Criteria for Judging Polyploidy 363 



After the amphiploids in Triticinae were produced in such large num- 

 bers, it was demonstrated that both monocotyledons and dicotyledons 

 were being doubled by the use of colchicine. 



A barrier in plant breeding had been removed or considerably 

 reduced by the discovery of a ready technique for making sterile hy- 

 brids fertile and estimating the effectiveness by seed production. In- 

 compatibilities such as failure to make hybridizations must now be 

 overcome. Some work on embryo culture has been used to excellent 

 ad^■antage. 



15.2: Appearance of Polyploids 



New leaves and stems that grow from treated sectors are usually 

 wrinkled, thicker, and darker green, and have coarser texture, as 

 compared with the untreated plants.^' '• ^^ An increase in thickness of 

 the tetraploid leaf can be judged by holding the leaves between 

 thumb and forefinger. By such methods a rough sorting of tetraploids 

 can be made among large populations of treated cultures. Those 

 that have not responded can be quite accurately eliminated. 



Specific marks on the leaves such as veins, hairs, and glands are 

 valuable references for the first sorting of possible changed types. 

 The outline of the leaves changes; they are usually shorter and more 

 rounded than the diploid leaves. 



Flowers of the tetraploid plants are larger (Fig. 15. IB) and more 

 compact than the diploid (Fig. 15.1/4). These changes were corre- 

 lated with chromosomal determinations (Fig. 15.1C,D). Tetraploid, 

 triploid, and diploid flowers form a decreasing series in size of flower. 

 These proportionate changes are illustrated for watermelon strains. 

 At the tetraploid level, optimum size is reached, and beyond that 

 point the increase in sets of chromosomes actually reduces the size 

 of the flower. Among the best varieties of Iris, polyploids are favored 

 over diploids.^" The increase in size of flower has been a goal for the 

 improvement of ornamental species. 



A tetraploid plant has a more rugged appearance, looks sturdier, 

 and has certain giant-like features. Usually the rates of growth are 

 slower, but even the final growth does not produce a plant as tall as 

 the diploid. Among polyploid watermelons, the vine remains green 

 over more days than among diploids, disregarding disease factors. 

 Another difference between the stems of diploids and those of tetra- 

 ploids is the shape of the apex as viewed in longitudinal section (cf. 

 Chapter 14) . 



15.3: Fruit and Seed 



The development of larger seeds from tetraploid lines is a con- 

 sistent macroscopic characteristic that has been confirmed for hun- 



