DEATH AND SENILE DECAY 223 



long as the conditions are favourable, without any weakening or death of 

 individual cells occurring. This is, however, only possible for a short 

 time, since the consumption of the food and the accumulation of excrete 

 products create external conditions which soon retard growth and 

 ultimately produce death. 



It is very probable that the embryonic cells of an asomatophyte 

 would ultimately die under otherwise optimal conditions, if growth was 

 mechanically prevented, so that division and rejuvenescence no longer 

 occurred. No certain proof has, however, yet been attained, and the fact 

 that cells of Spirogyra, Saccharomyces, and also the growing apices and 

 cambium of somatophytes remain living in plaster-casts says nothing, for 

 these experiments extended only over a few weeks, months, or years, 

 whereas the somatic cells in many trees with soft wood may live for as 

 long as a hundred years *. 



Evidence of the limited life of embryonic cells in the absence of growth 

 and division is afforded by the fact that the dormant buds and resting 

 cambium of trees ultimately die, though often only after a long period of 

 time 2 , and that seeds and spores lose the power of germination in from 

 a few to many weeks, months, or years. When yeast-cells and bacteria 

 slowly die in nutrient solutions which permit a certain amount of metabo- 

 lism but no growth 3 , it is possible that this result is due to the direct 

 action of the external conditions, which when unfavourable may produce 

 death in embryonic as well as in somatic cells. 



The maintenance of the species is only possible by the growth and 

 rejuvenescence of embryonic cells. In asomatophytes death is produced 

 only by external conditions, which may be due either to the organism itself, 

 or to some other agency. A portion of the embryonic cells of somato- 

 phytes, however, always follow a line of development and differentiation 

 leading automatically to death, and hence it arises that the differentiated 

 organs of such plants always have a limited duration. Under natural 

 conditions only a limited number of the progeny of an asomatophyte 

 survive, so that in this case also the species is maintained by a portion of 

 the embryonic cells 4 . 



1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, pp. 240, 355 ; Nevvcombe, Botanical Gazette, 1894, 

 Vol. xix, p. 232. 



2 Hartig (Lehrb. d. Anatom. u. Physiol., 1891, p. 272) states that non-growing cambium may 

 remain living for dozens of years. 



3 Hansen (Meddelelser fra Carlsberg Laboratoriet, 1899, Bd. IV, p. 109) finds that some 

 species of yeast may live for seventeen years in a solution of saccharose which permits slow growth, 

 while vinegar bacteria may live for one to six years in beer (Hansen, 1. c., 1894, Bd. ill, p. 210). 

 Cf. also Bolley, Centralbl. f. Bact, 1900, 2. Abth., Bd. vi, p. 33. 



* The continuity of the germ-plasma is a primary condition for the maintenance of the species, 

 and hence it is immaterial whether one terms the species immortal, or applies the term only to 

 asomatophytes (Weismann, Leben und Tod, 1884). A species can only maintain itself under 



