368 SENESCENCE AND REJUVENESCENCE 



in the gametes alone. Apparently the sporophyte may arise from 

 any vegetative cell of the gametophyte. But the fact that apoga- 

 mous development of a sporophyte often begins as a transformation 

 of the sex organs either antheridia or archegonia, or is correlated 

 with the incomplete development or degeneration of the sex organs 

 or of the eggs, suggests that some sort and some degree of phys 

 iological correlation exists between apogamy and formation ol 

 gametes. It seems not improbable that the degree of individuation 

 is in such cases not quite sufficient to carry the organism through 

 the entire cycle, and the physiological isolation of vegetative cells 

 in the stages near maturity leads to reproduction of a sporophyte, 

 i.e., the vegetative cells have the same developmental capacity as 

 the egg, but are less specialized and so do not require fertilization. 

 Up to the present, however, other aspects of the process of apog- 

 amy have received much more attention than its physiology and 

 relation to the individuation of the organism in which it appears, 

 and any attempt at physiological interpretation must at present 

 be a mere guess. 



CONDITIONS OF GAMETE FORMATION IN THE SEED PLANTS 



In the mosses, ferns, and related forms the two generations, the 

 asexual sporophyte and the gametophyte which produces sexual 

 organs and gametes, are more or less distinct and separate organisms 

 with different morphological structure and different habit. In the 

 seed plants, however, the sporophyte generation has become by far 

 the most conspicuous feature of the life cycle, and the gametophyte 

 generation is reduced to the pollen grain and the embryo sac of the 

 flower. The flower is commonly defined as an axis or shoot of 

 which some parts bear sexual organs. The flower, like the vegeta- 

 tive shoots, arises from an agamic bud, but this bud is evidently 

 more highly specialized than the vegetative buds, for its parts are 

 variously modified and differentiated in various directions into the 

 parts of the flower. Moreover, the axis which produces the flower 

 usually does not continue to grow for a long time, or indefinitely, 

 but the growth is narrowly limited and the development of the 

 flower ends under the usual conditions in death. Evidently the 

 flower represents the most advanced or the highest stage in the 



