SECONDARY PAIRING 237 



Further, they are spherical at meiosis, and so should oppose less 

 resistance to movement through a viscous medium than when they 

 are long, as in mitosis (Lawrence, I.e.). Secondary pairing is not 

 found at meiosis in plants whose chromosomes are still long at this 

 stage (e.g., in Tiilipa). On the other hand, somatic pairing is 

 shown by small chromosomes when the larger ones do not show it 

 (Upcott, 1936). 



Secondarily paired bivalents are distinguished from multivalents 

 associated by chiasmata in four ways : (i) they lie evenly side by 

 side, in twos, threes or larger groups according to whether the 

 plant is a tetraploid, hexaploid or higher polyploid ; (ii) they 

 never " touch," except through collapse in fixation ; (iii) they 

 separate regularly into their daughter halves without interfering 

 with one another ; (iv) they show no association at the preceding 

 diakinesis (D., 1928 ; Lawrence, 1929). 



The distinction has a profound theoretical importance. It was 

 formerly held that the characteristic pairing of chromosomes 

 at meiosis was merely an exaggeration of the somatic pairing 

 observed at mitosis. But chiasma pairing and " somatic " pairing 

 are now seen side by side at meiosis, and the differences in their 

 effects are as clear as the differences in their causes. These will be 

 discussed later in more detail. In many polyploid species, where 

 the two phenomena occur side by side, it is of great importance 

 for genetic interpretation to tell one from the other, for chiasma 

 pairing indicates a closer relationship than secondary pairing. It 

 will have been seen that, while the separation of diploid and polyploid 

 is a convenient one, every possible degree of relationship must be 

 found within the gametic sets of different organisms. There must 

 be polyploids so ancient that the original relationship between their 

 chromosomes has been lost so far as pairing analysis can reveal 

 it. There must also be diploids having parts of their chromosomes 

 reduplicated, and therefore having internal relationships not unlike 

 those of polyploids. In fact, the behaviour of haploids, with other 

 considerations, makes it probable that this is true of all diploids in 

 varying degrees (t;. Ch. VIII). It follows, therefore, that aU slight 

 unevennesses of distribution of chromosomes on the metaphase plate 

 might conceivably be interpreted in terms of genetical relation- 



