The Experimental Polyploids 283 



outperform the l)est diploids. Statements that colchicine causes 

 "harm"^" to the plants are also difficult to iniderstand. 



A second principle well known to practical breeders is the use of 

 !ar2;e populations. If one starts with a few plants, his project is 

 doonietl Ijciore a start has been made. Two qualifications should be 

 stated in this respect. The self-fertilized species should be used with 

 more strains and fewer plants from each, while the cross-fertilized 

 types demand many plants, but these can be taken from fewer strains. 

 In both instances, large numbers of tetraploid genotypes must be 

 made as the material for future selection work.^'' Naturally, a few 

 jilants cannot serve as a substitute for mass production. 



Each successful tetraploid nuist eventually have genotypical bal- 

 ance. Through selection the relation between plant and its environ- 

 ment must be brought into an adjustment. i'' Practical breeders are 

 accjuainted -with the need for the all-around performance of more 

 than one characteristic. It is not enough to acquire disease resistance, 

 or some other quality, to the exclusion of those equally as impor- 

 tant.*^^ The new tetraploids are no exception in this respect. The 

 transfer of a specific gene for disease resistance must not be per- 

 mitted at the expense of the whole genotype which may be thrown 

 out of balance — that is, if success in a practical way is anticipated. 

 Therefore, the opportimities for selection begin with the polyploid, 

 and the difficulties are also started as we shall learn in subsecjuent 

 sections. 



The genetic traits of the polyploid are an accumulation of those 

 contributed by the diploid. It does not follow that a very good diploid 

 \vill always give rise to the best polyploids. But there is this rule 

 to be observed that a polyploid, like the diploid, is a plant with 

 genetic traits that segregate and respond in selection according to the 

 same rules as the diploid. 



In judging the chromosomal ninnbers of natural species, there is 

 a law of optimal numbers above or below which the maxinunn per- 

 formance or adaptation cannot be expected. The polyploid series of 

 Phleum is a good example.'**'> Those types with best characteristics as 

 polyploids were found in the ninnbers 6 X '- an<^l 1 1 X 7. One cannot 

 expect to achieve success by doubling a tetraj)loid, so the di|)loid species 

 are needed for a start. Chromosomal doubling of natural tetrajiloids 

 in cotton from 52 to 104 chromosomes creates very weak and poor 

 plants; obviously this exceeds the optimum nimiber.^ There is, how- 

 ever, another point to be remembered: If the number of diverse 

 genotypes can be increased during the process of doubling high num- 

 bers with plants having good fertility, vigor and growth are possible. 

 Merely stating that the numbers cannot be above a certain value is 

 too limiting. In nature the natural polyploids are combinations of 

 two or more genomes that can I)e recognized. For example, the hexa- 



