CYTOLOGY AND TAXONOMY 237 



while the Pachystachya group comprises one triploid with 21, two 

 tetraploids with 28, and one hexaploid with 42. In the genus Triticwn 

 (wheats) the species of the einkorn group have 14 somatic chromosomes, 

 those of the emmer group 28, and those of the spelta group 42. The 

 highly important agricultural wheat species belong to the spelta group. 

 In the violets {Viola) the morphological characters determining the 

 sections are not well correlated with chromosome number, other factors 

 e^•idently having been more important in differentiation within this genus. 

 In the sedges (Carex) the species have a great variety of chromosome 

 nimibers that do not form any definite type of series, although it is 

 possible that they represent a combination of several definite series with 

 their modifications. 



Within a species there is sometimes a small amount of variation in 

 chromosome number. In lists based on many collections and counts 

 there frequently appears a "variety" with some multiple of the number 

 characteristic of the species. Its rarity and association mth the preva- 

 lent tvpe may indicate recent origin. In other cases its distribution and 

 characters suggest its independence as an established subspecific unit. 

 Most often such plants are tetraploids, though higher multiples sometimes 

 occur. Aneuploids are very rare. 



Within a family the related genera often reveal their relationship in 

 their chromosome numbers. Sometimes their numbers are the same. 

 When different numbers occur, these usually do not distinguish the genera 

 as clearh^ as they do related species in a genus. Very often a polyploid 

 series runs through a considerable group of genera, with one or more of 

 the multiples of the basic number appearing in each genus. In the 

 Ericaceae (heaths), for example, at least ten of the genera show one or 

 more numbers of the series 6, 12, 18, 24, 36, and 48, with 24 as the com- 

 monest somatic number. Four of the genera, together wdth a genus 

 in the neighboring Empetraceae, show 26 somatic chromosomes. In 

 the Malvaceae (mallow family) as many as five polyploid series are 

 represented, with 5, 6, 7, 11, and 13 as the basic numbers. These series, 

 singty or in combination, are more or less distinctive of certain groups 

 of genera within the family. In the Leguminosae (pea family) pol^^ploidy 

 appears far less frequently. Of the genera studied, 42 are completely 

 diploid, 18 are more than one-half diploid, none is predominantly poly- 

 ploid with a few diploids, and 10 are completely polyploid with the 

 occasional exception of one species. Polyploidy in this family, although 

 occurring in about 23 per cent of the species examined, does ser\'e to 

 differentiate certain related genera more clearly than is usual in the 

 angios perms. 



Chromosome number becomes less valuable as an indicator of relation- 

 ship when groups larger than genera and families are considered. Thoie 



