CHAPTER X 



THE AGE CYCLE IN PLANTS AND THE LOWER ANIMALS 



Any consideration of the age cycle and particularly of rejuve- 

 nescence in plants would be incomplete without reference to a 

 remarkable book. Considerations on the Phenomenon of Rejuvenescence 

 in Nature* by the German botanist Alexander Braun, published in 

 1851. The book is remarkable, not only as a consideration of re- 

 juvenescence, but as one of the pre-Darwinian statements of a 

 theory of evolution. This work became known to me only after 

 I had attained definite conclusions on the basis of experiment, and 

 it has been a matter of very great interest to discover to how great 

 an extent Braun had anticipated in his views the results of experi- 

 ment. He regards reproduction in the broadest sense and pri- 

 marily cell reproduction as the basis of rejuvenescence, describes 

 and discusses dedifferentiation, and recognizes clearly the important 

 fact that the vegetative life of plants is in most cases a series of 

 reproductions. In fact, the conclusions reached in the present 

 chapter are in many respects essentially those of Braun, but modi- 

 fied and brought into relation with modern physiological conceptions 

 and with my own experimental results on the lower animals. 



INDIVIDUATION AND AGAMIC REPRODUCTION IN THE 

 LIFE CYCLE OF PLANTS 



According to the conception of individuation discussed in the 

 preceding chapter, every growing tip, together with the region 

 which it dominates, is in greater or less degree a plant individual. 

 All except the simplest plants therefore are in reality, as botanists 

 have repeatedly pointed out, asexual colonies consisting of a larger 

 or smaller number of individuals which are not completely isolated 

 from each other. In animal colonies such individuals are commonly 

 known as zooids, and for convenience the individuals in a plant 

 colony may be termed phytoids. In most plants there is evidently 

 also some degree of individuation of the colony as a whole, for new 



1 Betrachtungen iiber die Erscheinung der Verjiingung in der Natur. 



237 



