232 FUNDAMENTALS OF CYTOLOGY 



to the next generation through the egg cytoplasm but is not transmitted 

 by the male parent because the male gamete brings no functional cyto- 

 plasm into the egg at syngamy. If the condition were under the differen- 

 tial control of nuclear factors, it would be transmitted equally well by 

 male and female gametes, since the nuclear contributions of the two are 

 equivalent. There are other plants in which chlorophyll characters of 

 this non-Mendelian type are inherited from the male parent as well as 

 from the female, indicating a participation of the male gamete's cyto- 

 plasm in the formation of the zygote. 



There is much concerning the inheritance and development of chloro- 

 phyll characters that is not well understood. The cytological mechanism 

 of variegation is particularly obscure in cases like the above, especially 

 where the color pattern does not coincide with the pattern of tissue devel- 

 opment. There is much more to be learned about plastids, the nature of 

 the cytoplasmic differential factor, and the causes of differentiation in 

 general before such problems can be solved. 



Conclusions. — The subject of this chapter leads us back to a concept 

 stressed in early pages of the book, viz., that of the living individual as an 

 organized protoplasmic system with many specialized regions contributing 

 to the orderly activity of the whole. Accounts of the remarkable role 

 of the nucleus in heredity like that in the more recent chapters sometimes 

 suggest that the nucleus is the sole arbitrary determiner of the proto- 

 plast's activities and their consequences, the cytoplasm being merely a 

 complex organic culture medium in which it performs its functions. 

 Whatever may have been the historical origin of the nucleus-cytoplasm 

 type of organization — and what it was w^e should like very much to know 

 — the fact that the cytoplasm participates in at least the development of 

 characters is now obvious. 



That the cytoplasm also shares in determining what kind of characters 

 shall develop is evident in the phenomena reviewed in this chapter. 

 Nuclei never develop alone : it is always a nucleocytoplasmic S3^stem that 

 undergoes development. When the type of cytoplasm associated with 

 the nucleus is sufficiently altered, as in certain wide crosses, the charac- 

 ters are also altered, showing the importance of nucleocytoplasmic inter- 

 action in character development. Hence in pure lines the cytoplasm at 

 least contributes to the similarity of individuals, whether these are 

 brothers or parent and oft'spring, and similarity is a principal feature in 

 heredity. The visible results of a strongly differential action of the cyto- 

 plasm sometimes observed in reciprocal interspecific crosses, and the fact 

 that in attempted crosses between very distantly related organisms the 

 nucleus and cytoplasm will not interact at all, indicate that the cytoplasm 

 is in some measure responsible for the differences between those organisms. 

 They also suggest that if such very wide crosses were successful and the 



