24 



THE AMERICAN NATURALIST [Vol.LVI 



cultures, is a line of argument applicable not alone to the 

 pair Wiry and Poinsettia. It also leads us to consider 

 Rolled an acolyte of Sugarloaf, and Strawberry an aco- 

 lyte of Buckling. The possibility of acolytes being caused 

 by modifying Meiidelian factors is being investigated. 



1 have said that the Poinsettia nnitant ga\e curious 

 color ratios in its offspring (4 and i^, tal)l(' _). Tlie evi- 

 dence seems conclusive that Poinsettia has its extra clirc^- 

 mosome in the set which carries the fa('t(>i-> i or piii-pIc and 

 white flower color. A heterozygous I'oiii-cnia !iia>- liave 

 one dose or two doses of the dominant pui'iilc tlowcr color. 

 The offspring of Poinsettia (like th()>c r,r t lir ( I Inl. ■ i, it will 

 be remembered, are part normals and part nintatits. If 

 a Poinsettia parent is duplex for ])nrplc, its iioinial olT- 

 spring show 8 purples to 1 white, while its i'oln-ettia 

 offspring are all purples. Tf the Poinsettia parent is 

 simplex for purple the ratio for the in>nnal oft^i)ring is 

 5 purples to 4 whites, and for Poinsettia offspring is 7 

 purples to 2 whites. The back crosses are also distinc- 

 tive. By similar reasoning we believe the Cocklebur mu- 

 tant has its extra chromosome in the set which carries 

 genes for presence or absence of spines on the capsules. 



The evidence is especially good for Poinsettia, since the 

 color classes can be recognized in the seed])an. Tsino' a 

 Poinsettia which arose in a purple line t'rotti Wa^hiimton. 

 I). ( '.. we ci-ossi^d it with a white line ot' similar ap|)ear 

 aiice also from Washington, and, without going ontside of 

 the>e Iwo lines, have synthesized Poinsettias of all the 

 possi])le com])inations of color factors and have made 

 nearly all the possible combinations of crosses between 

 them. The results with tlie Washington lines are in ac- 

 cord with what would be exjiected from a i-andom assort- 

 ment of 3 chromosomes in the set containing the i)urple- 

 white color factors. In a certain group of Poinsettias 

 simplex for purple in which the J chromosomes bearing 

 the white factor nii-hi ha\e brought in, so far as we 

 knew, either from the white Washington stock, or from a 

 distinct white line from Mrlnrt, (l.-rinany, the color ration 

 in the offspring of some {>a rents were according- to cnleii- 



