AGE CYCLE IN PLANTS AND LOWER ANIMALS 245 



the progressive changes. Undoubtedly also the character of 

 metabolism determines a more rapid senescence with less 

 capacity for regression in some plants than in others, and the 

 work of Klebs and many other investigators on the effect of 

 nutritive and other external conditions indicates that these 

 also influence the rate of senescence and the character of 

 differentiation. 



The process of differentiation of the plant cell is apparently not 

 fundamentally different from that of the animal cell. It consists 

 in the development of relatively stable structural features, the depo- 

 sition of relatively inactive substances in the cytoplasm or on its 

 surface, in many cases substances, such as starch, w^hich may serve 

 as nutrition under other conditions. The accumulation of fluid in 

 vacuoles is also a very characteristic feature of differentiation in 

 plant cells. In general here, as in animals, the process of differen- 

 tiation involves a decrease in the proportion of the chemically 

 active "undifferentiated" protoplasm. 



THE OCCURRENCE OF DEDIFFERENTIATION AND REJUVENESCENCE 



IN PLANT CELLS 



It was pointed out above that the formation of a new vegeta- 

 tive tip by the embryonic tissue of the plant must involve a new 

 individuation and some slight degree of physiological rejuvenes- 

 cence. But the occurrence of dedifferentiation, even among the 

 higher plants, is not limited to such changes as this. Cells which 

 have clearly lost their embry6nic character and have undergone 

 more or less morphological as well as physiological change have 

 been repeatedly observed to undergo dedifferentiation and become 

 embryonic, both in appearance and in behavior. E.xperiment has 

 demonstrated again and again that among the lower plants every 

 cell, or almost every cell, of the plant body may be capable of giving 

 rise to a new individual with all the capacities of the individual 

 which develops from the egg. Among the liverworts and in many 

 of the ferns the cells of the prothallium very generally retain the 

 capacity to give rise to new individuals, either when physically 

 isolated by section, or when physiologically isolated by growth of 

 the prothallium, removal of the growing tip. or other conditions, 



