APOMIXIS 



The evidence of variation is worth returning to. It has been 

 revealed most comprehensively in some apomictic stocks of common 

 descent — for we must not give them the name of race or species 

 intended for biparental groups — in Taraxacum. The range is not 

 unlike that in Oenothera. Some of the variation is attributable to the 



oc^8 



Species in Tap.axacum 



EXUAL 



A 

 p 

 o 



M 

 I 



C 

 T 



I -subsejiiUQl- I 



\ 



\ 



(5x;-(A^BK:))erc. 



3ac-A 



Fig. 68. — The origin of apomictic from sexual species in Taraxacum and their 

 subsequent Umited and irreversible evolution under conditions of sub-sexual repro- 

 duction, i.e. by recombination, loss and polyploidy at the suppressed meiosis. The 

 unbalanced forms are homologous in different triploid series and are increasingly 

 dwarfed with increasing unbalance (based on Sorensen and Gudjonsson, 1946). 



mere effects of the crossing-over without reduction seen at the 

 suppressed meiosis : the variants have the normal triploid complement 

 of 24. Others are due to grosser aberrations. Chromosomes are 

 lost to give types with 23, i.e. one short. And these are recog- 

 nizably of the 8 expected kinds, thus giving 8 parallel variants in 

 each group (Fig. 67). Or again both meiotic divisions may fail 

 so that the chromosome number is doubled and plants with 48 



267 



