188 SECTIONAL ADDRESSES. 



which the Meudehan factors stand is constantly before tlie mind of the 

 breeder, but we are only now on the threshold of investigation in this 

 direction, and it is doubtful whether we can as yet give a certain answer 

 in any single instance. Still less are we able to say what the actual 

 elements or units which undergo segregation may be. In the case of 

 such allelomorphic pairs as pm-ple and red sap colour or white or 

 cream plastid colour it may be that the difference is wholly qualitative, 

 consisting merely in the formation or non-formation of some one 

 chemical substance. But the majority of characteristics are not of 

 this hard-and-fast type. Between some the distinction appears to be 

 one of range — to be quantitative rather than, or as well as, qualitative 

 in nature, and range must mean, presumably, either cumulative effect 

 or a force or rate difference. It may well be, for example, that with 

 some change in physiological equilibrium accompanying growth and 

 development, factorial action may be enhanced or accelerated, or, on 

 the other hand, retarded or even inhibited altogether, and a regional 

 grading result in consequence. Range in a character is not confined to, 

 though a common characteristic of, individuals of cross-bred origin. It 

 may be a specific feature, both constant and definite in nature. For 

 example, a change as development proceeds from a glabrous or nearly 

 glabrous to a hairy condition is not of unusual occurrence in plants. 

 In the Stock such a gradational assumption of hahiness is apparent no 

 less in the homozygous form containing a certain weak allelomorph 

 controlling surface character, when present with the factors for sap 

 colour, than in those heterozygous for this or some other essential 

 component. We see a similar transition in several members of the 

 Scrofhulariacece — e.g., in various species of Digitalisi, in Antirrhinum 

 majus, Antirrhinum Orontium, Anarrhinum pedaturn, Pentstemon, and 

 Nemesia. In perennials an annual recuiTence of this change of phase 

 tuay be seen, as in various species of ]"iola and in Spircea Ulnmria . 

 It is somewhat curious that the transition may be in the same direction 

 — from smoothness to hairiness — in forms in which the dominant-reces- 

 sive relation of the two conditions is opposite in nature, as in Matthiola 

 on the one hand and Digitalis purpurea nudicaulis on the other. Mani- 

 festation of the dominant characteristic gradually declines in the Fox- 

 glove, while it becomes more pronounced in the Stock. In some, per- 

 haps all, of these cases the allelomorphs may stand for certain states 

 of physiological equilibrium, or such states may be an accompanying 

 feature of factorial action. A change of ])hase may mean an altered 

 balance, a difference of rhythm in interdependent physiological pro- 

 cesses. In the case, for instance, of a certain sub-glabrous strain of 

 Stock in which the presence of a single characteristically branched 

 hair or hair-tuft over the water-gland terminating the midrib in a 

 leaf otherwise glabrous is an hereditary character, it is hardly conceiv- 

 able that there is a localisation in this region of a special hair-forming 

 substance. It seems more probable that some physiological conditio ji 

 intimately connected with the condition of water-content at some 

 critical period is a causal factor in hair production, and that this con- 

 dition is set up over the whole leaf in the type, but in the particular 

 strain in question is maintained only at the point which receives the 



