432 BOTANICAL GAZETTE [DECEMBER 
sive mitoses with their peculiar distribution of the chromosomes, is 
similar in all essentials to the reduction division in the higher plants 
and in Dictyota. 
The mitoses in the tetraspore mother cell have certain peculiarities 
that deserve special consideration. The first mitosis is followed so 
rapidly by the second that there are no resting nuclei organized between 
the two divisions. In this respect the history of sporogenesis resembles 
that of Pallavicinia reported by Moore (50, 51), but there is this 
difference, that in Polysiphonia the granddaughter chromosomes 
present in the second mitosis are formed before the first and within the 
original membrane, and the organization of the four granddaughter 
nuclei takes place simultaneously. However, the distribution of the 
granddaughter chromosomes is clearly effected through two mitoses 
and two sets of spindles, so there is never present a quadripolar 
spindle such as was described by FARMER (28, 29, 30, 31) for Palla- 
vicinia and some other forms of Hepaticae, and has been called 
in question by Davis (19) and Moore (50, 51). 
ALTERNATION OF GENERATIONS. 
Alternation of generations—Judging from the studies of Poly- 
siphonia presented above, the male and female plants with their 20 
chromosomes are gametophytes. The union of the male and female 
nuclei results in the fertilized carpogonium with the double number 
of chromosomes (40), marking the beginning of a new phase, the 
sporophyte generation. This fusion nucleus gives rise to a series 
of mitoses in the central cell of the cystocarp, all characterized by 
the double number of chromosomes, and consequently sporophytic in 
character, and carpospores are finally formed. The carpospore 0m 
germination presents the same number of chromosomes (49), and the 
successive mitoses following contain this number, so that the sporeling 
developed from the carpospore is still a part of the sporophytic phase. 
It may never be possible to grow such sporelings to maturity under 
experimental conditions, but it is evident that the plant developed 
from the carpospore must have nuclei with 40 chromosomes, until 
there is some marked change in the life history. The only vegetative 
form of Polysiphonia with 40 chromosomes is the tetrasporic plant, 
from which it must be inferred that the tetrasporic plant arises from 
