W. Neilson Jones 85 



In the case of certain characters, however, there is complete 

 dominance in the hybrids irrespective of whether these characters are 

 derived from the maternal or paternal sides (e.g. the thickness of the 

 leaf, the possession of a swollen end-cell by the calyx hairs and the 

 occurrence of purple spots on the inside of the corolla tube). 



It would seem probable that cases where a character is completely 

 dominant in the hybrid, and where it is exactly intermediate between 

 its expression in the parents, are simply the end terms of a continuous 

 series. Also, if there is a tendency for the reciprocals to differ, one 

 might expect it to be most evident in cases where the dominance is not 

 very definite. 



In fact the degree of dominance of any character (i.e. the degree to 

 which any character is expressed in an individual heterozygous for that 

 character), and the difference occurring between the reciprocals in the 

 expression of a character, are possibly two quite distinct phenomena 

 resulting from entirely different causes. Varying degrees of dominance 

 are found in varietal crosses and the phenomenon does not appear to be 

 of much significance. It is in the latter pecLdiarity — the difference 

 between the reciprocals — that most interest lies. 



The first explanation that suggests itself is that the cytoplasm of 

 the egg-cell has an influence on the subsequent development of the 

 embryo. This influence may be due to hereditary determinants being 

 carried by the cytoplasm. For example, if the cytoplasm of the egg- 

 cell is provided with a certain kind of plastid, the seedling resulting 

 from fertilisation of this egg-cell will contain these plastids. If however 

 the cytoplasm of the egg-cell does not contain the plastids, then the 

 resulting offspring will be devoid of them whatever the nature of the 

 individual from which the male gamete is derived. 



Sap colours might be inherited similarly by means of vacuoles, if 

 there is any truth in the tonoplast theory of Went and de Vries. 



The hereditary behaviour of " Albomaculata," varieties of Mirabilis 

 Jalapa, Antirrhinum, &c., is commonly accounted for on some such 

 basis as the above \ 



But much more subtle determinants, such for example as specific 

 enzymes, might be carried by the cytoplasm of the egg-cell. These 

 would ensure that the course of metabolism followed would be that of 

 the female parent : and such cytoplasm might provide an environment 



' See Baui', Einfilhrung in die experimentclle Vererbungslehre, 1911, p. 163, &c. ; cf. 

 however Baur's hypothesis regarding white-leafed Pelargouium, p. 167. 



