20 



THE AMERICAN NATURALIST [Vol.LVI 



Of the balanced forms there are even-balanced or 

 stable, and odd-balanced or unstable types. In the even- 

 balanced diploid, which is the normal jimson weed, the 

 two chromosomes in each set go to opposite poles by the 

 ordinary process of disomic reduction, and the plants 

 breed true for chromosome number. Partly for the same 

 reason, the even-balanced tetraploid, which is our " New 

 Species," breeds essentially true. The triploid, on the 

 other hand, is odd-balanced and therefore unstable, since 

 in the trisomic disjunction in each set two of the three 

 chromosomes go to the one pole and one to the other, the 

 process taking place at random. Through the operation 

 of chance, therefore, gametes of different chromosomal 

 number will be formed, and simple and double mutants 

 as well as diploids will occur in the offspring. The rela- 

 tion may be seen from the pollen of the three balanced 

 types under the same magnification (Fig. 3), where the 

 photograph at the left (a) shows a field of pollen from a 

 diploid; that at the right, (c) with larger grains, pollen 

 from a tetraploid ; while that above (h) shows pollen from 

 a triploid. Pollen from a triploid is not only character- 

 ized by a large proportion of empty grains, but also hy a 

 great diversity in the size of the grains brought about 

 by the differences in the number of chromosomes which 

 they contain. 



Theupi.cr left ban.] Ii,-uiv of th.' unbalanced types (Fig. 

 2) has one rxtra clii-dinosoinc in \\\v lower right-hand set, 

 indicated by the arrow, giving 1 trisome, and 11 disomes 

 in this nucleus, and its formula may be written {2n -f 1). 

 Such a simple mutant is the Globe— simple because only 

 one set is affected. If another set has the extra chromo- 

 some-say the set on the right-instead of the one with 

 the arrow, this extra chromosome would cause the plant 

 to assume the characters of, sav, the Coeklebur nuitant. 

 It is ol)vions that since tliero aro V2 s(^ts in Dafura and 

 eael,s..t,navhav..an.xtra.hronH..oHH..th.n.a,vlJnn,- * 

 tants with tin. h.nnnla (iJ. 1 ) tl.Mnvticallv ,,ns.,hi.. 

 Through the process of disjunction in thes. V2 mutants, 

 half of the gametes should contain the extra chromosome, 



