36 Oenetical and Cytological Studies on Pisum 



work has confiimed the occuirence of an association of four chromosomes 

 in the semi-sterile plants, whereas fertile sister plants are found to have 

 the usual seven bivalents. A more intensive study of the cjrfcology of 

 these ring-forming types has revealed the occurrence of other arrange- 

 ments of the four chromosomes than those described in the earlier note, 

 and it is felt that a more complete description of the cytology of these 

 plants is desirable. 



Methods. 



Whole flower buds were fixed by being placed for 30 seconds in 

 Carnoy's fixative and afterwards left for 18-24 hours in La Cour's 

 modifications of Flemming's solution, 2B and 2 BE (La Cour, 1929). The 

 fixative 2B gave the better results. Sections were cut at 12 to 18ju, and 

 stained with Gentian Violet iodine. 



Observations. 

 Somatic chromosomes. 



The somatic chromosomes of several strains of Pisum have been 

 studied by Dr C. D. Darlington. Fig. 1 shows the somatic chromosomes 

 of (a) a cultivated variety, (&) Th. 10, and (c) a fertile Fg plant from a cross 

 between Duke of Albany and Th. 7. The morphology of the chromosome 

 complement is different in the different forms. The type (a) has a pair 

 of chromosomes with a trabant, whereas the other two forms have no 

 trabants. Secondary constrictions are also present in at least two pairs of 

 chromosomes in the type, but they cannot be distinguished in the other 

 plates. 



Meiosis. 



It has not yet been possible to make an intensive study of prophase 

 and diakinesis stages in Pisum; the observations have been confined to 

 metaphase and later stages. The pollen mother cells of various plants of 

 Pisum have been examined, including types and rogues, F-^ plants from 

 crosses between types and rogues, F^ plants from crosses between Th. 10 

 and Early Giant, and F^ and ^4 plants from crosses between Th. 7 and 

 Duke of Albany. With the exception of semi-sterile F3 and F^ plants 

 from the crosses involving Th. 7, the plants examined have seven pairs 

 of chromosomes at the first meiotic diAdsion. The semi-sterile plants have 

 five pairs of chromosomes and an association of four, whereas fertile 

 sister plants have seven pairs. The chromosome pairs have certain 

 characteristic shapes, a fact which has been previously discussed by 

 Hakansson (1931) in his study of chromosome association in crosses 



