AGE CYCLE IN PLANTS AND LOWER ANIMALS 251 



there is no question either of individuation or of possible age changes 

 involved, the reproductive changes being due primarily to the 

 action of the external conditions. 



As a matter of fact, however, these data when considered in 

 their proper relations to individuation and the age cycle are readily 

 interpreted and are directly in line with what we know of other 

 forms. Spore formation is, at least in most cases, a more specialized 

 reproductive process than vegetative reproduction, and therefore 

 might be expected to occur in later stages of development than the 

 latter. Moreover, since spore formation usually consists in the dis- 

 integration into single, independent cells of a parent body or part 

 already formed rather than in the growth of a new individual or 

 part, we should expect it to occur when the rate of metabolism in 

 the plant is low as compared with the rate in vegetative growth and 

 reproduction. Such a low rate of metabolism may result, either 

 from aging of the individual or part, or from the action of external 

 conditions. If the conclusions reached from the study of the lower 

 animals are applicable to the algae and fungi, and the facts seem 

 to indicate that they are, the plant under certain conditions may 

 remain indefinitely in the vegetative condition, because the differ- 

 entiation and senescence in each individual is balanced by the 

 rejuvenescence occurring in each vegetative reproduction. Under 

 other conditions senescence may overbalance rejuvenescence, and 

 the plant individuals undergo progressive development and senes- 

 cence, their rate of metabolism undoubtedly decreases, and sooner 

 or later the disintegration of the plant or parts of it into spores 

 occurs. The results of Klebs's experiments indicate that this con- 

 dition may be induced in the plant either by lowering the rate of 

 metabolism directly by low temperature, lack of oxygen, lack of 

 nutritive salts, etc., or by loading the cells with organic material. 

 While it is impossible from the data at hand to furnish a complete 

 demonstration, it appears highly probable that the effect of the 

 various conditions used by Klebs in his experiments in inducing 

 spore formation is either to bring about a natural senescence in the 

 plant by the accumulation of inactive substances or to decrease its 

 rate of metabolism so that a physiological condition like that 

 attained in natural senescence is brought about. In short, by 



