350 SENESCENCE AND REJUVENESCENCE 



high rate and the increase in rate in the pistil as a whole are asso- 

 ciated with the reproductive processes concerned in the formation 

 of the ovules and embryo sacs within them, processes which involve 

 much more extensive growth than the development of the pollen 

 grain. The volume and weight of these parts in relation to total 

 volume and weight of the pistil increases as development goes on, 

 and this change is undoubtedly sufficient to account for the increase 

 in respiratory rate in the whole pistil in those cases where it occurs. 

 Probably the rate of respiration in the embryo sac decreases as its 

 development proceeds. Fertilization occurs and embryonic devel- 

 opment begins in the seed plants without any considerable period 

 of rest, and this fact may also play a part in determining a high 

 respiratory rate in the pistil during the later stages of its existence. 

 Determinations of the rate of oxygen consumption or production 

 of carbon dioxide or other metabolic products have not been made 

 for different stages of gametic development in animals, but as 

 regards the egg there can be little doubt that the rate of metabolism 

 decreases as development proceeds and that in the fully developed 

 egg very little chemical activity is going on. The male gamete, on 

 the other hand, usually shows very great motor activity, often con- 

 tinuing over a long period of time, and at first glance there may 

 seem to be little reason for regarding it as a physiologically old, 

 highly specialized cell, approaching death. It must be remem- 

 bered, however, that, except in some of the less highly differentiated 

 male cells of the unicellular organisms and the lower plants, the 

 motor activity of the sperm is wholly or in large degree due to 

 external stimulation. In this respect the sperm resembles volun- 

 tary muscle. In both cases the fully differentiated cell or tissue is 

 capable, when stimulated, of a very high rate of reaction, perhaps 

 higher than that in the sperm mother cell or the embryonic muscle 

 cell, but it is certain that the self-determined inherent rate of meta- 

 bolic change without stimulation in the differentiated cell is a much 

 more exact measure of its physiological condition or its stage of 

 senescence as compared with the embryonic cell. In the " resting" 

 muscle and in the motionless spermatozoon the metabolic rate is 

 undoubtedly lower than in the undifferentiated cells from which 

 they arose. 



