246 SENESCENCE AND REJUVENESCENCE 



and this change in behavior in all cases undoubtedly involves a 

 greater or less degree of rejuvenescence. 



Even in the seed plants new growing tips which are capable of 

 developing into new, complete plants and producing sex cells often 

 arise from cells which have undergone visible differentiation. In 

 Begonia, for example, the formation of so-called adventitious buds 

 from epithelial cells of the leaf has been observed, and in many 

 other plants new individuals develop from cells which are far from 

 being embryonic. The cells concerned in such cases lose their 

 differentiated character and return to the embryonic condition, 

 resume growth and division, and enter upon a new developmental 

 cycle. The formation of meristematic, or embryonic, tissue from 

 the parenchyma of the leaf petiole and from other differentiated 

 tissues has also often been noted. The transformation of inflores- 

 cence into vegetative shoots has been experimentally induced by 

 Klebs and others in various plants, and its occurrence in nature 

 has been repeatedly observed. One case described by Winkler 

 ('02) in a species of Chrysanthemum deserves brief mention. In the 

 disk flowers the style formed the stem and the stigma gave rise 

 to two leaves like normal upper leaves of the species. The 

 embryo sac developed and the pollen was capable of germination, 

 but the embryo died at an early stage. The corolla became green, 

 the vascular system increased and branched, and stomata appeared. 

 In this case the flower evidently underwent a partial transformation 

 into a vegetative structure, and this change must have involved 

 some considerable degree of dedifferentiation and rejuvenescence. 



In fact, the occurrence of dedifferentiation among plants has 

 been demonstrated beyond question. 1 Certainly in the plant, as 

 in the animal, senescence is associated with specialization and 

 differentiation of cells, and it is just as certain that dedifferentiation 

 is accompanied by rejuvenescence. Moreover, the increased activ- 

 ity in growth and division of the cells concerned, as well as their 



1 The following references will serve as an introduction to the extensive bibli- 

 ography of the subject: Brefeld, '76, '77; Burns and Heddon, '06; von Faber, '08; 

 Goebel, '08, pp. 141-65; Heim, '96; Hildebrand, '10; Jost, '08, Vorlesung 26; Klebs, 

 '03, '060, 'o6&; Kohler, '07; Kreh, '09; Magnus, '06; Miehe, '05; Noll, '03; Regel, 

 '76; Riehm, '05; Schostakewitsch, '94; Tobler, '02, '04; Vochting, '85; Winkler, 



'02, '07. 



