108 CARNEGIE INSTITUTION OF WASHINGTON. 



plates tend to have an equal number, but inequality is not uncommon^ 

 and the rarer the greater the inequality. It also appears that homolo- 

 gous chromosomes are attracted, even if each group has an abnormal 

 number, like 3 or 4. 



When triploid is crossed by normal diploid the offspring are peculiar 

 in respect to assortment in the metaphase of pollen-mother cell. Thus 

 in 24 plants the chromosomes separated as 12:12; in 31 plants as 

 12 : 13; in 10 plants as 12 : 14 or 13 : 13. Since one might expect the 

 chromosomes to separate as 15 : 15, it is clear that many chromosomes 

 must have been eliminated somewhere in the process of hybridization. 

 Such eliminated chromosomes are found in the pollen-mother cells 

 and form micronuclei, which come to lie in microcytes. Of the few 

 progeny of the numerous crosses of tetraploid by normal where the 

 assortment 18 : 18 is to be expected, in five cases it was 12 : 12; once 

 12 : 13, and in three cases it ranged from 12 : 24 to 18 : 18. Here 

 again chromosomes must have become lost. There is a strong ten- 

 dency to restore the normal chromosome number. The progeny of 

 triploid by tetraploid mutants were few. Three plants assorted 

 24 : 24 or 23 : 25; one plant gave 23 : 24. The expected 21 : 21 was 

 not found. Here again internal adjusting factors are present. 



An attempt has been made to arrange mutants in order of size of 

 the trisomic group upon which each mutant depends. The result is 

 tentative and is as follows: Mutilated (largest), Glossy, Buckling, Cock- 

 lebur, Sugarloaf, Spinach, Microcarpic, Ilex, Wiry, Poinsettia, Globe, 

 Reduced. Rolled and Elongated have not yet been included. 



A beginning has been made in the determination, in trisomic mu- 

 tants, of the average numbers of microcytes with eliminated chromo- 

 somes; and also the numbers of double-sized pollen-grains with a 

 double number of chromosomes. Also, the difference in nature of 

 the extra chromosome in the simple trisomies is being correlated with 

 the range of variation in size of the mature pollen-grains, with normal 

 diploids, triploids, or tetraploids; the volume of the pollen-mother 

 cells, or of the pollen-grains, has been shown to be proportional to the 

 number of chromosomes; but it has not yet been shown that this rule 

 holds when only one chromosome is doubled. Of the 12 possible 

 pentasomic tetraploids, only a few have been as yet studied cyto- 

 logically. Of the 12 possible simple tetrasomic diploids only one has 

 as yet been identified, and only 1 of the possible 12 hexasomic tetra- 

 ploids. The simple trisomic tetraploid is of especial scientific interest, 

 because of the effect of the loss of one chromosome, which causes the 

 abortion of half of the pollen. 



It may be added that Mr. Belling's analysis, given above, of the 

 cytological conditions accompanying Datura hybrids has been of the 

 greatest possible assistance in the interpretation of the originally 

 baffling phenomenon of mutation in Datura. 



