184 ORIGIN OF NON-SEXUAL CHARACTERS 



When a gamete with 8 chromosomes unites in 

 f ertihsation with a normal gamete with 7 the zygote 

 has 15. The lata mutants having an odd chromo- 

 some are almost completely male-sterile, and their 

 seed production is also much reduced : but this 

 partial sterility cannot be attributed entirely to the 

 odd chromosome because semilata, which has also 

 15 chromosomes, does not show the same degree of 

 sterility. 



Other cases occur in which the number of chromo- 

 somes in the somatic cells is double the ordinary 

 number — namely, 28 — and others in which the 

 number is 21. The normal number in the gamete, 

 7, is considered the simple or haploid number, and 

 therefore the number 28 is called tetraploid. This 

 doubling of the somatic number of chromosomes is 

 now known in a number of plants and animals. It 

 occurs in the CEnothera mutant gigas. The origin of 

 it has not been clearly made out, but it must result 

 either from the splitting of each chromosome or 

 from the omission of the chromosome reduction. 

 In many cases the more numerous chromosomes are 

 individually as large as those in normal plants, and 

 consequently the nucleus is larger, the cell is larger, 

 and the whole plant is larger in every part. But 

 giantism may occur without tetraploidy, and vice 

 versa. In the CEnothera gigas the rosette leaves are 

 broadly lanceolate with obtuse or rounded tips, 

 more crinkled than in Lamarckiana, petioles shorter. 

 The stem-leaves are also larger, broader, thicker, 

 more obtuse, and more crinkled than in Lamarckiana, 

 The stem is much stouter, almost double as thick, 

 but not taller because the upper internodes are 

 shorter and less numerous. It is difficult to avoid 



