82 THE CHROMOSOMES 



of the chromosomes reduplicated in other members 

 of the chromosome set ; in these cases the small 

 reduplicated sections may pair at zygotene and 

 form chiasmata later, so that there are a few bivalents 

 at the first meiotic division. ^i Plants like this are 

 clearly not true haploids ; they are really inter- 

 mediate between the diploid and the true haploid 

 condition. 



Meiosis in Polyploid Organisms 



Where there are more than two homologous 

 chromosomes of each kind in the somatic set three 

 or more may become paired at zygotene — but never 

 more than two at any one point. Thus in a triploid 

 if we consider three homologous chromosomes Aj, 

 Aa and A3, Ai may pair with Ag in one region and 

 with A3 in another, but Aj, A 2 and A3 are never 

 associated together in the same region. ^^^ On the 

 other hand, Aj may pair with Ag throughout its 

 entire length (to form a bivalent) in which case A3 

 will be left unpaired and form a univalent. Thus in 

 a triploid organism trivalents, bivalents, and univa- 

 lents may be found in the same nucleus. Similarly, 

 in a tetraploid quadrivalents may be found in addi- 

 tion to the three other types and in higher polyploids 

 quinquevalents and hexavalents may also occur. All 

 associations of more than two chromosomes can be 

 spoken of collectively as multivalents. The frequency 

 of formation of multivalents in polyploids varies a 

 great deal and apparently depends in part on the 

 length of the chromosomes and in part on the 

 rapidity of zygotene pairing. ^'^ Where the chromo- 

 somes are long and pairing is slow the probability of 

 multivalent formation is high, where the chromo- 

 somes are short and pairing is rapid it will be low. 

 Other factors such as the ratio of the volume of the 

 chromosomes to the volume of the nucleus and the 

 arrangement of the chromosomes at zygotene (ran- 

 dom or polarized — see Chap. IV) also affect the fre- 



