WHITE— STUDIES OF INHERITANCE IN PISUM. 



>41 



in weight that of the small grandparent (I.), of the large grand- 

 parent (IV.) and those with seeds weighing on the average either 

 more (III.) or less (II.) than the average weight of the F^ seeds. 

 The Fo results from about 12 F^ plants were: 



The Fo progenies of the 12 or more F^ plants were similar in 

 composition, only those with the largest numbers giving the extreme 

 variants. It is not clear as to whether parents, F^'s, and F^'s were 

 grown under the same conditions, and in one case at least the F^'s 

 and Fo's appear to have been obtained in different years. In my 

 own experiments, seed size is quite sensitive to environmental dif- 

 ferences, peas of the same pure line being almost twice as large 

 under certain conditions than under others. The effect of environ- 

 mental changes also varies with dififerent varieties. 



In crosses between large- and small-seeded varieties made at 

 the Brooklyn Botanic Garden, the F^ generation generally has as 

 large seeds as the large-seeded parent, while crosses of large seed 

 X intermediate (true breeding) seed has given in F^ intermediates. 

 In studies of such a character as seed size or weight, which has so 

 many true breeding variations, a marked difference in results from 

 crossing of different varieties is to be looked for, and while some 

 of these crosses should give simple results, in other cases results 

 of the most complex character are to be expected. 



Interpretation. 



Crossing data on this character are too scanty to give much 

 time to interpretation. Tschermak (86), while not definitely com- 

 mitting himself, is inclined to interpret his results as due to several 

 factors, possibly four, though by combining groups I., II. and III., 

 IV., a ratio varying from 3.5:1 to 4 : i is secured. One of the 

 many objections to considering seed weight to be determined by the 

 presence or absence of a single factor is the breeding true in F3 



