Oct. 22, I917 
Inheritance Studies in Pisum 
173 
stuck together when mature, is regarded by the Hagedoorns (1914) as 
being modified by the factors for flower color and glaucous foliage and 
stems. In working over Vilmorin’s data (1913), the present writer 
concluded that these data were more simply explained by regarding 
factors (Bl) (3) and S’(30) as partially coupled or linked and by assum¬ 
ing certain of the F 2 combinations to be adversely affected by weather 
conditions. The variety with stuck-together seeds (“chenille”) has 
emerald foliage, but emerald varieties are known with free seeds. 
RELATION OF PISUM FACTORS TO EACH OTHER IN INHERITANCE, 
WHETHER LINKED (COUPLED) OR INDEPENDENT 
The studies of Bateson (1909), Pellew (1913), Surface (1916), Tanaka 
(1913), Emerson (1911), Vilmorin (1913), Gregory (1911), Doncaster 
(1913), Pearl (1912), Altenburg (1916), and especially of Morgan and 
his coworkers and students (1915), Sturtevant (1915), Bridges (1914), and 
Muller (1916) make it increasingly evident that the various hereditary 
characteristics of organisms and their determiners or factors are inherited 
in more or less closely linked groups, instead of independently, as was 
supposed in the early Mendelian studies, these groups being held together, 
perhaps, by chromosomes. As shown by Cannon (1903), Strasburger 
(1911), and others, at least some varieties of the common pea have seven 
pairs of chromosomes; and a wide cytological investigation, now in 
progress, will determine how true this is for all species of Pisum. On the 
assumption that seven pairs of chromosomes are the number usually 
present in all varieties, the number of independently inherited groups in 
Pisum, on the chromosome hypothesis as developed by Morgan and his 
students (1915), should be seven. Should accurately checked data show 
that more than seven of these groups occur, for instance, eight or nine, 
either the varieties used possess more than seven pairs of chromosomes or 
the chromosome theory as now held, so far as peas are concerned, would 
need revision or would be disproved. The chromosomes of peas, as com¬ 
pared with those of Drosophila spp., however, are long; and from this fact 
one might assume the linear distances between many of the factors to be 
greater. This being true, greater opportunities for crossing over would 
occur, and when the number of cross-overs approached 50 per cent, 
greater difficulty would be found in securing sufficiently accurate data 
to distinguish between ratios involving linkage and those showing only 
independent inheritance. 
Table III shows the totals of F 2 ratios from crosses involving the pres¬ 
ence and absence of eight factors [A, B, (Fa), I, (Ee), G, R, (Tl)], two of 
which [R and (Tl)] belong to one group. Each factor has been tested out 
in crosses involving all its possible combinations with the other seven, 
the results indicating seven independently inherited groups in Pisum. 
Hence, in Table III all the factors of one group have a common number, 
as, for example, R and (Tl) meaning that both R and (Tl) belong to group 
7. The F 2 generation totals in most cases are fairly large and dependable, 
