318 



Gene Action 



the control of cell division in the very young leaves so that some 

 forms have more and smaller cells whereas others have fewer 

 but larger cells. The results of the different number of cells 

 are leaves of different shape. Whereas superficial studies would 



Fig. 89. Leaf types in the common nasturtium and their development: 

 (1) a juvenile leaf; (2) an acutely lobed leaf of the genotype II uu; (3) a 

 roundly lobed type, UU—', (4) the orbicular type, L— uu. (Courtesy Dr. 

 W. G. Whaley in the American Journal of Botany.) 



tend to have us speak of a gene "for round leaves/' a more 

 accurate statement would be a gene ''for more cells per unit of 

 area in the sinuses of juvenile leaves." 



Genes and Cytoplasm 



Although the exact role of the cytoplasm in heredity is not 

 known, there is much evidence that the genes interact with the 

 cytoplasm in such a way that the cytoplasm may be changed 

 as the result of the action of genes. The individual begins his 

 or her development under the influence of a certain cytoplasmic 

 pattern found in the egg, but his or her own genes may so 

 influence the cytoplasm that it would have a different pattern 

 in the eggs of subsequent generations. 



The coiling of snails illustrates the role of the maternal cyto- 

 plasm in certain cases. In the gasteropod mollusks, the mature 

 shell is twisted into either a right- or left-handed spiral. It has 

 long been known that the type is determined by the position 

 of the mitotic spindle at the second or perhaps even at the first 

 division of the egg. Thus, at a very early stage of ontogeny, 

 the type of spiral is determined. The inheritance of this char- 

 acter is interesting for, although it appears to be dependent 



