FIFTEEN- AND SIXTEEN-CHROMOSOME OENOTHERA MUTANTS 99 
Bartlett ('15&, p. 141) says: ''Recent discoveries are making it very 
clear that mutative changes in the chromosome number occur fre- 
quently, and that such changes are always associated with a modi- 
fication in the morphological characters of the plant. In other words, 
certain mutations are probably dependent upon, or, at any rate, closely 
associated with, visible changes in the nuclear mechanism. We have 
every reason to believe, therefore, that the different chromosome 
numbers of different species were acquired simultaneously with the 
acquisition of other specific characters." 
Gates ('15&), as earlier quoted, states that whenever a germ-cell 
having 8 chromosomes fertilizes a normal germ-cell (and he would 
doubtless concede the reverse as well), a new form is produced and that 
one of the most important factors determining the nature of the char- 
acters of the new form is probably the peculiar combination of chro- 
mosomes received. 
It is now quite certain that whenever an offspring is derived from a 
14-chromosome form by means of the union of an 8- with a regular or 
irregular 7-chromosome cell, the offspring will invariably differ from 
the parent in somatic character as well as in somatic chromosome 
number. Likewise, it has been shown that whenever an offspring is 
derived from a 15-chromosome plant by means of the union of two 
7-chromosome cells, it will invariably differ from the parent in somatic 
character as well as in somatic chromosome number. We have seen, 
however, that unsurmountable difficulties are soon encountered when 
one attempts to explain mutation on a strictly chromosomal basis. 
We may now return to the question of the probable number of 
chromosomes which were present in 0. semilata de Vries. 
Having found that 0. semilata de Vries and 0. semilata Gates are 
distinct types. Gates ('15a, p. iii) concludes regarding the former that 
"since it bred true it probably had 16 chromosomes," and adds that it 
will therefore be understood that de Vries's form "is another mutant 
which probably had 16 chromosomes." 
Whether or not 0. semilata de Vries had 16 chromosomes is in itself 
a matter of small importance, since the strain has died out and this 
particular mutant type may never reappear; but the questions which 
Gates's statements raise are of considerable interest. 
scintillans reproduce the characters of the mutant parent, indicates, in the opinion 
of the writer, that the extra female chromosome degenerates less frequently in strong 
biennials than in the less vigorous annuals and that S-^-y unions occur more frequently 
than 7+7, when biennials are selfed, for the simple reason that relatively fewer female 
7-chromosome gametes are produced. 
