552 
E. RUGGLES GATES AND NESTA THOMAS. 
that they may also have aberrant chromosome numbers, but 
this has not yet been proved, except in gigas. It is also to 
be expected that some of the offspring of semi lata or of 
fertile lata plants contain 16 chromosomes, but such plants 
have not yet been observed. 
As already ^^ointed out, lata and semilata form an almost 
continuous series, all, however, having certain features in 
common, and all having 15 chromosomes. They may also, 
of course, possess various minor characters in addition. 
The presence of red or white midribs on the leaves, e. g., 
appears to be a simple Mendelian character-difference. But 
the relation between lata and semilata is not at present 
clear. The variability of the lata-semilata series may 
depend upon the fact that the extra chromosome belongs to 
a different pair in different cases. Since there are seven 
pairs of chromosomes, we should then expect seven more or 
less distinct lata -like types. But there is at present no 
evidence that the plants having 15 chromosomes can be 
divided in this way. 
An equally plausible hypothesis would be that any chromo- 
some can constitute the extra one in any member of this 
series of plants, and that the external variability exhibited 
by them depends upon certain chromosomes not being dis- 
tributed whole during meiosis. Though such processes have 
already been described, there are difficulties also with this 
view, for it involves the rather unlikely assumption that the 
chromosomes of CEuothera are undifferentiated in their 
potentialities. The cause of this external variability, and 
particularly of the difference in fertility, leaf shape and 
habit between lata and semilata therefore remains for the 
present obscure, though semilata stands midway between 
lata and Lamarckian a. 
AVhat has been made clear, howe'ver, is a constant associa- 
tion between the extra chromosome and the lata-like cou- 
stellation of external characters, and we believe this to be 
the first case in plants in which such a relationship has been 
shown to exist. A comparison with the sex chromosomes in 
