340 PERMANENT HYBRIDS 



differences in the behaviour of different species. He found that the 

 two kinds of pollen grains produced by a heterozygote could often 

 be distinguished by their size and the shape of their starch grains. 

 Often one kind was entirely inactive, or, owing to its smallness, 

 relatively feeble in growth. Again, in some species he found that, 

 instead of the megaspore at the top (micropylar) end of the row 

 of four, formed in maturation, always giving the functional embryo- 

 sac as it does in a homozygous species such as CE. Hookeri, the 

 megaspore at the other end often pushed its way round the upper 

 three and usurped the position of the micropylar megaspore. 

 This competition is known as the " Renner effect." Now, if the 

 first division of the mother-cell is the reduction division which 

 separates the two complexes, it will be seen that this competition 

 between top and bottom cells is in effect a struggle for predominance 

 between the two complexes. Where half the embryo-sacs are 

 formed from the micropylar cell and half from the chalazal cell it 

 is natural to suppose that they are all of one complex type which 

 has passed equally often to the top and bottom pole at the first 

 division. The genetic results agree with these conclusions. (E. 

 muricata, which has half its pollen inactive and half its potential 

 embryo-sacs overridden in competition, produces functional pollen 

 of only one kind, with the curvans complex, and functional embryo- 

 sacs almost all of the other kind, with the rigens complex. Hence, 

 when this species is crossed with a homozygous species such as 

 (E. Hookeri reciprocal hybrids differ, one being a curvans hybrid, 

 the other a rigens hybrid (Table 54). Such a species as this can 

 breed true and with scarcely any loss of fertility, although it must 

 be heterozygous in the same degree as many interspecific hybrids. 



The assumption of the Renner complex, therefore, enables us to 

 describe the phenomena of inheritance and variation in (Enothera 

 in mendelian terms, and as an independent description it is un- 

 assailable. But now its mechanism has been analysed by the study 

 of the behaviour of the chromosomes in the hybrid species. This 

 shows its relationship with other systems and helps us in explaining 

 them. 



(ii) Ring-formation in CEnothera. Cleland (1922 et sqq), first 

 showed that the association of the chromosomes at meiosis has 



