Appendixes 313 



heterozygous condition, and he argues that observations of 

 de Vries on Veronica, and of MacDougal on Oenothera 

 indicate the possibility that this process may be reversed, 

 a heterozygous branch becoming a homozygous dominant. 

 (Correns, Ber. Deut, Bot. Ges., xxviii. 19 10, p. 433.) 



The whole problem is evidently one of great obscurity, 

 but the facts suggest that a clear understanding of the 

 genetics of these variegated forms may give light on the 

 nature of segregation in general. 



This paper of Correns should be read in connexion with 

 that of Erwin Baur on the periclinal variegation of Pelar- 

 gonium (Zts. ind. Abstam., i. 1909, p. 330) "^j in which one 

 of the most notable of recent advances in genetic physiology 

 is made. Plants with green inside and white epidermis and 

 sub -epidermal layers have exclusively albino offspring, while 

 plants in which the inside is white and external layers green 

 have exclusively green offspring. Similarly when the 

 external layers are golden the offspring are greens and 

 goldens, just as when derived from wholly golden plants 

 (see p. 253). [The last two statements I have myself con- 

 firmed experimentally, but with the alb -marginal ae I have 

 hitherto failed to obtain offspring.] The characters of the 

 cells of the sub-epidermal layer thus indicate the characters 

 of the germ-cells. In view of this discovery of Baur's we 

 are led to speculate whether the sub-epidermal layer of any 

 heterozygote is not in reality a patchwork of cells, bearing 

 or not bearing a given factor, and that according as this 

 patchwork is coarse, or fine (as in an emulsion) we obtain 

 aberrant or expected numbers in our ratios. It must be 

 remembered that external appearance is only an imperfect 

 guide to the genetic properties. For instance Tropaeolum 

 with variegated leaves breeds nearly if not quite true in 

 this respect, though we might expect its germ-cells to 

 be green, yellow, or mosaic in nature. If in the 

 mixtures of substances capable of forming coarse or fine 

 emulsions we have a true picture of the heterozygous 

 state, such phenomena as those observed by Correns for 

 the green and variegated branches become capable of 

 representation without any great straining of the facts. 

 Segregation on this view is regarded as a phenomenon 

 * See also ibid. iv. 19 lo, p. 81. 



