373 
and of other related Primula Hybrids. 
has its parallel in the Oenotheras. The parent type, O. Lamarckiana , 
possesses 14 (2 x) and 7 (x) chromosomes ; its mutant, O. gigas , possesses 
28 (ix) and 14 (x) chromosomes. This fact was first observed by Lutz 
( 31 ), who counted 28 or 29 chromosomes in the roots. The numbers 14 (%x) 
and 7 (x) seem to be characteristic of this series of Oenothera ; they are 
found in O. lata , in O. nanella , in the mutant O. rubrinervis , and in the 
hybrid arising from the cross O. lata x O. Lamarckiana. 
O. gigas appeared first in de Vries’s experimental garden in 1895 
amongst a crop of O. Lamarckiana which was known to have bred true 
for three previous generations ( 7 ). It arose without any intermediate stage, 
and the plants, grown from self-fertilized seeds, were with one exception 
pure gigas . Thus a new elementary species can ‘ appear without any 
obvious cause in a single individual and be absolutely constant from the 
very first ’ ( 7 ). O, gigas reappeared in 1 898 from the seeds of a plant of 
O. sub tin ear is, which had itself arisen from the Lamarckiana family ; and 
again in 1899 from a cross between O. lata and O. hirtella. Thus, like 
P. kewensis (seedling form), O. gigas has originated from more than one 
source. Gates ( 17 ) has worked out the cytology of the pollen mother-cells 
of O. gigas in detail, and he believes that the doubling of the chromosome 
number is to be regarded merely as a duplication of the set of chromosomes 
present in O. Lamarckiana. ‘ There is no evidence that any new unit 
characters have been added, or that anything really new has come into the 
germ plasm.’ Whereas in P. kewensis the acquirement of the double 
number of chromosomes is apparently associated with the change from 
a sterile to a fertile condition. 
P. kewensis (fertile) makes no attempt to form a second contraction, 
and in that respect resembles its parent P.floribunda. Another cytological 
characteristic peculiar to P. kewensis (fertile) is the temporary joining 
together of two of the bivalent chromosomes in the first meiotic division, 
resulting in a large quadrivalent chromosome, an association which is main- 
tained until the univalent chromosomes, after having collected at the 
equatorial plate, separate off to the poles of the spindle. Further, P. kew- 
ensis (fertile) has a remarkable diakinesis in which the chromosomes are 
much contracted, a stage which has never been seen in the other Primulas. 
Lastly, the nuclei of the pollen mother-cells of P. kewensis (seedling) are 
considerably larger than are those of its parents. Thus it affords another 
example of the size of the nucleus in relation to the number of its contained 
chromosomes. 
A rest occurs between the last premeiotic and the first meiotic 
division as in the other Primulas. During this and the subsequent stages 
the nucleolus buds most vigorously (PI. XLIII, Fig. 72). There is a close 
synaptic knot, which leaves a large nuclear space. The buds at first stain 
faintly, but they gradually become chromatic in their staining reaction. 
