Genetic Factors 



439 



leaves, and its flowers are larger. Haploid plants are universally smaller 

 than diploid ones. Triploids may be intermediate between tetraploids and 

 diploids but are often indistinguishable from the latter. 



Genetic differences may sometimes determine the effect of polyploidy 

 on plant size. Flax ( Linum usitatissimum ) has been selected commercially 

 in two directions, toward the production of linseed oil and of flax fiber. 

 Pandey (1956) compared the tetraploid with the diploid forms of both 

 types of plants in this species and found that in the linseed type the An 

 is a gigas form whereas in the flax type it is actually smaller than the 2n. 

 The linseed tetraploid grows faster than its diploid but the flax tetraploid 

 grows more slowly. The two tetraploids also show certain morphological 

 differences. 



SHOOT 

 APEX 10 



LEAF 10 



SHOOT 

 APEX 10 



LEAF 10 



Q 10 mm. 



Fig. 19-10. Shoot apex and leaf primordium in Zea mays. Left, diploid; right, tetra- 

 ploid. Difference in size is due entirely to larger cells of the tetraploid. (From Ran- 

 dolph, Abbe, and Einset. ) 



The origin of size differences in members of a polyploid series has 

 been studied developmentally in a few cases. The apical meristem of the 

 tetraploid is always broader than that of the diploid. Sometimes it is 

 relatively flat, as in Vinca rosea (Cross and Johnson, 1941), or it may 

 be the same shape as the diploid but doubled in size, as in maize 

 (Randolph, Abbe, and Einset, 1944; Fig. 19-10). In the development 

 of the cucurbit fruit, Sinnott and Franklin (1943) found that the ovary, 

 from primordium to the time of flowering, was almost twice the volume 

 in An as in 2n and thus was proportional to the volume of its cells (Fig. 

 19-11). In these plants, growth of the ovary into the fruit is chiefly by 

 cell expansion. This second phase of growth is much less extensive in An 

 than in In, so that the mature fruit is almost the same size in both, as 

 are the cells of which it is composed. In these cases the flower is "gigas" 

 but the fruit is not. 



