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INTRODUCTION TO CYTOLOGY 



two species of such a group frequently have but one or two chromosome 

 pairs more or less than one of the numbers of the multiple series. From 

 this it is to be inferred, as suggested by McClung (1905, 1907), that there 

 is a relationship of some sort between the constitution of the chromosome 

 complement and the externally visible taxonomic characters. Certain 

 illustrative examples will now be cited. 



Plants. In the Leguminosae most of the species which have been 

 examined possess either 6, 12, or 24 pairs; Pisum has 7. Common 

 numbers in the Rosaceae are 8, 16, and 32; some species have 6. 

 In a new study of a large number of species of Rosa Tackholm (1920) 

 finds the fundamental haploid chromosome number in this genus to 

 be seven rather than eight. The various species of Chrysanthemum 

 have 9, 18, 27, 36, and 45 pairs. In both Triticum (Sakamura 1918) and 

 Avena (Kihara 1919) the pairs number 7, 14, and 28 (Fig. 136). From 



a b c 



FIG. 136. The chromosomes of 3 species of Avena. a, A. strigosa; 14 chromosomes, b, 

 A. barbata; 28 chromosomes, c, A. sterilis; 42 chromosomes. (After Kihara, 1919.) 



these and many other similar cases it is inferred that tetraploid species 

 have been derived from diploid species in much the same way that 

 (Enothera gigas with its 14 pairs has been observed to arise from (E. 

 Lamarckiana with seven; that triploid species have arisen either by 

 dispermy or by a union of diploid and haploid gametes as in the semigigas 

 group of (Enothera mutants; that hexaploid species have in turn arisen 

 from the triploid ones; and that those forms with numbers not belonging 

 to the regular multiple series have resulted from further irregularities in 

 chromosome behavior. Among such irregularities are the failure of the 

 two members of a homologous pair to separate at reduction (non-dis- 

 junction), and the segmentation of certain chromosomes at their points 

 of constriction (Sakamura 1920; Kuwada 1919). Changes in chromo- 

 some number are thus looked upon as an important factor in the origin 

 of new species. 



The above conclusion is supported further by the observations of 

 Sakamura (1918) and Kihara (1919) on wheat. Sakamura finds that the 

 one-grained wheats (Triticum monococcum) have 14 chromosomes (dip- 

 loid), the emmer wheats (T. dicoccum, T. polonicum, T. durum, and T. 

 turgidum) 28 (tetraploid), and the spelt wheats (T. spelta, T. vulgar e, 



