342 INTRODUCTION TO CYTOLOGY 



numbers. Thus in Brassica the gametic numbers are 9, 18, 10, 20, and 

 19.^° In Rumex the species of the Eulapathum section show gametic 

 numbers of 10, 20, 30, 40, 50, 60, and 100, while those of the Acetosa 

 section show 7, 8, 9, 10, 20, 21, and 24. ^'^ Another such genus is Viola.'^^ 



Of special interest is the relation sometimes exhibited between 

 chromosome number and taxonomic division into subgenera. An 

 outstanding example is Triticum, in which the gametic numbers are 7 in 

 the einkorn group of species, 14 in the emmer group, and 21 in the spelta 

 group. ^^ Similar correlations are shown in a number of other genera. ^^ 



In contrast with the above are certain homoploid genera, such as 

 Pinus, with 12 pairs of chromosomes in every species yet studied: Quercus, 

 also with 12; Lilium, with 12; Philadelphus, with 13; Epilobium, with 18; 

 and Mecostethus, with 12.^° 



Numbers in Related Genera. — Related genera often show "related" 

 chromosome numbers. In the large systematic group Bicornes (Ericales) 

 the gametic numbers found in the various genera are 6, 12, 18, 24, 36, 48; 

 also 8, 16, 13, 23j and 26. In the Antirrhinese the numbers are 6, 7, 8, 9, 

 and 12; here there appear to be related pairs of genera which differ by 

 one pair of chromosomes. The Orchidaceae have 8, 11, 12, 14, 16, 17, 20, 

 and 28, with 16 and 20 most frequent. ^^ Zea Mays and certain species of 

 Euchlcena and Sorghum have 10 pairs. Perennial species of the latter two 

 genera are tetraploid, and it is of interest to find that an induced tetra- 

 ploid from a diploid annual Euchlcena is perennial (Randolph, 1932). 



Heteroploidy within a Species. — Individuals or races within a species 

 often differ characteristically in chromosome number; in many cases such 

 differences have been seen to arise in the breeding plot. In Zea Mays, 

 whose normal diploid number is 20, individuals have also been found with 



i» Karpechenko (1924a, 1928), Shimotomai (1925), Morinaga (1929), and others. 



11 Roth (1906), Kihara and Ono (1926), Ono (1926 et seq.), Jaretzky (1927a6, 

 19286). 



12 J. Clausen (1927, 1929a), Gershoy (1928). 



i^Sakamura (1918), Kihara (1919 et seq.), Sax (1918 et seq.), Nikolaewa (1920a, 

 1924), de Mol (1924), Watkins (1924, 1925, 1927, 1930), Bleier (1926 et seq.), Percival 

 (1926), Aase and Powers (1926), Aase (1930), Kagawa (19276), W. P. Thompson 

 (1925 et seq.), and others. Cf. Schulz (1913), Zade (1914, 1918), and Vavilov and 

 Jakushkina (1925) on relationships. See also footnote 8, p. 366. 



" E.g., Valeriana (Senjaninova, 1927a), Gossipium (Denham, 1924o6), Bursa 

 (S. E. Hill, 1927), Pobjgonum (Jaretzky, 19286). 



15 Chamberlain (1899), Ferguson (1901, 1904), and others on Pi7ius; Ghimpu 

 (19296/, 1930), Jaretzky (1930), Friesner (1930), and others on Quercus; Bangham 

 (1929) on Philadelphus; H&kansson (19246), Schwemmle (1924), Michaelis (1905), 

 and Johansen (1929) on Epilobium; McClung (1923, 1924) on Mecostethus. For 

 Lilium and other cases, see the lists of Tischler (1927a, 1931). 



16 Hagerup (19286) on Bicornes, Heitz (1927a) on Antirrhineae, Hoffman (1929) on 

 Orchidacese. See also Langlet (1927a, 1932) on Ranunculaceae, Jaretzky (1928a, 

 1932) and Manton (19326) on Cruciferaj, Fernandes (1931) on Liliaceae, Wanscher 

 (1931, 1932) on Umbelliferae, and Tschechow (1932) on Trifoliese. 



