CHAPTER 9 



Regeneration 



In the preceding chapters there have been considered those morpho- 

 genetic phenomena which manifest themselves in normal development. 

 Polarity, symmetry, differentiation, and the wide variety of correlative 

 manifestations evident as the plant and its parts progress from embryo to 

 maturity are all indications of the orderly control of growth processes 

 which is the visible aspect of biological organization. But the progress of 

 development is not always unimpeded. Accident and injuries of various 

 sorts may happen to the growing plant which remove a part of its tissues 

 or divide it into two or more incomplete portions. In nature this may 

 result from the attacks of fungi, insects, or higher animals; from mechani- 

 cal injuries of many sorts; or from unfavorable conditions which impede 

 the functions of its organs. The changes that follow such injuries, losses, 

 or functional disturbances often throw much light on morphogenetic 

 activities, and one of the most fruitful methods of studying developmental 

 processes has been to observe the consequences which follow their ex- 

 perimental disturbance. Indeed, this is the only way at present by which 

 many of these processes can be investigated at all. 



This field of morphogenetic research is not as active today as it was in 

 earlier years, and many of the most important papers in it go back to 

 some decades ago. For a review of the earlier literature the student is 

 referred to McCallum (1905), Kupfer (1907), and Goebel (1908). Ban- 

 ning ( 1955 ) has discussed some of the recent work. 



An important fact which such studies reveal is that the organism shows 

 a tendency to restore or replace parts that have been removed and thus 

 to produce again a complete individual. To this general process, which 

 includes a wide range of developmental phenomena, the term regenera- 

 tion is commonly applied. Regenerative activities are much more common 

 in plants, with their less highly organized bodies, than in animals and can 

 often be subjected to a more complete developmental analysis. 



Regeneration can be brought about not only by the removal of a part 

 but by isolating it physiologically from the rest of the plant. In a young 



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