Size Relationships 449 



When a duplex plant is crossed with a nulliplex the ratio of off- 

 spring is bA : la. If a duplex plant is self-fertilized, the ratio 

 is 35^ : la. Such ratios are not expected among diploids nor are 

 they to be expected among pure allotetraploids, which, as we see 

 in the next chapter, behave at meiosis essentially as diploids. 

 Since pure allotetraploids produce only bivalents and autotetra- 

 ploids produce some quadrivalents, quadrivalent formation has 

 been suggested as a criterion for autotetraploidy as opposed to 

 allotetraploidy. Unfortunately, it has limitations, for some un- 

 doubted autotetraploids do not form quadrivalents, and some 

 plants that are in part auto- but also in part allotetraploids do 

 form quadrivalents. Dawson, therefore, has suggested that tetra- 

 somic inheritance in the progeny of a tetraploid and the extent 

 to which it occurs are better criteria. One note of caution needs 

 to be interjected. Even in a pure autotetraploid, true tetrasomic 

 inheritance occurs only for genes that are so near the centro- 

 mere that practically no crossing over occurs between them and 

 the centromere. Although we cannot consider chromatid cross- 

 ing over in polyploids in this book, we should say that such cross- 

 overs disturb these tetrasomic ratios and even produce aaaa 

 plants from the cross AAAa X cicicia or when a tetraploid with 

 gene A in the triplex condition is self-fertilized. 



Size Relationships 



In many respects autopolyploids are larger than the diploids 

 from which they are derived, whereas the corresponding haploids 

 are smaller. These size differences may be manifested in general 

 body structure, or in specific organs and structures, or in func- 

 tions, or in more than one quantitative feature. For example, 

 in Nicotiana Langsdorffii, H. H. Smith found that in a series con- 

 sisting of haploids, diploids, triploids, tetraploids, and octoploids 

 there was an increase with each additional set of chromosomes 

 in the width of the tube of the corolla, in the ratio of the width 

 to the length of the leaves, in the thickness of the parts of the 

 plant, in lateness of maturity, and in the size of the cells through- 

 out the plant, including pollen, microsporocytes, guard cells, and 

 cells in the root tips and in the leaves when viewed in cross- 

 section. From the haploid to the tetraploid there was a general 

 increase in the size and sturdiness of the plants and their organs, 

 but the octoploid was very abnormal. It was somewhat dwarfed, 



