160 THE THEORY OF THE GENE 



that are balanced, all the chromosomes are united in pairs 

 (gemini), while in those polyploids with odd numbers and 

 even in some of those with even numbers (taken to be hy- 

 brids) only 7 (or 14) gemini are present, the rest of the 

 chromosomes being single in the first maturation division. 

 In other words, when there are four, six, or eight chromo- 

 somes of each of seven kinds they conjugate in twos, as 

 though these types were diploid. Whatever their origin 

 may have been, the chromosomes never conjugate in 

 fours, sixes, or eights. In these polyploids, the conjugants 

 separate at the first maturation division, half going to 

 each pole. At the second division each chromosome di- 

 vides, and half of each goes to one or the other pole. The 

 germ-cells, whether pollen or ovules, thus come to con- 

 tain half the original number of chromosomes. Hence, if 

 they propagate sexually, the characteristic number is 

 maintained. 



Another group of roses is regarded as hybrid by Tack- 

 holm, because the changes that take place in their germ- 

 cells show them to be unstable forms. Some of these have 

 21 chromosomes, hence are triploids. In the early matu- 

 ration stages of the pollen mother cells there are 7 biva- 

 lents (gemini) and 7 single chromosomes. At the first 

 division the 7 bivalents divide and 7 go to each pole; 

 the 7 single chromosomes do not divide and are distrib- 

 uted at random to the poles. Hence several combinations 

 are possible. The type is unstable in this respect. At the 

 second maturation division, all the single chromosomes 

 divide, whether they come from the earlier bivalents or 

 from single chromosomes. Many of the resulting cells 

 degenerate. 



In other hybrids there are 28 chromosomes (4 times 7), 

 but these are not classified as true tetraploids by Tack- 

 holm, because the behavior of the chromosomes at the 

 time of conjugation indicates that there are not four of 



