 

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



ever, followed by the formation of more elongated 

 cell forms. 



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

 that regeneration in plants and animals shows the great- 

 est difference, for in the plant no regeneration of this 

 kind takes place. The dissimilarities between animals 

 and plants in the matter of growth and development 

 throw some light upon the subject, for nearly all plants 

 grow continuously while most animals reach maturity 

 and subsequently cease active growth. Further, in 

 animals the germinal matter is stored up in the gonads 

 from which it is liberated under special circumstances, 

 while in vegetables the germinal matter seems to be 

 widely distributed throughout the structure and merely 

 concentrated at the flowers and at the buds. 



