CHROMOSOME NUMBERS AND THEIR ALTERATION 



20- 



tetraploid nucleus. When the influence of the drug has declined suffi- 

 ciently for normal nuclear and cell divisions to occur, development of the 

 tissues continues with the altered chromosome number. 



The characters exhibited by tetraploid plants derived by doubling 

 as described above often serve to distinguish them from their diploid 

 relatives (Figs. 152, 155). In many instances they are stockier in habit. 



wXwi 



Fig. 153. — Stoniates and epidermal cells of diploid, tetraploid, and octoploid Nicotiana 

 hybrids. {After W. H. Greenleaf.) 



darker green in color, bear larger flowers and seeds, and have larger nuclei 

 and cells. With a hand lens one can often tell the polyploids from the 

 diploids by the size and arrangement of the stomates (Fig. 153). In 

 mature vegetative organs such as leaves the cells may be larger and fewer 

 than in the diploids; sometimes they are not. In the meristematic 



W^,e#-* 



* .' 



Fig. 154. — Synaptic configurations in heteroploid plants, a, trivalent at zygotene in 

 triploid tulip, b, postdiplotene trivalent in triploid tulip, c, quadrivalent at zygotene in 

 tetraploid hyacinth, d, postdiplotene quadrivalent in pentaploid tulip. {After TV. C. F. 

 Newton and C. D. Darlington.) 



tissues of maize the number of cells is the same as in the diploids, but 

 with the greater cell size is associated the development of larger organs. 

 There are also physiological differences between diploids and tetraploids. 

 Some tetraploid plants are able to grow well in a wider range of ecological 

 habitats than the related diploids. Fruits borne on tetraploid tomato 

 plants derived from dii:)loids have a higher vitamin C content than those 



