252 SENESCENCE AND REJUVENESCENCE 



controlling the rate of vegetative reproduction, the rate of metab- 

 olism, or the accumulation of inactive material, it is possible 

 to determine whether the plant or the phytoid shall remain indeti- 

 nitely in the vegetative stage and physiologically young, or whether 

 it shall attain the physiological condition characteristic of an older 

 plant and produce spores. There are, however, certain cases which 

 apparently cannot be accounted for in this way: for example, 

 Klebs finds that when the alga Oedogonium is cultivated at a low 

 temperature a rise of a few degrees induces spore formation, but 

 when cultivated at a higher temperature a rise in temperature has 

 no such effect. As regards this case, it is probable that the degree 

 of individuation at the low temperature is so slight that when an 

 increase in metabolic activity occurs with a rise in temperature the 

 cells become independent before their activity is subordinated or 

 controlled by the increased degree of individuation. 



To sum up, there is good reason to believe that algae and fungi 

 may undergo senescence and rejuvenescence like the lower animals, 

 and that the different forms of reproduction are characteristic of 

 different stages in the life cycle. But since reproduction and 

 consequently rejuvenescence are characteristic of younger as well 

 as older stages, it is possible to control and modify the course of 

 the life history in a great variety of ways. This possibility of con- 

 trol does not prove that these plants have no definite life cycle: 

 it indicates merely that progressive and regressive development 

 can be determined experimentally in the plant, as in the animal. 



As regards the spores themselves, there can be no doubt that 

 extensive reconstitution and rejuvenescence occur in their forma- 

 tion. In the case of the motile zoospores characteristic of many 

 forms (Figs. 104, 105, 106), this is conspicuously the case, for the 

 zoospore is a free-living, unicellular organism and bears Httle resem- 

 blance to the plant from which it arises. This new individuation 

 of the zoospore from the physiologically old vegetative stage 

 involves reconstitutional changes which result in a simpler and 

 more primitive kind of individual than the vegetative form. This 

 change must be associated in some way with the change from the 

 multicellular or multinucleate to the unicellular or uninucleate 

 condition. In the development of the vegetative form from the 



