Regeneration 231 



bean plant, for example, if the epicotyl is decapitated the buds in the 

 axils of the cotyledons will grow into shoots, thus replacing the leafy 

 shoot that would have grown from the epicotyl; but these buds may also 

 be induced to grow by chilling a portion of the epicotyledonary stem and 

 thus preventing the interchange of stimuli or substances between it and 

 the tissues below ( Child and Bellamy, 1919 ) . 



In general, the more simple and undifferentiated a plant is, the more 

 completely will it restore missing parts; and the more specialized and 

 differentiated it is, the less regenerative capacity it will show. Early devel- 

 opmental stages are thus more likely to regenerate readily than later ones, 

 and groups lower in the phylogenetic scale than those of higher position. 

 Ability to regenerate is often completely lost. 



The origin of regenerative powers in plants and animals is sometimes 

 explained as the result of natural selection, much as in the case of other 

 traits. That a long process of competition and selection conferred the ability 

 to repair almost every type of injury seems unlikely. However this may 

 be, the developmental activities in regeneration are not essentially dif- 

 ferent in their origin and control from those which occur in normal de- 

 velopment. In both, the production of a single, complete individual is the 

 final result. The factors involved in regeneration seem neither more nor 

 less difficult to understand than those in normal ontogeny, nor do they 

 require a fundamentally different explanation. Both are manifestations of 

 general developmental control, a fundamental self-regulation in the indi- 

 vidual. Both seem to be the result of the same formative process. 



Begeneration is a term that has been variously defined. The author pro- 

 poses to use it here in the broadest sense, as the tendency shown by a 

 developing organism to restore any part of it which has been removed or 

 physiologically isolated and thus to produce a complete whole. This 

 covers processes from wound-healing to the reproduction of adventive 

 structures and vegetative multiplication, in which many different activ- 

 ities are involved. 



In general, the method of restoration of lost parts is different in plants 

 and in animals. Animal tissues are composed of thin-walled cells, usually 

 able to divide and often to migrate. Most plant cells at maturity are 

 relatively thick-walled and ordinarily do not divide or grow further, 

 though it has been shown that many retain the power to do so. Begenera- 

 tion in animals, therefore, consists largely in a reorganization of the re- 

 maining portion of the organ or body. In plants, on the contrary, this type 

 of regeneration is limited to meristematic or rapidly growing regions or 

 to the relatively rare cases where cells become embryonic again. Much 

 more commonly, at least in the higher types, replacement of lost parts 

 results from the growth of dormant buds or primordia or the development 

 of new ones. Such primordia arise from cells in the plant body which 



