CONDITIONS OF GAMETE FORMATION 365 



than for spore formation. According to Klebs, these differences 

 in concentration concern primarily the nutritive substances, but 

 it seems probable that the protoplasm of the cells may also be 

 involved. I have endeavored to show that vegetative reproduction, 

 in consequence of the regressive changes associated with it, retards 

 or inhibits the progress of senescence (pp. 237-55). The conditions 

 which bring about the formation of gametes in Klebs's experiments 

 decrease or check vegetative growth, and the cells of the plant 

 accumulate organic substance and so attain a condition of greater 

 physiological age with a lower rate of metabolism than during active 

 vegetative reproduction. Apparently spore formation occurs at an 

 earher, and gamete formation at a later, stage of this process of 

 senescence. 



In the algae and fungi, with their low degree of individuation, 

 certain parts of the plant may under certain conditions become old 

 while others remain young, and in such cases gamete formation 

 and vegetative reproduction may occur simultaneously, the one in 

 the older, the other in the younger, parts. The results of Klebs's 

 experiments do not then indicate that the plant has no definite life 

 history, but merely that because of its capacity for vegetative 

 reproduction it can be prevented indefinitely from attaining the 

 later stages. But when it, or a part of it, attains these stages, the 

 more specialized reproductive processes appear, and the formation 

 of gametes is apparently characteristic of a more advanced stage 

 than spore formation. Even after gamete formation, however, 

 the plant does not necessarily die, but under the proper conditions 

 may resume vegetative reproduction or spore formation. In those 

 cases where gametic reproduction may be induced before vegetative 

 reproduction has continued for any considerable length of time it is 

 probable that the conditions bring about premature aging, and the 

 plant very soon attains a certain physiological state which under 

 other conditions may arise only after a long time or not at all. 



The process of aging in these lower plants is then very inti- 

 mately associated with external conditions. Under certain con- 

 ditions progressive senescence and gamete formation, under others 

 a balance between senescence and rejuvenescence, with continuous 

 vegetative reproduction, may occur. A Ufe cycle exists as a 



