Genetic Factors 437 



developing in such a tissue will be a tetraploid or higher (Jorgensen, 

 1928). In mosses and ferns gametophytes may be regenerated from 

 diploid tissue under favorable conditions (p. 234) and are thus diploid 

 in character. From them tetraploid sporophytes may arise. Haploid 

 sporophytes in higher plants have been produced by various chemical 

 and physical treatments and sometimes occur in twin seedlings ( Christen- 

 sen and Bamford, 1943). These cases prove that the differentiation into 

 sporophyte and gametophyte does not result simply from difference in 

 chromosome number. 



The primary effect of a multiplication of chromosome number is an 

 increase in the volume of the nucleus and the cell. Most other distinctive 

 traits of polyploids follow from this one. The relationship between num- 

 ber of chromosome sets and cell size is not always a simple proportional- 

 ity, however. A study of such a series as that in Datura stramonium (Sin- 

 nott, Houghtaling, and Blakeslee, 1934), for example, where In, 2n, 3n, 

 and 4n plants can be compared directly, shows that the increase in cell 

 size is different in different tissues. In epidermal cells it is not far from 

 1:2:4:8. In xylem cells the increase is a little greater, but in the paren- 

 chymatous cells of the fundamental tissue in the petiole, the tetraploid 

 is usually much more than eight times the diploid. 



In such cases, each added chromosome complement does not simply 

 add an amount proportional to the increase in chromosome number but 

 multiplies cell size by a certain amount. In other words, the addition is 

 geometric rather than arithmetic. This is evident to some degree in the 

 pedicel cells of Datura but particularly in the large ones of the petiole. 

 It is well seen in mosses, where von Wettstein ( 1924 ) produced diploid 

 gametophytes by regeneration of protonemata from sporophyte (2n) 

 tissue so that haploid and diploid gametophytes could be directly com- 

 pared. When this was done, Tobler (1931) found that the increase in 

 cell size of diploid over haploid was different for different races and 

 that in crosses between them it was a character which seemed to segregate. 

 The effect of polyploidy may be different in related species ( H. H. Smith, 

 1943). 



In some polyploid series, like that reported by Harriet E. Smith (1946) 

 for races of Sedum pulchellum with two, four, and six chromosome sets, 

 cell size increased with number of sets. In many cases, however, mem- 

 bers of such a polyploid series in nature do not differ appreciably in cell 

 size. An observation of von Wettstein's (1938) may indicate the reason 

 for this. He grew a diploid race of Bryum, which he named Bryum 

 corrensii, from a regenerated diploid protonema. It had large leaves and 

 cells about twice the haploid size and was quite sterile. Under vege- 

 tative propagation its size gradually became reduced until after 11 

 years it had returned to a practically normal condition as to leaf and 



