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CARL D. LA RUE AND H. H. BARTLETT 
The same explanation does not hold for the difference between the 
two kinds of f. typica, for vegetatively they are equally vigorous. 
May not the yellow seedlings which occur in progenies of mut. hilonga 
give a clew to an understanding of the situation? These yellow seed- 
lings constitute a mutational type in which chlorophyll formation 
can not take place, and therefore a type which can not persist more than 
a few days after germination. It does not require a very great effort 
of the imagination to conceive of physiological defects that might 
originate by mutation and that might operate disadvantageously to 
the organism possessing them at an even earlier stage in the life cycle 
than failure to produce chlorophyll. May not the aborting seeds in 
the polymorphic progenies represent one or more physiologically 
defective classes of mutations, of which the zygotes are unable to 
develop into mature embryos? Pending cytological study of the 
abortive seeds, such a hypothesis seems to us much more plausible 
than the alternative hypothesis that they are typica zygotes, eliminated 
by some unknown selective process that leaves to develop the intrin- 
sically weaker zygotes of the several mutational types. 
Matroclinic Inheritance in the Mutation Crosses 
In 1915 a complete series of mutation crosses was made, involving 
f. typica and the three well-known mutations. One parent plant of 
each form served for self-pollination and for crossing with the three 
other forms. Each cross was made reciprocally. Two of the twelve 
crosses, mut. semialtaX mut. dehilis and mut. bilongaXmut. debilis, 
failed, but the remaining ten were in varying degrees successful, and 
progenies of all were grown in 1916. The reader will find the four 
parent plants of these crosses indicated by asterisks in figure i, and 
may determine by reference to Table I that all gave rise to uniform 
progenies in the following generation. It will be observed that the 
phenomenon of mass mutation had not occurred in the direct line of 
descent of the individual of f. typica chosen as a parent. Both the 
semialta and the bilonga parents belonged to first generation progenies 
from primary mutations ii. e., mutations derived directly from f. 
typica, and not from one of the other mutations). The former type 
arises only as a primary mutation, but the latter is frequently derived 
as a secondary mutation from mut. debilis. The individual of mut. 
debilis used as a parent was an actual primary mutation in a poly- 
morphic progeny, chosen because, at the time the other plants were in 
