530 CANNON: STUDIES IN PLANT HypsriDs: 
SUMMARY 
The main points in this paper may be briefly stated as follows : 
1. Both of the hybrids studied and reported on in this paper, 
Fillbasket x Debarbieux and Express x Serpette, were of the sec- 
ond generation, that is, their grandparents were pure races of 
peas; both of the hybrids exhibited variation after the Mende- 
lian law. 
2. The hybrids are fertile. 
3. Both of the hybrids matured their spores quite as the pure 
ancestral forms did. The first maturation mitosis was heterotypic, 
the second homotypic. 
4. The reduced number of chromosomes is seven, the somatic 
number fourteen. 
5. No abnormal mitoses were observed. 
6. In the anaphases of the last sporogenous division of both 
hybrids, and of the pure form Fillbasket, the chromosomes were 
associated in pairs; this condition was seen also in one nucleus of 
Fillbasket x Debarbieux that was in the nuclear plate stage. The 
paired condition is probably not the result of chance; further 
work, however, which the writer hopes to carry on, will be neces- 
sary definitely to determine this point. 
CONCLUSIONS 
If the physical basis for the Mendelian variation of hybrid 
peas was not demonstrated by a single histological study of them, 
certain conclusions which touch more or less directly the causes 
of the variation may nevertheless be legitimately although ten- 
tatively drawn. In the first place it has been shown that the vari- 
ation of the peas, in the third generation at least, is due entirely 
to the union of gametes which originate and develop quite the 
same as do those of the pure ancestral types, or in other words, 
abnormalties and irregularities of nuclear divisions do not form the 
basis for the variation of these hybrids. 
What then is the probable structural basis for the Mendelian 
variation of the hybrid peas ?. As has already been intimated, I 
think it is fairly well shown that this basis is the same as that of 
the variations in the pure forms, and if so an understanding of the 
latter should give the key to explain the former as well. But 
