Genes Affecting Form -317 



the ovary grows considerably faster in length than in width, and 

 in this plant more mitotic spindles run lengthwise or nearly so 

 than in any other direction; and in Trichosanthes, the "snake" 

 gourd in which the ovary grows much more rapidly in width 

 than in length, an even higher percentage of the spindles are 

 oriented parallel to the long axis of the developing fruit. When 

 there is a tendency for the spindles to be oriented, those in meta- 

 phase and anaphase are even less oriented than those at telo- 

 phase. Apparently the ultimate position of the spindle is de- 

 termined by the polarity of the cytoplasmic body of the cell, but 

 the spindle shifts its position somewhat until it settles down at 

 telophase to the final position. This cytoplasmic polarity of the 

 cells of the ovary is a determining factor in the ultimate shape 

 of the ovary. It is, of course, itself under genie control. How 

 the "shape" genes affect this polarity of the cytoplasm is not 

 easily determined. 



Some interesting studies of a similar nature were made by 

 W. G. and C. Y. Whaley on leaves of the common nasturtium 

 of the garden, belonging to the genus Tropaeolum. [Mature 

 leaves of several shapes and sizes were found. In the juvenile 

 condition, all types have leaves with prominent lobes and sinuses. 

 In one type whose genetic constitution is II uu, the lobes enlarge 

 at about the same rate as the sinuses, and the mature leaf is 

 deeply lobed and looks like merely an enlarged version of the 

 juvenile leaf. AVhen the dominant allele U is present, as in 

 II U- plants, the sinuses grow somewhat more rapidly than the 

 lobes, and the mature leaf is roundly lobed in outline. In the 

 presence of gene L, as in L- U- and L- uu plants, the growth of 

 the sinuses is very much more rapid than of the lobes and the 

 mature leaves are orbicular (Fig. 89). The presence of L and 

 U in the same plant also appears to produce a greater absolute 

 growth. Studies of cell size in these three types indicates that 

 in the youngest juvenile leaves there are more cell divisions 

 in the sinus regions of the roundly lobed and orbicular types 

 than in the acutely lobed type, and, therefore, there are more 

 cells. Later, during the period of cell expansion, all the cells 

 of a given tissue tend to expand to approximately the same size. 

 Consequently, the leaves which have more cells per unit of area 

 in the sinuses have shallower sinuses w^hen mature and some- 

 times become rounded. The direct action of the gene seems to be 



