EXTRA CHROMOSOMES BL.\KESLEB 447 



Class 7 are the tertiary (2n+l) types formed by segmental inter- 

 change between nonhomologous chromosomes. Their ends therefore 

 are from different primary chromosomes. Judging from interchanges 

 involving three chromosomes, it appears prol)able that more than two 

 chromosomes may take part in the ft)rmation of a tertiary chromo- 

 some. Moreover, the fact that the interchange in several cases is 

 known not to involve exact halves of the two chromosomes involved 

 is furtiier evidence that the number of tertiaries theoretically possible 

 is much larger than the 2G4 given in the table. A considerable num- 

 ber of chromosomes have been identified as tertiaries but tlie mor- 

 phological characters of eight (2n+l) tertiary forms have been 

 studied in cultivation. 



Class 8 has two extra chromosomes but they are both in the same 

 set. The (2n + 2 Gl) shown in Plate 9 is an example. 



Class 9 also has two extra chromosomes in the same set but one 

 of these extras is a primary and the other a secondary. 



Class 10 includes the double (2n + l + l) types which have two extra 

 primary chromosomes in different sets. Examples have been given 

 in Plates 11 and 12. 



Class 11 comprises the double types which have extra chromo- 

 somes in different sets, one of the extras being a primary and the 

 other an unrelated secondary chromosome. 



Class 12 comprises the double types in which one of the extras is 

 a primary and the other a tertiary chromosome. 



Class 13 comprises the rare group in which there are three extra 

 chromosomes, each in a different set. One form of this type has 

 been surely identified by its chromosomes. It was too feeble in 

 growth to give offspring. 



Class 14 comprises types in which one chromosome has been 

 dropped out. Such types would be expected only as abnormal 

 branches on otherwise normal plants. A number of deficiencies of 

 this kind have been found but so far as analyzed they have been 

 caused by a dropping out of one of two chromosomes (Rl or Pn). 

 Apparently a sex cell (In) can not live unless all the 12 different 

 chromosomes are present. Hence, deficiencies would not be expected 

 directly from sexual reproduction. 



Class 15 is similar to class 14 in that one branch is a chromosomal 

 deficiency. In class 15, however, the plant begins as a secondary 

 (2n+2/2) type and the chromosome that is eliminated is a pri- 

 mary related to the secondary chromosome. Thus a " Dwarf " type, 

 which has a 17 • 17 chromosome in addition to two 17 • 18 chromo- 

 somes, lost one of the 17 • 18 chromosomes in forming an abnormal 

 branch. 



Class IG includes the compensat ng types in which the compensa- 

 tion is between a secondary and i tertiary chromosome; while in 

 class 17 the compensation is between the two tertiaries. 



