144 Gates, Tetraploid Mutants and Chromosome Mechanisms. 



of the i-//hr/ct/lt/.i- mutant showed that it was heterozygous, resulting 

 from union between a mutated and a normal rubrinerris germ cell, 

 because in later generations it has split into rubricalyx and nibri- 

 iH'rris in a 3 : 1 ratio. There were no intermediate types, and this 

 fact together with the ratio are clear proofs of its origin as a 

 monohybrid 3 *). 



Conclusions. 



The purpose of the present paper is to consider giantism and 

 tetraploidy in Oenothera from a comparative standpoint, in order 

 to reach a better understanding of its nature and meaning in rela- 

 tion to evolution. Incidentally it has been necessary to refer to a 

 number of other phenomena of mutation, and the main conclusions 

 arrived at may be set forth as follows: 



1. A survey of the species or races having tetraploid (4 X) or 

 higher chromosome numbers, shows that stet. oO such cases are 

 now known in plants and 7 in animals, the about having been greatly 

 increased in the last three years. On the other hand, not a single 

 triploid wild species is known, although in some cases (e. g., Droscra 

 rotinidi[oUa X D lonyifolia diploid and tetraploid species intercross, 

 producing triploid hybrids. Triploid species can not be expected 

 to occur in nature, since their chromosomes are not all paired and 

 therefore (especially when the X number of chromosomes is odd) 

 the meiotic processes will result in varying numbers of chromo- 

 somes in successive generations, leading to the sterility of many 

 individuals, and finally to the gradual diminution of the chromo- 

 somes of surviving individuals to the diploid number. 



2. The occurrence of triploid mutants in Oenothera, as shown 

 by Stomps and Miss Lutz, is obviously due to the union of a 

 diploid with a haploid germ cell. But this does not prove that 

 the tetraploid mutant, 0. gigus, originates from the union of two 

 diploid germ cells of 0. Lantarekiana. It is at least equally probable 

 that gigas originates, at least in some cases, as I have suggested, 

 from the apogamous development without fertilization, of an un- 

 reduced megaspore mother cell having (4 X) 28 chromosomes. This 

 conclusion is based on a) the actual observation of such a megaspore 

 mother cell in 0. Lamarckiana by Geerts and the failure to observe 

 the omission of reduction in the pollen mother cells, though thou- 

 sands of times more of the latter than of the former have been 

 studied. On the other hand, the occasional occurrence of 4-lobed 

 pollen grains in Lamarckiana , points to the probability that diploid 



34) It is interesting to observe that Nilsson uses the occurrence of many 

 types in the offspring of gigas, as an argument for its origin by the bringing 

 together of many independent units, and yet applies the same reasoning to rubri- 

 calyx in whose offspring only two types appear in a simple monohybrid ratio. 



