568 



BOTANY OF THE LIVING PLANT 



Pea has 14 chromosomes, the Gooseberry 16, Maize 20 and the Tomato 

 24 ; these, of course, are the diploid numbers, each chromosome being 

 present in the sporophyte cells in duplicate. The actual chromosome 

 number, though characteristic of a particular species, or race, is 

 apparently not in itself of special significance ; that is to say, there is 

 no correlation between chromosome number (or chromosome size) on 





M.long. 



kin 



I w A B q C z c 3 d) d 2 d 3 0I4 



M.ten, 



till 



M.monsir. j 



IllilfMf 



A B C| C2C3 c^c^c^oU 



Fig. 434. 



Somatic chromosome complements (karyotypes) of three species of Muscari— 

 M. longipes, M. tenuiflorum and M. monstrosum. (From Sharp's Introduction to 

 Cytology, by courtesy of the McGraw-Hill Book Co., Inc. After Delaunay, 1926.) 



the one hand, and taxonomic position or grade of evolution on the' 

 other, at any rate in so far as the major taxonomic groups are con- 

 cerned. The constancy of the chromosome number, however, is im- 

 portant as evidence that the chromosomes, though not ordinarily 

 distinguishable as such except at nuclear division, do nevertheless 

 retain their individuality during the resting stage. Further support 

 for this view is provided by the many cases in which the several pairs 

 of chromosomes are visibly — i.e. morphologically — distinct ; an 

 example is shown in Fig. 434. The type of chromosome complement 

 characteristic of a race or individual has been termed its karyotype. 

 Comparison of karyotypes within limited circles of affinity has thrown 

 light on problems of taxonomy and evolution (see below under " poly- 

 ploidy," pp. 581-584). 



Hybridisation. 



If the gametes involved in producing a succession of generations were 

 uniform throughout in their origin and hereditary constitution, the 

 organisms produced might be expected to remain constant. But the 



