418 Morpho genetic Factors 



Large pumpkins differ from small gourds, for example, simply because 

 they grow for a longer time (Sinnott, 1945b, and p. 16). The actual 

 growth rate of these two varieties in terms of compound-interest growth is 



the same. 



Inherited size differences are also related to cellular characters (p. 34 ) . 

 Most of them are due to differences in number rather than size of 

 cells, large size being the result of more cell divisions during develop- 

 ment. Less frequently the period of cell expansion is more extensive in 

 the larger forms and their cells are consequently larger, though usually 

 not in proportion to body size. 



In the many cases where there are inherited differences in cell size, 

 it is usually not the size of the meristematic cells that is different but the 

 amount of increase that occurs after division ceases. Thus the fruits of 

 large races of pumpkins have much larger cells than do small gourds, 

 but this difference is not evident at the meristem. Some of it appears in 

 the growth of the young ovary but most of it during the enlargement of 

 the ovary in fruit development (Sinnott, 1939). Sugar beets have much 

 larger cells (and leaves) than do table beets, but only in their mature 

 structures. The meristematic cells are much the same size in both. 



Many cases have been found in which there is not a gradation be- 

 tween large and small types but the small ones are so markedly dif- 

 ferent as to be regarded as somewhat abnormal dwarfs. In most such 

 plants there is a single gene difference from normal which seems to 

 control one important growth factor. A number of these occur in maize, 

 and the auxin relations of some have been worked out (p. 377). Some 

 dwarfs are small-celled but a few have cells larger than those of normal 

 plants. There are also a number of gigas forms which are due to gene 

 differences. Large and small types are also frequently related to chromo- 

 some number (p. 438). 



Differences in height may result from mutations that alter a determi- 

 nate type of growth to indeterminate. These have been found, for ex- 

 ample, in tobacco (Jones, 1921) and maize (Singleton, 1946). Each 

 shows single-gene segregation with normal determinate plants. The dif- 

 ference between bush and climbing varieties of beans, also due to a 

 single gene, is really a difference between determinate and indeterminate 

 growth. 



Another important effect of genes on size is to be seen in cases of 

 hybrid vigor or heterosis. The Fi plants, in crosses between parents that 

 are homozygous or essentially so, are often much larger and more vigor- 

 ous than either parent (Fig. 17-1), and this fact has wide economic ap- 

 plication, especially in maize. The difference is closely associated with 

 heterozygosity and disappears with inbreeding. Various suggestions have 

 been made to account for it— the stimulating effect of the heterozygous 



