46 Genetical and Cytological Studies on Pisum 



the chromosomes has been indicated, and not a breakage at the chiasmata 

 {MattMola, Philp and Huskins, 1931). 



The association of four chromosomes. 

 Pollen mother cells of semi-sterile plants usually have five bivalents 

 and a ring of four chromosomes at metaphase. The ring is caused by 

 each chromosome pairing at one end with one chromosome and at the 

 other with another. Exceptions sometimes occur, presumably due to the 

 failure of chiasmata between some of the segments concerned in the ring. 

 Four chiasmata, one for each pair of segments, are necessary for the 

 formation of a closed ring. If chiasmata are present between only three 

 pairs of segments a chain of four chromosomes is formed. This occurs 

 less frequently than the ring. Chiasma formation between two pairs of 

 segments will give rise to a chain of three chromosomes and a univalent, 



^s^ 



Fig. 7. Metaphase plate from a semi-sterile plant 696^/30. The chromosomes which usually 

 form a ring are represented by two univalents of unequal size, x 5000. 



a condition which has not been actually observed, or to two "pairs" of 

 chromosomes, which have been seen in a few instances. When chiasmata 

 are formed between one pair of segments only, six pairs of chromosomes 

 and two univalents result. Such a condition has been observed in one 

 case illustrated in Fig. 7. It will be noticed that one chromosome pair in 

 this figure has two of its ends free, and the univalents are of unequal size. 

 There is little doubt that these are the four chromosomes which usually 

 form the ring. 



The appearance of the ring is determined partly by its orientation, 

 and partly by the number and distribution of chiasmata. The number 

 of chiasmata usually varies from four to six, but in one exceptional case 

 (Fig. 9) as many as ten chiasmata were present. Chromosomes are very 

 often connected by a single subterminal chiasma, but single terminal 

 chiasma sometimes occur. In most rings and chains one pair of segments 

 is united by two chiasmata, one interstitial the other terminal. Fig. 6 a 



