I9I5] 



BARTLETT—MASS MUTATION 



451 



The numerical data for mut. selacea 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. setacea 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 lauts. fortnosa 

 and albicans, as well as 

 from the insignificant 



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 



Fig. 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. 



