Genetic Factors 435 



of stomatal mother cells, initials of trichosclereids, and many other ex- 

 amples of unequal cell division the same differential distribution of cyto- 

 plasm, before division, is evident. 



In such cases, unlike the coenocytes, the differences which arise in the 

 cytoplasm cannot become distributed beyond the limits of the original 

 mother cell. If in each cell division, however, there were a quantitatively 

 or qualitatively unequal distribution of cytoplasm between the two 

 daughter cells, cellular differentiation would result. It seems reasonable 

 to suppose that many cell divisions are thus cytoplasmically unequal 

 even though the differences are not visible and may be at the submicro- 

 scopic or chemical level. Such inequality would provide the necessary 

 cytoplasmic basis for differentiation. 



If differentiation proves to be primarily a matter of cytoplasmic distribu- 

 tion, the mechanism by which this distribution is controlled must evi- 

 dently be one of the major problems of morphogenesis. In some cases 

 the cause may be ascribed to polarity. Certainly polarity is involved, as 

 we have seen, in many axes other than the major one of the plant body 

 and is evident in many developmental patterns. It may be that the mecha- 

 nisms which are effective in the extreme and conspicuous cases of un- 

 equal and polar cell division may also be involved in all differential di- 

 visions (Bunning, 1958). 



That the cytoplasm contributes to the determination of developmental 

 processes through inheritance is clear from a number of facts, particularly 

 in cases where the offspring of reciprocal crosses are unlike. Where the 

 offspring tends to resemble the maternal parent this difference is evi- 

 dently due to that which only this parent contributes to it, the cytoplasm. 

 In traits where plastid differences are involved the influence of the 

 cytoplasm is clear, since the plastid primordia are carried in it. In other 

 cases it is more difficult to see what the mechanism of transmission is. 



Crosses in Epilobium, where reciprocal hybrids are often markedly dif- 

 ferent in size, have been studied intensively. Lehmann (1936) showed 

 that in such cases the smaller hybrids have a lower concentration of auxin 

 than the larger ones. When Schlenker and Mittmann (1936) applied 

 auxin to the smaller plants their size was considerably increased. These 

 facts suggest that something carried in the cytoplasm stimulates the 

 synthesis of auxin. It may be, as has sometimes been suggested, that 

 sensitivity to auxin is determined by the genes but that auxin synthesis 

 is carried on in the cytoplasm. Michaelis (1938) disagrees with Lehmann 

 and believes that the facts can be explained by specific interactions be- 

 tween genes and cytoplasm. This Epilobium work, however, and the 

 great body of evidence obtained by von Wettstein (1924) and his col- 

 leagues in experiments with mosses show that the genes are not inde- 

 pendent in their effects but that what they do is determined to a con- 



