96 Experiments with Primula sinensis 



In the foregoing types the colour extends into the root-stock and 

 roots, and in the faintly coloured forms its presence is much more easily 

 detected there than in the stem, where the colour of the sap is masked 

 by the green colour of the chlorophyll. 



The plant represented in fig. 6 is the " Ivy-leaf." In this form the 

 colour can be recognized most readily in the young petioles, and it also 

 appears, though more faintly, in the pedicels. In older leaves the 

 colour may bf noticed at the base and sometimes along the edges of 

 the leaf-stalk. It does not appear to extend to the root-stock and roots. 



Outline of the inheritance of stem-colour. 



In its general outlines, the inheritance of stem-colour is simple. 

 Thus, the red-stem crossed with a green-stem gives an F^ in which 

 the red-stemmed offspring are either approximately 9 in 16, or 3 in 4, 

 according to the constitution of the green-stemmed parent. The full 

 colour crossed with the faint colour (fig. 5) gives, in F^, 3 of the former 

 to 1 of the latter, and similarly the faint colour behaves as a simple 

 dominant to the complete absence of colour. 



Although the character of the stem in " Sirdar " is, in its lighter 

 shades, not very different in appearance from that of other faintly 

 coloured types, the inheritance of stem-colour can be most simply 

 explained if the " Sirdars," which appear in certain F^s, are regarded 

 as forming a part of the fully coloured population, lacking, however, in 

 the factor {F) which effects the even distribution of the colour in the 

 stems and flowers. We have then a factor {R) for colour, and epistatic 

 to R, and without effect in its absence, a distributing factor F^. In 

 order to provide for the existence of the forms with some faint 

 colour in the petioles we require to assume the existence of another 

 factor {Q) determining this character, which is independent of R and F 

 and is unaffected by them, except in so far as the faint colour is not 

 discernible when R and F are present. 



In crosses between plants with the lower grade of stem-colour and 

 those without colour, the last factor (Q) only comes into play, and the 

 3 : 1 ratio is obtained in F^ (Table, p. 98, II.). Since " Sirdars " have 

 only occurred in my experiments in cases in which "Snowdrift" was 



1 The use of a so-called distributing factor is intended merely as providing a simple 

 means of formulating the observed results. The relation which subsists between the 

 " Sirdar" types and the self -colours is probably different from that which obtains between 

 flakes and self-colours (p. 122). 



