CYTOLOGY AND TAXONOMY 235 



Cytological Evidence of Relationship. — Cytological evicU^ice of valiu^ 

 to the taxonomist is of four chief kinds: chromosome number, chromo- 

 some morphology, chromosome behavior in crosses, and aben-ations in 

 reproduction. 



Chromosome Number. — The number of chromosomes has now been 

 ascertained in many hundreds of species of animals and plants. The 

 reported numbers have been assembled in lists which every worker in 

 this field must have ready to hand. Unfortunately, the usefulness of 

 many original reports is impaired by a lack of completeness in the 

 taxonomic designations and by failure to make permanent records of the 

 plants in the form of herbarium specimens or photographs. Furthermore, 

 the reliability of the numbers varies, for in many cases the coun,ts have 

 been made upon too few or sometimes atypical specimens. As the 

 science of cytotaxonomy develops further, it is obvious that more care 

 A\dll be required with respect to these points. 



One does not look far in a general list of chromosome numl^ers without 

 being struck by the fact that the species of a genus tend strongly to show 

 numbers that bear some characteristic arithmetical relation to one 

 another. This is most evident where the numbers constitute a regular 

 series of multiples. In other genera the numerical correlations are less 

 complete, and in still others no significant correlation can be detected 

 between numerical and specific differences. Such diversity is, of course, 

 to be expected in view of the many ways and degrees in which chromo- 

 some numbers may be altered. Such ways have been described in 

 previous chapters. It is to be borne in mind that it is primarily the kinds 

 of genes present that determine characters, the number of chromosomes 

 in which they are carried being a secondary factor in producing diversity. 



A few examples of such numerical relationship will now be listed. In 

 the pond lilies of the genus Nymphaea the species stellata, lotus, odorata, 

 Candida, and gigantea have the following somatic numbers, respective!}' : 

 28, 56, 84, 112, c. 224. It may be suspected that the basic number, 

 or original monoploid number, for this genus is 7, but no species with a 

 somatic number of 14 has .yet been reported. In the genus Plantago, 

 the plantains, the 40 species examined include 24 diploids AAith 1 2 somaticr 

 chromosomes, four diploids with 10, one diploid with 8, one di))loid 

 with 18, eight tetraploids with 24, one tetraploid \Y\i\i 36, one octoploid 

 with 48, and one sixteen-ploid with 96 (Fig. 170). Here it appears that 

 there is more than one basic number, and it is of interest to observe that 

 the sections of the genus can be arranged in three groups: (1) those 

 comprising onlj^ diploids with six as the basic number, (2) those containing 

 both diploids and tetraploids with this basic number and types with a 

 lower basic number, and (3) those containing polyploid species onh'. 

 The original basic number is probably 6, mth 5, 4, and 9 as derivatives. 



