STUDIES ON THE PLANT CELL. VI. 



BRADLEY MOORE DAVIS. 



SECTION V. CELL ACTIVITIES AT CRITICAL PERIODS OF 

 ONTOGENY IN PLANTS. 



WE shall discuss in this paper the behavior of the protoplasm 

 at a number of critical periods in the life history of plants when 

 the organism passes from one phase to another of a fundamen- 

 tally different character. At such times great changes take 

 place in the potentialities of the cells which inaugurate the new 

 developments, changes that are generally most conspicuously 

 shown in the structure of the nucleus. Some of the most inter- 

 esting events of cell and nuclear history take place at these 

 times, as would be expected from the importance of the phe- 

 nomena. We shall treat the material under the following heads : 

 (i) Gametogenesis, (2) Fertilization, (3) Sporogenesis, (4) Re- 

 duction of the Chromosomes, (5) Apogamy, (6) Apospory, (7) 

 Hybridization, (8) Xenia. 



i . GAMETOGENESIS. 



The events of gametogenesis are clearly known for the higher 

 plants but there is some confusion and almost no detailed infor- 

 mation in the accounts of the thallophytes where the nuclei are 

 very small and the details of the mitoses preceding the forma- 

 tion of sexual cells exceedingly difficult of study. 



There is complete agreement among all investigators that the 

 mitoses which precede the differentiation of gamete riuclei in 

 spermatophytes, pteridophytes, and bryophytes are typical karyo- 

 kinetic figures not differing essentially in the behavior of the 

 chromosomes from the mitoses generally characteristic of the 

 gametophyte generation. This information is based upon a 



