CHROMOSOME NUMBERS AND THEIR ALTERATION 205 



hexaploid and triploid, or tetraploid and diploid, rather than diploid and 

 monoploid as in the types selected for discussion in foregoing chapters. 



A nucleus (or tissue, etc.) with some number other than an exact 

 multiple of the monoploid number is anewploid. When the number is a 

 little lower than some multiple, it is hypoploid; when it is a little higher, 

 it is hyperploid. Obviously, a number falling between the diploid and 

 triploid numbers, for example, may be called either hjqDerdiploid or 

 hypotriploid. 



A chromosome complement in which heteroploidy is due to the 

 multiplication of a single kind of genome (or of some of its members) 

 is said to be autoheteroploid ; whereas one in which specifically different 

 genomes or members are combined, as in an interspecific hybrid, is 

 aUoheter opioid. Although this distinction cannot always be sharply- 

 drawn, it is of considerable importance, as will appear later. This 

 chapter deals with the first type of heteroploidy. The second type is 

 discussed in the following chapter. 



Unfortunately, authors have not agreed in their use of symbols 

 denoting chromosome numbers. At present it seems best to let x and 

 2x stand, respectively, for the gametic and zygotic nvimbers in the life 

 cycle, regardless of whether the organism is heteroploid or not. The 

 syml)ols n and 2n have often been used in the same sense, but the present 

 tendency, which should be followed in the interest of uniformity, is to 

 use n for the true monoploid number (one genome) , 2n for the true diploid 

 number (two genomes), 3w for the true triploid number (three genomes), 

 etc. Obviously, there are cases in which one cannot determine without 

 special study whether a gametic complement of x chromosomes is made 

 up of one, two, or more genomes. In some of the literature the symbol h 

 ])enotes the basic number, or true monoploid number. 



Tetraploidy, — Next to diploid plants, tetraploids constitute the com- 

 monest chromosomal type in natvu'e. The doubling of the chromosome 

 number occurs in two principal ways. The first of these is by somatic 

 doubling, in which the chromosomal division cycle and the spindle 

 mechanism lack their normal correlation, so that the divided chromosomes 

 at the close of the resulting aberrant mitosis are enclosed in one nucleus 

 instead of two. If this aberration occurs in a very young embryo, the 

 whole plant into which it develops has the tetraploid number, whereas 

 its occurrence at a later stage results in a plant with tetraploidy in one 

 or more branches or other portions. The second way is by ameiosis, 

 in which failure of haplosis leads to the formation of diploid spores and 

 gametes, a union of two such gametes then giving a tetraploid plant. 



The principal methods used for inducing tetraploidy artificiall}'- are 

 temperature treatment, decapitation, and treatment with colcliicine. 

 When, for example, young maize ears are kept unusually warm during 



