F. Keeble and E. F. Armstrong 287 



Although there can be no doubt but that this broad relation exists 

 between oxydase and potentiality of pigment formation, it is not to 

 be concluded that all varieties of P. sinensis have identical oxydase 

 content. 



Simple observations and experiments suffice to show that the extent 

 of distribution of oxydase diflFers in different varieties. For example, in 

 the superficial tissues of the flower-peduncle of Sirdar, oxydase is limited 

 exclusively or almost exclusively to the epidermal layer; whereas the 

 corresponding tissues of other green-stemmed varieties give a well 

 marked oxydase reaction in both epidermal and sub-epidermal layers. 

 There is good reason to believe that chloroplasts act as inhibitors of 

 oxydase -formation in a cell and it is noteworthy that the sub-epidermal 

 layer of Sirdar is specially rich in chloroplasts. Again in red-stemmed 

 varieties, the petioles, which together with the roots appear to be the 

 members richest in oxydase, are so rich in both chromogen and oxydase 

 that pigment occurs in practically every cell ; in green-stemmed plants, 

 though the roots are rich in oxydase, the cortical cells of the petioles 

 give no oxydase reaction. These phenomena and others to be 

 referred to immediately lead us to the opinion that such localisation 

 of oxydase as occurs in P. sinensis is at least in large measure a 

 phenomenon of inhibition. We shall see in a later .section, that, as 

 postulated by Mendelians, flower colour may be inhibited. We know 

 that the result of crossing a dark red- and a reddish-stemmed plant 

 is the production of a reddish ^i, and that the F^ generation from 

 this cross consists of 3 reddish : 1 dark red-stemmed ; that is, dark red 

 stem behaves as a simple recessive to reddish stem. Nevertheless, as 

 we have just seen, red stem differs from reddish stem in possessing 

 pigment in certain cortical and stelar tissues which are not pigmented 

 in the reddish-stemmed plants. This being so, it would be expected 

 that the presence of pigment would be dominant to the absence and 

 hence that red stem would be dominant to reddish stem. Since the 

 reverse is the case we can scarcely escape the conclusions that reddish 

 stems contain an inhibitor and that this inhibitor is not powerful enough 

 to suppress pigment formation altogether though it suffices to suppress 

 it in certain tissues. 



It remains to ask whether partial inhibition of pigment formation is 

 to be attributed to inhibition of the action of oxydase on the chromogen 

 or to inhibition of the processes which lead to the formation of chromogen. 

 We are not in a position to answer this question with respect to stem 

 colour though, as we show in the following section, we can answer it 



20-2 



