SEXUALITY IN PLANTS—JOHNSON. 395 
multiplication, while the sporophyte is a third, a new generation which 
has risen by specialization of the zygote. There isin the cormophytes 
then an antithetic alternation of the two most recently evolved 
phases of the life cycle, while the only clear trace of the primitive 
nonsexual phase is found in the halved number of chromosomes, 
which is reverted to by a process of chromosome reduction at some 
point in each life cycle. 
In the two decades since this famous pronouncement of Stras- 
burger’s was made, chromosomes have been counted in the different 
developmental phases of nearly all groups of plants. These counts 
have shown that wherever there is sexual fusion there is also, at some 
other point in the life cycle, a reduction of the double number of 
chromosomes so formed to the single number characteristic of the 
gametes. In all cormophytes and many thallophytes this reduction 
occurs at sporogenesis. 
The investigation of the complementary phase of the chromosome 
behavior, the doubling of the number at fertilization, has during the 
past two decades also led to extremely mteresting results. 
The earlier workers on sexual nuclear fusion apparently believed 
that the paternal and maternal nuclear materials became intimately 
mingled soon after contact of the nuclear walls. Thus Klebahn 
(1892) described the chromatin nets of the two nuclei as gradually 
merging into one in Cidogonium, and Shaw (1898) described the 
same process in Onoclea. It is true that Guignard (1891) had noted 
that, in Lilium and Fritillaria, the male and female reticula remain 
distinct until the prophase of the first nuclear division of the embryo. 
Later research, however, showed that the paternal and maternal com- 
ponents remain. distinct till much later than this; in fact, that the 
chromatin elements from the two parents do not really fuse at all 
during the process of fertilization. On the contrary, it seems quite 
likely that all through the development of the sporophyte the chro- 
mosomes from the two sources retain their identity and individuality. 
Thus Blackman (1898) and Ferguson (1901) say that in the fusing 
nuclei of Pinus the two chromatin nets never lose identity, and that 
at the first mitosis of the embryo each constituent gives rise to its 
own group of chromosomes. This independence of the two chro- 
matins at fertilization has since been seen in a number of species, 
and it is now believed to persist throughout the life of the sporophyte. 
The double number of chromosomes is present at each mitosis of this 
generation, and these chromosomes sometimes occur in pairs and are 
assumed to consist of a paternal and a maternal chromosome each. 
In certain plants also, according to Overton (1909), Gregoire (1910), 
Stout (1912), and others, the individuality of the chromosomes of the 
resting nucleus, postulated by Strasburger in 1894, is morphologically 
discernible. De Vries (1903) emphasized this fact that the sporo- 
