244 SENESCENCE AND REJUVENESCENCE 



of a plant which show a low degree of individuation, and conse- 

 quently frequent vegetative reproduction, are capable of continu- 

 ing their vegetative activity for a long time or even indefinitely, 

 without indications of senescence of the meristematic tissues, while 

 the length of life is usually more or less definitely Umited in plants 

 or phytoids with infrequent or no vegetative reproduction. In 

 various plants with subterranean stems, such as the flags and 

 rushes, for example, the stem shows repeated vegetative reproduc- 

 tion, giving rise to the aerial shoots, and its length of hfe is appar- 

 ently unhmited. The aerial shoots, however, show a much higher 

 degree of individuation with Kttle or no vegetative reproduction, 

 and their length of life is short. 



In spite of the occurrence of nuclear and cell division and 

 vegetative reproduction, however, the vegetative tips and other 

 meristematic tissues of many plants show indications of progres- 

 sive changes. The shoots produced from later buds may differ 

 in character from those of earlier origin, the later leaves often differ 

 in form and structure from the earher and the rate of growth may 

 decrease until growth finally ceases (Diels, '06; H. M. Benedict, 

 '12, '15). The relations between vegetative growth and reproduction 

 and the formation of tubers, bulbs, bulbiUi, and other individuals or 

 parts which contain nutritive reserves also indicate the occurrence 

 of progressive changes, of a real life history, although Vochting 

 ('87, '00) and others have shown that the formation of reserve- 

 bearing structures, like other features of the Hfe history, can be 

 controlled experimentally by retarding or accelerating the pro- 

 gressive development of the plant with the aid of external con- 

 ditions. There is much evidence in favor of the view that the 

 change from vegetative reproduction to flowering is connected with 

 advancing senescence and specialization of the meristematic tissues 

 of the plant (see chap. xiv). 



The conclusion to which the various lines of evidence point is 

 that senescence is a characteristic feature of the vegetative life of 

 plants, but that it is not an uninterrupted, continuous process. 

 The low degree of individuation in plants determines a high fre- 

 quency of agamic reproduction, which brings about a greater or less 

 degree of rejuvenescence and so balances more or less completely 



