CYTOLOGICAL ASPECTS OF HYBRIDITY 



221 



from one parent with one of the other is hkewise allotetraploid.) Amplii- 

 diploid hybrids are of special importance because they are usually fertile, 

 occur rather widely among angiosperms in nature, afford clues to the 

 relationship of certain species, and open a new path to the improvement 

 of cultivated plants. 



Amphidiploidy commonly arises through a doubling of the chromo- 

 some number in the somatic cells of a diploid hybrid. Such doubling may 

 occur spontaneously, and in numerous cases it has been induced with 



Fig. 164. — Hybridity and polyploidy in Calendula. From right to left: flower heads 

 of C. suffruticosa, C. officinalis, a diploid officinalis X suffruticosa hybrid, a tetraploid 

 officinalis X suffruticosa hybrid resulting from colchicine treatment. {Courtesy of C. 

 Weddle.) 



colchicine. If it takes place in the zygote very soon after syngamy, the 

 whole plant is amphidiploid; if later, e.g., in a bud, only a branch or other 

 portion shows this condition. The most valuable feature of these plants 

 is the increased fertility that many of them show over the diploids from 

 which they arose. It is largely in the hope of conferring some degree of 

 fertihty upon sterile hybrids with desirable combinations of characters 

 that the doubling technique is applied in plant-improvement programs, 

 and in many cases the results have been successful. In addition, the 

 plants obtained may show certain characters associated with tetraploidy 

 itself, such as sturdier habit or greater flower size (Fig. 164). They also 

 frequently exhibit pronounced hybrid vigor. 



