F. Keeblk and C. Pellew 61 



thick stem: that these factors are Mendelian in their inheritance; 

 being dominant respectively to short intemode and thin stem factors. 

 Half-dwarf peas are of two kinds. One kind, represented by Autocrat, 

 owes its semi-dwarfness to lack of the long intemode factor. In the 

 absence of this factor, the thick-stem factor cannot effect more than a 

 sturdy, medium growth. The other kind of semi-dwarf lacks the thick- 

 stem factor, and, in the absence of this factor, the long intemode factor 

 cannot build the stem-segments of a sufficient length to produce tallness 

 in the plant. It may be urged that this, after all, is but a common-sense 

 view of the way in which growth in length is effected : that only plants 

 with long internodes among annuals are likely to be tall ; and that only 

 when stems are sturdy may internodes reach their full length. This 

 may well be and it is certainly not a reproach to Mendelism that it 

 may lead to the discovery of the obvious which, without the method, 

 remains obscure. 



The conclusions which we have reached as to the gametic constitu- 

 tion of tall, semi-dwarf and dwarf peas may be summarised thus : — 



Tall = TL. 



Semi-dwarf = tL or Tl. 

 Dwarf = tl. 



In a cross described by Lock (1905) we have what seems to us an 

 interesting confirmation of this view of the chief factors involved in 

 stature of Pisum. Lock's comment on the case is as follows {op. cU. 

 p. 414): — "This cross seems to afford an example of remarkable inten- 

 sification of both the allelomorphic characters of the same pair, viz. 

 tallness and dwarfness — the former in F^ and both in F. and later 

 generations." The cross in question was one between Satisfaction — a 

 variety which at Peradeniya grew to an average height of 46 feet — 

 described as of robust growth (which we may take to mean thick stem), 

 and with internodes of an average length of 1'74 inches, and a Native 

 Pea of less than 3 feet in height, but varying much in different years, 

 with thin stem, and internodes of an average length of 1^ inches. In 

 the four plants of the F^ generation, the internodes were of an average 

 length of 2*4 inches — longer than in either parent — the height of the 

 ^1 plants was about 6 feet, and the number of internodes was the same 

 as in Satisfaction. It would seem that in the increased length of 

 intemode of the Fi plants (an average of 24 inches as compared with 

 the 174 inches of Satisfection) is seen the influence of thick stem on 

 an intemode which, when combined with thin stem, is of an average 



4—2 



