Oct. 22, I917 
Inheritance Studies in Pisum 
169 
Peas are easily crossed and the flowers but little affected by mutilation. 
The set of seed in good weather is, at least, 60 to 75 per cent. Of 200 or 
more varieties grown in the greenhouse in winter in 4-inch pots and in 
the open bench, none have failed to blossom and set seed. 1 
THE FACTORS OF PISUM 
Mendel (1866) 2 , Tschermak (1911), Correns (1902), Bateson (1909), 
Darbishire (1911), Lock (1905), Pellew (1913), Keeble (1910), Vilmorin 
(1911, 1913), Hoshino (1915), White (1916), and others have demon¬ 
strated, on the presence and absence hypothesis, the existence of over 
35 factors in the genus Pisum, these obviously forming only a small part 
of the full factorial Pisum complex. If the experimental data secured 
by the above-mentioned investigators are interpreted on the "paired 
allelomorph” hypothesis, the "absence” of each factor is to be regarded 
as a recessive factor, and the number of demonstrated factors is thus 
increased to over 70. In Table II the factors are listed according to the 
* ‘presence and absence” hypothesis. The presence and absence of these 
35 factors gives rise, under certain specific environmental conditions, 
to over 70 differential Pisum characters. 
The detailed experimental data by which these factors were demon¬ 
strated and the references to the literature, together with much other 
summarized data on the genetics of peas, are given in No. II of this 
series of studies on Pisum 3 ; hence need not be repeated here. Suffice 
it to say that in many cases the existence of these factors has been proved 
by a large amount of experimental data, including large numbers of F 2 
and F s generation individuals, results from the back-crossing of F x and 
homozygous F 2 and F 3 individuals with the ancestral "parent” types 
and with each other, reciprocal crosses, etc. With the exception of 
factors 11, 15, 17, 26, 27, and 33, the data are especially complete and 
satisfactory. In several cases studies on a single factorial difference have 
involved observations on over 20,000 individuals. In a large number of 
the cases the studies involve a thousand or more individuals. Most of 
these factors (except factors 13 and 24) have been isolated by other 
workers, but in many cases the writer has repeated their experiments 
with similar results. 
In collecting these factors different symbols from those originally 
assigned them by their discoverers have often had to be given, in order 
to make them intelligible (Table II). Since the letters of the alphabet 
1 The writer is much indebted to his assistant, Miss M. Mann, and to Miss S. Streeter, a graduate 
student of the Brooklyn Botanic Garden, for assistance in collecting these data, Mr, Montague Free, of 
the Garden staff has also helped collect and classify the data. 
2 Bibliographic citations in parentheses refer to " Literature cited,” p. 178-180. 
8 White, O. E. inheritance studies in pisum. n. the present state op knowledge op. he¬ 
redity and variation in peas. Brooklyn Botanic Garden Contrib. 19. (Compilation and correlation 
of all the genetic data on Pisum up to 1916, with critical discussion and full bibliography. Read by title 
before the American Philosophical Society, May 4.1917. Not yet published. Manuscript copy in Brooklyn 
Botanic Garden Library.) 
