M. Wheldale 165 



increase the concentration of aromatic glucosides is one which can only 

 be solved by quantitative estimation. I am at present engaged in 

 experiments in connection with this point. 



Application to Mendelian Factors. 



A question which now arises is how this hypothesis I have for- 

 mulated fits in with our knowledge of the relationship between colour- 

 varieties and the type from which they are derived. 



In the first place I shall deal with a variation which, though com- 

 paratively rare, may be most closely connected with the reactions 

 controlled by the glucoside-splitting enzymes. 



There are anthocyanic species which have given rise to varieties 

 having some organ or part fully pigmented with anthocyanin, whereas, 

 in the type, the same organ or part is unpigmented or only slightly so. 

 The following are examples : — 



Flower. The type in Bellis pei'ennis, Cyclamen persicum, Primula 

 acaulis, P. elatior, Cheiranthus cheiri, Crataegus oxyacantha, 

 Achillea millefolium, is either without, or is only slightly 

 tinged with, anthocyanin, but fully coloured varieties are 

 known. 



Leaf. Fagus sylvatica, Coryllus Avellana, Beta vulgaris, Atriplex 

 hortensis, Perilla nankinensis. Carina indica, Plantago major, 

 Brassica sp., Lactuca saliva, produce red-leaved varieties. 



Fruit. The orange and banana have varieties in which the flesh 

 and pericarp respectively are pigmented with anthocyanin. 



We may assume that the coloured varieties arise through the loss 

 of some factor from the type, and in some cases it has been shown that 

 the coloured variety is recessive to the type. If the petals and leaves 

 of the coloured varieties are examined microscopically, it is found that 

 the pigment is invariably limited to the epidermal cells, and it is 

 reasonable to suppose that the loss of the factor is also limited to the 

 epidermis. Previously (19) I have termed this unknown factor a reductase 

 or inhibitor, but if the views I have expounded in the present paper 

 are correct, the appearance of pigment in the epidermis might be 

 explained on the supposition that the enzyme controlling hydrolysis 

 and synthesis of the glucoside is absent from this tissue. Hence the 

 chromogen is free from sugar and can be oxidised. In the type the 



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