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THE AMKRICAX X. I TCl^ALIST [VOL. LV 



from the homozygous dominants by different intensities 

 of pigmentation. Simplex heterozygotes when selfed 

 throw offspring with 5 dominants to 4 recessives among 

 the normals, and 7 dominants to 2 recessives among the 

 Poinsettias; while duplex heterozygotes should give a 

 ratio of 8 : 1 among the normals, and all dominants among 

 the Poinsettias. There is evidence which seems to indi- 

 cate that the mutant Oocklebur is conditioned by dupli- 

 cation in the chromosome set which carries both the fac- 

 tors for spiny capsules and also those for number of 

 nodes. If this is actually the case, we must assume that 

 these two factor pairs are loosely linked in the same 

 chromosome with about 50 per cent, crossing over, since 

 they ap]iear to segregate independently of each other. 



^yv have been discussing duplication of a single mem- 

 ber in Olio of the 12 chromosome sets. On the lower 

 I)art of the chart (Table II) is represented the only plant 

 we have yet found with an extra chromosome in every 

 one of its 12 sets. Such a plant is triploid. What its 

 breeding behavior will be, can not be told before another 

 season. If the chromosomes assort at random, the 

 gametes theort'licnlly should have the chromosome num- 

 bers indicated in the chai t. and the counts which my col- 

 league, Mr. In'liini;', has made from figures in pollen 

 mother cells are not inconsistent with the distribution 

 of the theoretical frequencies. One might expect such 

 triploid plants to give rise to individuals intermediate 

 between triploids and mutants of the Poinsettia type ; in 

 other words to mutants with duplication of chromosomes 

 in 2, 3, 4, etc., up to duplication in all the 12 sets. Such 

 compound mutants we have not yet been able to surely 

 identify in our cultures; but we have never before this 

 past season had a triploid plant, which from the wide 

 range of gametic types in its egg cells would seem a 

 likely source of such mutations. 



Tetraploid plants have been disciisse<l at > csterday 's 

 session of the Botanical Society of Aiiieric;i. They rep- 

 resent a further duplication over those of tlie triph)i(ls 



