SEXUALITY IN PLANTS—JOHNSON. 397 
as he says, “‘at or soon after fertilization.’”’ He evidently regards the 
connections between sporophytic chromosomes referred to above as 
affording ample opportunity for any interchange of material or 
“influences”? between the chromosomes. Gates does not say just 
when the parental chromosomes are first paired up after fertilization 
nor give the evidence for this. He fails also to explain the fact, 
upon which practically all workers seem agreed, that the constituent 
chromosomes of the diploid pair are associated with each other in a 
more intimate way than are the chromosomes of any other mitosis 
in the life cycle. 
VI.—ALTERNATION AND CHROMOSOME NUMBERS IN THE ALG, 1896. 
We have already seen that an attempt was made in the third 
quarter of last century to interpret the life histories of certain thallo- 
phytes, especially among the alge, in terms of the alternating gen- 
erations discovered by Hofmeister among the archegoniates. The 
basis of comparison was the occurrence of a sort of polyembryony 
at the germination of the sexually produced oospore in these alge. 
There was much uncertainty, however, concerning the exact corre- 
spondence of phases in the two groups, and even as to whether the 
alternation was of the same sort in the two groups. 
With the promulgation of Strasburger’s view (1894) regarding the 
significance of the reduction of the chromosome number in the life 
cycle, botanists felt that they would now be able to distinguish 
the phases of a real alternation of generations wherever chromosomes 
could be counted. A number of workers therefore followed out 
cytologically the details of development and conjugation of the 
sexual cells; and the germination of the zygote in various alge. 
The work of Chmielewski (1890) on Spirogyra, and of Klebahn 
(1891) on desmids showed some indications of a reduction process at 
the germination of the zygote in these forms, though chromosomes 
could not be counted. Not till very recently was it demonstrated 
for one of these, Spirogyra, that the chromosome number is actually 
reduced at this time. Tréndle (1911) has counted chromosomes of 
Spirogyra and finds that there is a real reduction here, and that three 
of the first four nuclei formed in the zygote degenerate, the fourth 
remaining as the nucleus of the single embryonic plant formed. 
In a study of the green alga Coleochete, Allen (1905) showed that 
the chromosome reduction occurs with the beginning of germination of 
the zygote. Hence the group of zoospore-producing cells arising 
from the latter is not to be regarded as a sporophyte, as had often 
been maintained. Allen thus eliminated the only ancestral prototype 
of the bryophyte sporogonium that the antithetic alternationists had 
been able to discover among the green alge. 
