I 88 STEBBINS 



with many diploid and tetraploid species, plus some hexaploids and octoploids. 

 So far as is known, all the polyploids behave like amphiploids, and they prob- 

 ably have arisen from interspecific hybrids. At least one of them, B. arenarius, 

 contains chromosomes of different sizes and, judging from its external mor- 

 phology, is probably derived from an intersectional hybrid. Although the 

 intricate interrelationships between the species of these polyploid complexes 

 have been only partly analyzed, the evidence at hand suggests that they 

 are nearly as recent in origin as AegUops and have had a similar evolutionary 

 history. 



The third and largest section, Bromopsis, is highly developed on all three 

 continents. It consists of perennials, mostly bunch grasses, but includes one 

 of our most valuable sod-forming pasture species, B. inermis, the familiar 

 "brome grass" of our middle western states. Although they occupy a great 

 variety of habitats in the Temperate Zone, the majority of the species of 

 this section grow in forested or shady areas and prefer good soil. The interre- 

 lationships of the species of Bromopsis are imperfectly known, but their 

 chromosome numbers, which range from diploid (14) to octoploid (56), sug- 

 gest that highly developed polyploid complexes exist in this section, par- 

 ticularly in Eurasia. 



The final two sections of Bromus, Ceratochloa, and Neobromus consist 

 entirely of polyploids and appear to represent the relics of polyploid com- 

 plexes which were formerly more extensive. Ceratochloa contains three or four 

 species which are hexaploid {2n = 42) and endemic to South America, plus 

 about four octoploids {2n = 56) distributed primarily in North America, 

 and one 12-ploid species {2n = 84) found in the southwestern United States. 



The hexaploid South American species are all closely related to each other 

 and are typified by "rescue grass" {B. catharticus), a lush pasture grass of 

 subtropical regions which "rescues" livestock by its fine growth in winter. 

 They have no living relatives with chromosome numbers lower than 42. They 

 are not descended from diploids or tetraploids of the Bromopsis section, as 

 is evident from the relatively primitive structure of their fertile scales (lem- 

 mas) and fruits (caryopses), plus their considerably smaller chromosomes 

 which in several different artificial hybrids have been shown to be completely 

 non-homologous to those of sect. Bromopsis. The cycle of polyploidy which 

 built up their numbers probably occurred in the early or middle part of the 

 Tertiary period, involving diploid and tetraploid species which are now com- 

 pletely extinct. 



The later evolution of sect. Ceratochloa came about through hybridization 

 between hexaploids of the rescue grass type and species belonging to other 

 sections of Bromus (fig. 7). The manner of chromosome pairing in hybrids 

 has shown that the octoploids of North America, which include the well- 

 known mountain brome {B. morginatus) and California brome {B. carinatiis) 

 of our western states, contain 42 chromosomes essentially the same as those 



