R. P. Gregory 101 



of course a matter of much greater practical difficulty. The point has been studied with 

 some care in the cross ("Snowdrift" x "Ivy-leaf" ^). The Fi has a faint trace of colour 

 in the young petioles; the F^ consists of (1) plants with full colour in the young petioles, 

 which grade through rather lighter forms to (2) those in which faint colour in the 

 petioles can be recognized with certainty; and these again grade, through doubtful 

 forms, to (3) those in which no colour can be detected. Precise numerical results cannot 

 be given, but so far as can be judged the constitution of the families can be fully explained 

 without the assumption of any other factors than those for colour and for its partial 

 suppression. 



B. Flower-Colours. 



The various colours exhibited by Primula sinensis may be classified 

 as (1) full colours, which jjaay exist either in the self or in flaked 

 patterns (Plate XXXI, figs. 56 — 59), and are always associated with 

 fully coloured stems; (2) "Sirdars" (Plate XXXI, figs. 44, 45), in 

 which the characteristic distribution of the full colour is associated with 

 a definite type of stem-colouring; and (3) pale colours (Plate XXXI, 

 fig. 46) which occur only on green or faintly coloured stems. 



White flowers may occur in association with stems of any kind, and 

 may be dominant or recessive to colours. The dominant whites owe 

 their character to the possession of factors which inhibit the development 

 of colour in the flower (see under "Inhibition," p. 105). 



The full colours and " Sirdars " may be sub-divided into blues, 

 magentas and reds; in the pale class, however, no distinction of this 

 kind can be drawn, for the pale forms which correspond with the 

 magenta full colours (and give magenta offspring when crossed with 

 a red) are quite indistinguishable to the eye from those which 

 correspond with the red class (and give only red offspring when 

 crossed with reds). 



Colours belonging to all these classes appear in the offspring of 

 certain hybrids; the sharpness of the separation between the various 

 classes of full colours varies, however, in different cases, and though in 

 the majority the classes are fairly readily distinguished, in others 

 intermediate forms occur. Whether these intermediate forms are 

 always heterozygous cannot yet be said ; in the few experiments in 

 which they have been tested they have proved to be so^ 



' The "Ivy-leaf" used in the experiments on stem-colour was heterozygous for the 

 pallifying factor. Hence the appearance of "Ivy-leaf" here as the parent lacking the 

 pallifyiag factor, and previously as the parent bringing in that factor. 



' A plant with red stigmas, which probably belonged to the red class but had flowers 

 of a colour somewhat intermediate between the magenta and red classes, has since proved 

 to be homozygous for its type of colour. 



