R. P. Gregory 75 



in opposite directions in the two cases, are to be regarded as merely 

 accidental, or whether they may have some significance, either in 

 connexion with observed differences in the fertility of the various 

 unions between plants of different form, or in other ways (p. 83). 



Colour. The colour of the stems and flowers in the coloured races 

 is due to the presence of coloured sap. The colour may be absent 

 from the flowers, which are then white, or from the stems, which are 

 then green. Colour, both in flower and stem, is presumably produced, 

 as in other cases, by the interaction of two or more complementary 

 factors. I have had no decisive case of the production of an F^ with 

 coloured flowers from the mating of two albinos, but Keeble and 

 Pellew^ record a coloured F-^ from the mating of the red-stemmed 

 "Snow King" with the gi'een-stemmed "Snowdrift."' Similarly as 

 regards the stem-colours, I have no example of the production of a 

 coloured F^ from the mating of two green-stemmed plants, but in two 

 cases (p. 97) heterozygous plants with coloured stems have given 

 unmistakably the ratio 9 coloured : 7 green stem. 



There exist .several distinct types of coloration, both of the stem 

 and of the flowers. Thus, the stem may be fully and evenly coloured 

 (Plate XXX, figs. 1, 2), or it may possess only a faint colour, which is most 

 easily recognized in the young leaves and leaf-stalks (Plate XXX, fig. 5). 

 The faint colour is, in some cases, an elusive character, and the plants 

 bearing it are only with difficulty to be distinguished from those 

 devoid of colour in the stem. The inheritance of these two kinds of 

 pigmentation of the stem may be explained most simply if we assume 

 the existence of two separate and independent chromogen factors, each 

 of which reacts with the common activator to produce, one the full 

 colour, the other the faint colour (p. 9(5). 



The colours of the flowers and stems are inter-related in such a way 

 that the more deeply coloured flowers never occur in conjunction with 

 stems wholly green. Flower-colours may then be divided into two 

 classes, namely, full colours, which are found only on plants having 

 fully coloured stems ; and pale colours, which occur on plants having 

 green or faintly coloured stems. White flowers may be associated with 

 stems of any kind. 



When the albino "Snowdrift" (Plate XXX, fig. 7) was cros.sed with 

 types having fully coloured flowers and stems, the F^ contained only 

 one real albino to every fifteen pigmented forms. These coloured forms 

 were of three kinds, (1) full colours on red stems, (2) a type known in 



' Jouin. of Genetics, Vol. i. 1910, p. 4. 



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