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: ` 
(1) 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 nuclei 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 
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