1915] BARTLETT—MASS MUTATION 451 
The numerical data for mut. setacea in all cultures have had to 
be based largely on the determination of very young plants, for 
many weak plants do not succeed in forming new roots after being 
transplanted. There is 
no difficulty in growing 
to maturity practically 
every individual of mut. 
formosa that germinates, 
and most of those of 
mut. albicans and mut. 
revoluta. It is the rule 
rather than the excep- 
tion, however, to -lose 
three-fourths or more of 
the setacea plants. 
They show some varia- 
tion among themselves 
which may possibly indi- 
cate that mut. sefacea is 
itself mutable and that 
more than one type is 
covered by this name. 
If so, only one type sur- 
vives in the part of the 
cultures which reaches 
maturity. 
The phenomenon of 
mass mutation 
From the results of 
the crosses between f. 
typica and muts. formosa 
and albicans, as well as 
from the insignificant 
Fic. 13.—Mut. formosa (Lexington E-5-206-51): 
the setiform leaf appendages show very clearly; 
the position of the plant in the pedigree may be 
determined from table VI. 
variation in the composition of cultures showing mass mutation 
regardless of the source of the pollen, it appears clear that the 
factors responsible for the mutational characters are carried in the 
