1 



440 Haploids and Autopolyploids 



usually more or less at right angles to the metaphase plate, 

 making polar views practically useless, whereas side views are of 



little value because the 2n 

 ^^v^ ^ i , number of chromosomes is 



^^^^/fg r^^^"*^^^/ ^° large that the individ- 



i\ jfy VV V*^^ ^^*^ ^^^ chromosomes become ob- 

 r y^^^ ^ Vk^** scured. In the endosperm, 



Fig. 124. Chromosomes from the ^^ ^^e other hand, only n 



endosperm of Finns Thunbergiana chromosomes are present, cell 



(left) and Thuja onentalis. The first divisions are numerous, and 



has twelve and the second eleven ^^^ chromosomes are readily 



chromosomes. (Redrawn from Sax , , . .., . , 



^nd S^x in the Journal of the Arnold observed m either side or 



Arboretum.) polar views by the smear 



technique (Fig. 124). The 



method is also very useful for the Cycadales when female 

 cones are available. 



Autotriploids 



Autotriploids are organisms that have three identical genomes 

 or, to put it somewhat differently, are primary trisomies for all 

 their chromosomes. Therefore, when they undergo the meiotic 

 divisions we should expect the same configurations for all their 

 chromosomes that we find for one of the chromosomes in a 

 primary trisomic. 



Autotriploids are rare in animals but have been found in a 

 great many plants. Drosophila triploid females with three sets 

 of autosomes and three X chromosomes are not very different in 

 general appearance from diploid females with only two sets of 

 autosomes and two X chromosomes. In general, however, they 

 are somewhat more robust and have larger cells in the wings. 

 Triploids of the male sex are not true males. They are discussed 

 in Chapter 29. 



This generally greater, robustness is a rather constant at- 

 tribute of triploids and is often revealed in plants by a some- 

 what larger size, more vigorous growth, and greater ability to 

 become adapted to a wider environment. Navashin's study of 

 triploids in Crepis affords an interesting example. He found 

 that triploids of three species had increased dimensions of both 

 cells and cell organs. The fruits were enlarged and the entire 



