Genetic Factors 445 



origin of root nodules in legumes. Von Witsch and Fliigel (1952) found 

 that meristematic tissue of Kalanchoe Blossfeldiana was diploid but that 

 as the leaves differentiated the mesophyll became polyploid, as could 

 be shown by wounding and observing mitoses in the wound callus. Inci- 

 dentally, the mesophyll was only 8-ploid if the leaf had developed under 

 long days but 32-ploid under short-day conditions. Steffen (1956) esti- 

 mated from nuclear volume that chalazal haustoria in Pedicularis were 

 96-ploid and micropylar ones 384-ploid. An illuminating study of endo- 

 mitotic polyploidy in the differentiation of the trichomes of angiosperms 

 has been made by Tschermak-Woess and Hasitschka (1954; Fig. 19-16). 



Somatic polyploidy has now been shown to be so frequent as to make it 

 probable that much of the differentiation of plant cells, so far as cell size 

 is concerned, is related to it, cell size being roughly proportional to the 

 degree of polyploidy. Exceptions have been found to this relationship, 

 and no firm generalization about it can yet be made. Obviously, too, 

 there is much more to differentiation than change in size. However, the 

 possibility certainly exists of learning much about the mechanism of 

 cellular differentiation and of developing what has been called "karyologi- 

 cal plant anatomy," a subject outlined in some detail by Tschermak-Woess 

 (1956). D'Amato (1952) has also reviewed the field of polyploidy in 

 differentiation, and Geitler's book (1953) discusses endomitosis more 

 fully. 



Other Effects of Chromosome Differences. There is sometimes a rela- 

 tion between cell size and total chromosome bulk. In 13 species of Crepis 

 differing in number and length of chromosomes Navashin ( 1931 ) meas- 

 ured total chromosome length at a comparable stage of mitosis, using 

 this as an indication of chromatin mass. In each species he plotted this 

 against the volume of comparable cells in the root meristem and found 

 a close correlation between the two, suggesting that the total bulk of 

 chromosome material affects the size of the cell (Fig. 19-17). In some 

 cases, notably in mutants of Primula and Phragmites (p. 35), both 

 chromosomes and cells are markedly larger than normal, and this is also 

 reflected in the size of the plant itself. Neither of these types is polyploid. 

 In some plants there are "accessory" chromosomes which seem to have 

 little or no genetic effect, but Miintzing and Akdik (1948) find that 

 their presence causes an increase in the size of stomatal cells. 



The influence of extra chromosomes was studied in Crepis tectorum by 

 Schkwarnikow ( 1934 ) . This species has four pairs of chromosomes called 

 A, B, C, and D, and four races were available in each of which one of 

 these chromosomes was represented by three instead of two members. 

 Plants in which B or C was present as a trisome had cells larger than the 

 normal diploid. Those in which A or D were the extra ones, however, 

 had smaller cells than normal. Here evidently something more than bulk 



