EXTRA CHROMOSOMES — BLAKESI.EE 437 



the white set. Those of Rolled and Su<rnrloaf are taken from Bell- 

 ini's fifjures, tliose of Polycarpic w^ere drawn by Doctor Berp^ner. 

 Since only like ends may become attached, the trisome of secondaries 

 may form a closed rinp: of three, but the chromosomes of a primary 

 never can form such a closed conli^uration. 



Plate G A, li, and (' shows mature plants, capsules, and seedlings, 

 of the three (2n+l) types under discussion. 



If we compare the primary Rolled with its two complementary 

 secondaries, Sugarloaf and Polycarpic — all three the result of single 

 extra chromosomes in the white set — we will note that each secondary 

 is extreme in certain characters while the primary i.s intermediate. 

 Thus, in Sugarloaf the capsule is large, a peculiarity due to the un- 

 balance brought about by two extra unshaded halves of the white 

 chromosome. In Polycarpic, the other secondary, the capsule is 

 small, a fact resulting from the presence of two extra shaded halves. 

 The primary Rolled, having extra both the shaded and the unshaded 

 half of this same white chromosome, is intermediate in capsule size 

 and shape. 



Similarly from the habit photographs (pi. 6) it will be seen that 

 the secondary Polycarpic is erect and has very narrow leaves. The 

 complementary secondary Sugarloaf is spreading and has broader 

 leaves. The primary Rolled is intermediate in respect to both these 

 characters. 



In a similar manner in the stippled set, we have a primary type 

 called " Poinsettia " (pi. 7, fig. 1) due to an extra stippled (17- 18) 

 chromosome and a secondary type called " Dwarf " (pi. 7, fig. 2) due 

 to the presence of an extra made up of two shaded halves (17 • 17) of 

 (he stippled chromosomes. The secondary with an extra made up of 

 two unshaded halvas of the stippled chromosome (18 18) has 

 not yet been discovered. Possibly it is not viable. From the char- 

 acters of the primary wc know something of the factors in the whole 

 stipi)led chromosome and from the characters of the single second- 

 ary (17- 17), we know" something of the factors in the shaded half. 

 By subtraction we can gain some idea of what characters the miss- 

 ing secondary should show when discovered and hence of what the 

 factors are in the unshaded (18) half of the stippled chromosome. 



Photographs of the capsules of the 12 primary (2n + l) types in 

 the Jimson weecl are given in Plate 8 below that of a normal (2n) 

 capsule. It will be noted that all are different. Capsules of the 

 secondaries are distinct from those of their jirimaries as well as from 

 ihose of other secondaries. Extra chromosomes have similar effects 

 upon other parts of the plant. E.xtra chromosomal material thus 

 bi-iiigs about distinct and sj>ecific changes in the appearance of the 

 plant in which it is present. This it does because of the genetic 

 2.8005—31 29 



