APOMIXIS 



Here a plant with 42 chromosomes can give progeny with 63 and 

 84 as well as 42 ; and a plant with 84 can give progeny with 42 and 

 63 and nearly 126 as well as with its own number. One plant with 

 72 even gave a vigorous seedling with 18. It is not surprising, there- 

 fore, that a world collection of 56 plants of Poa pratensis revealed 

 such a distribution as that shown in Table 24. It follows a normal 

 curve, produced evidently by continual variation from a mode 

 between 60 and 70. 



TABLE 24 



THE WORLD FREQUENCY DISTRIBUTION OF CHROMO- 

 SOME NUMBERS IN POA PRATENSIS (HARTUNG, 1946) 



Chromosome number 



40 50 60 70 80 90 



Frequency 



13 



25 



12 



This is an extreme instance of versatile reproduction, but great 

 numbers of normally sexual species are known in which exceptional 

 seeds reveal the same principle, since they contain twin embryos. 

 One of these embryos is usually a normal sexual diploid, while the 

 second is usually a haploid or a triploid which has arisen illegiti- 

 mately in competition with its lawful twin (Fig. 69). These examples 

 show that many plants, perhaps indeed individuals in nearly all 

 species of flowering plants, have the capacity for evading meiosis or 

 fertilization or both, and thereby giving rise, in the right circum- 

 stances of sexual sterility, to the obligatory apomict which cannot 

 do otherwise. 



In the normal sexual system, the hybridity optimum is related 

 to the mode of breeding. The inbreeder has low hybridity, the 

 outbreeder high hybridity, and the breeding system is genetically 

 adjusted to secure these ends. This relation between hybridity and 

 breeding system can be broken only by such extreme devices as 

 complex hybridity or the abandonment of sexual reproduction. 

 Complex hybridity and apomixis must have been derived from the 

 normal sexual system, within which we can also see that the 

 inbreeding system may replace outbreeding. What is it that 

 determines these changes ? What advantages do inbreeding and 



269 



