Other cases occur in which the number of chromosomes 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 _OEnothera_ 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 _OEnothera 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 the conclusion that the stouter character of the organs in this plant is causally connected with the increased number of chromosomes. Where the number of cells formed is approximately similar, as in two allied forms of plant in this case, the greater size of the cells would naturally give a stouter habit, but it is clear that large cells do not necessarily mean greater size. The cells of _Salamander_ and _Proteus_ are the largest found among Vertebrates, but those Amphibia are not the largest Vertebrates. It is curious to note how different are these discoveries concerning differences in the _number_ of chromosomes from the conception of Morgan that a mutation depends on a factor situated in a part of one chromosome.