Senile Changes in Leaves of Ceetain Plants 349 



The best review of the whole question is by Mobius (1897), who cites 

 at considerable length the various observations on the conditions of those 

 cultivated plants of to-day which have long been vegetatively propa- 

 gated. His conclusion is that no senile deterioration in such plants has 

 been proved to exist. He maintains that meristematic tissue is true em- 

 bryonic tissue in the same sense as is the tissue in the embryo sac, and 

 that there is no theoretical reason why propagation by cuttings should 

 be different in its results from that by seeds. He believes that it has been 

 shown by various published accounts that the banana, the grape, the date 

 palm, the fig, and the Chinese yam (Dioscorea batatas), have been propa- 

 gated for from two thousand to four thousand years by means of vegeta- 

 tive methods without degeneration. The number of writers which he cites 

 is very large and could be greatly increased, but practically none of these 

 reports can be considered as offering evidence of value. 



It is evident, therefore, that the problem is still unsolved. It must be 

 remembered that the lowest plants known to-day are apparently sexless, 

 and capable of vegetative reproduction only. There can be little doubt 

 of their ability to propagate their kind continuously over long periods 

 of time. Only in those plants and animals which have developed sexuality 

 and sexual reproduction, can the question of possible degeneration through 

 long-continued vegetative propagation be seriously raised. Undoubtedly 

 sexual reproduction efficiently rejuvenates protoplasm. Apparently it is 

 also true that rejuvenescence is accomplished in the primitive sexless 

 plants by an equally efficient method. In those higher plants that have 

 lost their sexuality, it is possible that the loss of sexuality has been ac- 

 companied by the regaining of a more prmiitive method of rejuvenescence. 

 It is in the seed-producing cultivated plants, the varieties of which are 

 vegetatively propagated, that the problem of the relative efficiency of 

 the two methods in rejuvenating protoplasm is presented with the least 

 degree of complication. 



First, therefore, such cultivated plants as the grape and the date should 

 be dealt with, and later the more complicated cases, such as the Dioscorea. 

 The records of the cultivation of the seed-producing fruits are not sufficiently 

 complete to assure beyond question that the varieties which have been 

 so long cultivated have not been replaced at intervals by seedlings. To- 

 day, for example, countless seedlings of Concord grapes are being grown. 

 Some of these develop into plants almost identical in vine and fruit with 



