Multiple Chromosome Numbers 389 



an egg having seven chromosomes, would give rise to a new- 

 individual carrying fifteen chromosomes in each cell; and 

 the supernumerary chromosomes would affect the develop- 

 ment of all the parts of the body. In some cases there may 

 also be double non-disjunction, which would result in the 

 carrying over of two extra chromosomes. 



Multiple Chro^nosome Numbers 



In several species of evening primroses, in the thorn- 

 apple or Jimson weed, in wheat, tobacco, and other groups, 

 there has been observed a multiple of the normal number 

 of chromosomes. Thus CEnothera gigas has 28 chromo- 

 somes, as against the 1 4 in lamarckiana. CEnothera semigigas 

 has 21 chromosomes. Since we conceive the chromosome 

 to carry or perhaps to be made up of the material units 

 called genes, all of the genes affecting the development of 

 the individual would be already present, for whatever they 

 may signify, in a single set of chromosomes. It is therefore 

 difficult to conceive how the doubling or trebling of the sets 

 of chromosomes can produce a change in type. We find 

 that in reality the presence of these multiple sets of chro- 

 mosomes is related to changes in all parts of the body, but 

 chiefly of a quantitative sort, that is in the size and vigor of 

 the plant as a whole. 



The appearance of multiple chromosomes would seem 

 to arise when an tgg, containing say seven chromosomes, is 

 fertilized by two sperm cells, each adding seven more chro- 

 mosomes to the total. That this does sometimes happen 

 has been actually observed in several species, although the 

 multiple chromosome numbers may arise in other ways. 



The presence of multiples of the basic chromosome 

 number, called polyploidy, has been reported in many groups 

 of flowering plants, both wild and cultivated. Among them 

 are the banana, the mulberry, the pineapple, varieties of 

 wheat, oats, sugar cane, maple, tobacco and dahlia. The 

 multiples run sometimes as high as eight or ten times the 



