440 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



in reference to this single highly inbred line. The two interchanged 

 chromosomes, when added to line 1 diploids (2n), produce (2n+l) 

 tertiary forms. " Wiry " (pi. 10, fig. 2) is a tertiary with the 1 • 18 

 chromosome extra and " Dwarf Sugarloaf " (pi. 10, iig. 1) is a ter- 

 tiary with the 17 • 2 chromosome extra. 



Three other types of races have been found in nature in which 

 two chromosomes differ from those in line 1 by interchange of seg- 

 ments. Each of these races should give two tertiary (2n+l) types 

 when properly combined with our standard line 1. 



In Table 1 (p. 449) are summarized the (2n + l) types thus 

 far identified, together with the factors (genes) already located in 

 particular chromosomes. The chromosomes are recognized microsco- 

 pically first by size and designated by the initials of tlie adjectives 

 Large (very), large. Medium ( + ), medium (-), Small and small 

 (very). Special modifications (humps) serve secondarily to fur- 

 ther distinguish chromosomes of the same size. 



In mutations called primary (col. 3) an extra chromosome is 

 added to 1 of the 12 ; this is like the other two of the group of three 

 (trisome) thus made. Its two ends are designated by numbers. 

 In mutations called secondary the extra chromosome is made up by 

 the doubling of only one-half, or terminal portion, of a chromosome. 

 In column 2, this half chromosome is indicated by the odd number 

 of the numbered ends of the primary chromosomes. Complementary 

 to this is the extra chromosome made up of the two even-numbered 

 ends. (Col. 4.) 



In mutations called tertiary the extra chromosome is made up of 

 united end portions from two nonhomologous chromosomes. These 

 are listed in column 5 ; each one twice. 



Secondaries have been discovered for all except the smallest two 

 chromosomes. For four chromosomes, both the primary and its 

 two complementary secondaries are known. (Cols. 2 and 4). The 

 tertiary chromosomes DS, Wy, and SE have been isolated from chro- 

 mosomal races found in nature, such as the " B " race discussed in 

 an earlier paragraph. They originated presumably through seg- 

 mental interchange, as also probably the tertiary chromosomes Mp, 

 ES, and X which were formed in cultures of our standard line 1. 

 The other named tertiaries have been induced by radiation treatment. 

 Of the tertiary chromosomes represented by figures in parentheses, 

 the ends are known, but their effects upon the plant, when present as 

 extras, have not been adequately studied. After a little practice the 

 primary, secondary, and tertiary (2n+l) types are easily recognized 

 without microscopic examination by the effects upon various parts 

 of the plant brought about by the presence of the extra chromosomes 

 or new combinations of fragments of chromosomes. 



