MUTILATION AND REGENERATION 387 



chambers and valves and when the somatic life is main- 

 tained solely through the circulating blood, the heart 

 cannot be dispensed with at all. 



REGENERATION IN PLANTS. 



This subject is best considered under two separate 

 headings: 1. The repair of damage; 2. The restoration 

 of lost parts. 



1. The repair of damage done to plants is effected 

 through changes in the cells injured but not destroyed. 

 The destroyed cells die, become brown and dry, and 

 drop off. The walls of the underlying cells then become 

 lignified or wooden and the more delicate cells below 

 thus protected. Such changes at the cut edge of a leaf 

 protect the remainder, which lives on in its deformed 

 state for a long time. In the case of tubers, as, for ex- 

 ample, potatoes, similar changes take place in the cut 

 surfaces and thus prevent destruction of the buds 

 which remain alive, so that cut fragments of seed potatoes 

 may be kept for several days before planting, the buds 

 remaining vital and beginning to grow when favorable 

 opportunities are afforded. 



The wooden stems of higher plants when super- 

 ficially injured are repaired by an active growth of the 

 living cells round about the seat of injury, forming a 

 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 



