EXTRA CHROMOSOMES — BLAKESLEE 



449 



Probably any other plant equally well adapted to breeding experi- 

 ments would f^ivc similar results when sufficiently investigated. 

 Evidence has been accumulating that select horticultural varieties, 

 which have been kept true to ty^w by vegetative propagation, are 

 frequently characteri/Anl by extra chromosomes. Pending detailed 

 analysis of such forms, it is logical to attribute their desirable char- 

 acters as much to the extra chromosomes which they possess as to 

 any individual factors in res[)ect to which they may differ from 

 the average, 



P^xtra chromosomes in unbalanced types are transmitted to only 

 part of the offspring. Such types, however, can be multiplied by 

 cuttings, grafts, and other vegetative methods of reproduction. By 

 such means horticulturists in the past have unwittingly propagated 

 chromosomal types which were oidy later recognized as such when 

 their chromosomes were studied. It is our belief that in the future 

 extra chromosomes will be consciously utilized as a source of de- 

 sirable variations in })lants of economic importance. 



Table 1. — List of primari<^s, secondaries, and tertiaries arranged by size of 

 chromosomes in the trisomic set 



[Ends of chromosomes are designated by numbers 1 to 24] 



Chromo- 

 somes 

 size class 



Secondary 



chromosomes 



and (2n+2/2) 



type 



Primary 



chromosomes 



and (2n+l) 



type 



Secondary 



chromosomes 



and (2n+2/2) 



type 



Tertiary 



chromosomes 



and (2n+l) 



type 



Oenes located in par- 

 ticular chromosome 



Py (ii). 



RI (1-2). 



Sg(2-2). 



Sm (3-3). 



08(3-4). 



1 St (5-5). 



M Un(7-7). 



M" Mt (9-9). 



Bk (5-6). 



£1(7-8)... 

 Ec(9-10°). 



Wd (ll-H).. 



Ck (1112°). 



At (6-6) 



'Th'(Td''To»)'. 



DS=2-17 



Wy=M8. 

 ES = 2-9. 

 Ph = 2-5. 

 ng=l-9. 

 trSg=12-ll-2. 



Mp=4-6 



(3-21). 

 (4-22). 



Mp = 4-6 



Ph = 2-5. 



Not named , Mc (1314) j Not named (14-14). 



(13-13). 



So (1615) , Rd (15-16).. 



Df (17-17).... Pn (17-18). . 



SE=9■20''.... 

 (10-19). 

 ES = 2-9. 

 Hg=l-9. 

 trSg=12-ll-2. 

 (11-21). 

 (12-22). 

 X-18-13 



Dv (19-19).... 



Sp (19-20°). 

 01 (21°-22). 



Ix (23-24)... 



DS-2-17 



Wy-1-18. 

 X-18-13. 



SE-9-20° 



(1019). 



trli- (20-19-23). 



(3-21) 



(4-22). 



(11-21). 



(12-22). 



trlx- (20-19-23). 



p in Wy. 



fw. Bz in 4 half. QS in 

 Us, half not yet de- 

 termined. 



MS in either Ec or Sp. 



in, tr, e, sky, in 12 half. 



to in 16 half, 

 c, wt in 17 half; p in 18 

 half. 



sb. MS in either Sp or 

 Ec. 



sc, bb, pi. 



The ends 10°, 12°, 20°, and 21° are characterized by terminal humps. 



