(EXO'l'lIElU MUT. LATA AND (E. MUT. SEMILATA. 
557 
Madrid contained a lata mutation, having lata-like foliage 
and the small flowers of biennis. 
These cases, together with the previous work, prove that 
the peculiar characters of lata and semilata are constantly 
associated with the presence of 15 chromosomes, even when 
combined with other characters derived by inheritance from 
14-chromosome individuals. 
'J'hese mutants with 15 chi-omosomes have acquired the 
extra one by the occasional distribution of two chromosomes 
of a pair to the same daughter-nucleus in the reduction divi- 
sion, such occurrences having been discovered by one of us in 
1908. The inconstancy of lata and semilata is explained 
by the behaviour of the extra chromosome, as is also the fact 
that lata x Lamarckian a gives both parent types in the F^, 
since lata produces some germ-cells having 7 and some 
having 8 chromosomes. The proportion of latas in the 
cross varies widely, from 4 per cent, to 45 per cent., the 
percentage being determined by the number of 8-chromosome 
germ-cells which mature. The fluctuation in this ratio is 
probably caused (1) by the environmental conditions at the 
time the germ-cells are being developed, and (2) by the 
physiological condition of the mother plant at this time. The 
various other hereditary peculiarities of lata and semilata 
are also explained by the presence of the extra chromosome. 
The cause of the variability in the lata- semilata series 
of forms is at present obscure, but it may depend on the 
irregular distribution of portions of chromosomes during 
meiosis. 
The extra chromosome is associated with the foliage and 
habit of lata or semilata in the same way that one of the 
sex chromosomes is associated with sex in such insects as 
Anasa tristis, with this difference, that in CE . mut. lata 
or semilata the extra chromosome is a triplicate of a pair 
already present, while in these Hemiptera the presence of 
the accessory chromosome in duplicate is constantly associated 
with the female sex. 
The extra chromosome in CEnothera resembles more closely 
