420 Lutz, Triploid Mutants in Oenothera. 



As previously mentioned in this report, I have determined the 

 exact number of chromosomes in 228 plants, and have found that 

 somatic chromosome number in Ocnothera is constant, so far as has 

 been observed. I have not kept a record of all the figures counted, 

 for in some root-tips they were so abundant that this was not 

 practical. In such cases only the best were recorded. However, an 

 effort was made to determine the number of chromosomes in every 

 figure that appeared at all countable, and it will be a very conservative 

 statement to say that somatic chromosome number has been exactly 

 determined in not less than 8000 metaphase figures, and probably 

 many more. Exclusive of certain very rare exceptions previously 

 referred to (and which will be fully described in later reports), the 

 number has been found constant for the individual in all cells of 

 the root-tips and floral tissues studied in which it was possible to 

 determine the number. Since the majority of these studies were 

 made from sections of root-tips, we can not explain the above 

 phenomenon on the basis of "the well-known variation in chromo- 

 some numbers in root tips". 



Although the divisions of the somatic cells may be said to be 

 constantly regular, and as a result, somatic chromosome numbers 

 practically invariable, we know from the researches of Gates (9, 10, 

 11, 13, 15), Davis (3, 4) and Geerts (16) that irregularities some- 

 times occur in the heterotypic mitosis of the Oenotheras', and from 

 the publications of Gates (13, p. 188) and Davis (4, p. 952) that 

 irregularities sometimes occur in the homotypic mitosis as well. 

 We must therefore turn to the germ cells for an explanation of 

 these mutant chromosome numbers. 



The derivation of 15-chromosome mutant offspring from 14- 

 chromosome parents can readly be explained as stated by Gates, - 

 by one chromosome passing to the wrong pole at the reduction 

 division of one of the two parents, and the cell having the larger 

 number of chromosomes uniting in fertilization with a normally 

 reduced cell. The origin of the two 15-chromosome lata offspring in 

 the culture of 0. lata X 0. giyas can also be explained as suggested 

 by Gates (13, 14), by the apogamous development of these 

 forms from female germ cells having the unreduced number of 

 chromosomes. However, it will be shown in the following paper 

 that there is some evidence to indicate that these two plants may 

 have derived their chromosome numbers from the fertilization of 

 8-chromosome eggs by 7-chromosome sperms (or 7-chromosome 

 eggs by 8-chromosome sperms), and that the latter may have 

 attained their reduced numbers in the manner described by 

 Geerts (16). 



The presence of 14 chromosomes in a second generation offspring 

 of 0. i/if/ris X 0. Lfrntarckiana described by Geerts (16) is explained 



