520 



Cytogenetics and Evolution 



Where erosion has broken the face of the cliff the two species can 

 come together and produce hybrids (Fig. 147). 



The Louisiana irises also illustrate ecological isolation and 

 hybrid swarms. About twenty years ago a great assortment of 

 different types was discovered, and the question was raised con- 

 cerning their possible hybrid origin. In an ecological study Vi- 

 osca showed that Iris julva, a species with a coppery red-colored 



a b 



Fig. 147. An ecological barrier in Tradescantia. (a) A cliff with T. 

 canaliculata growing above and T. subaspera var. typica below. (6) Hy- 

 brids (indicated by arrows) between these two species growing in a ravine 

 where the surface of the cliff is worn away sufficiently that the species can 

 come into close contact and hybridize. (Redrawn from Anderson and 

 Hubricht in the American Journal of Botany.) 



flower and a flower stalk 25 to 45 inches tall, was found in the 

 clayey soils on the banks of deltaic streams and on the flat lands 

 on the far sides of these deltaic ridges. It follows deltaic forma- 

 tions almost to sea level and therefore is often found in woods. 

 I. hexagona var. giganticaerulea has columbine blue flowers with 

 sepals 50 per cent larger than those of the other species. The 

 flower stalks are normally 40 to 60 inches tall, but are sometimes 

 even longer. This species is found only in low lands in rich, 

 mucky clay with a high water content bordering a marsh. In 

 many places the two species are not far apart, and the actual 

 difference in elevation may be only 2 or 3 feet. However, the 

 regions are ecologically so different that the two species can come 

 together only if an intermediate habitat is present. Such a habi- 



