102 RE. Clements 



western Texas, respectively. The wide interval is occupied in 

 part by the related monotypic Ungnadia speciosa of Texas, New 

 Mexico, and northern Mexico. Prunus nana-melanocarpa-de- 

 missa is found from Newfoundland to California, the wide- 

 spread melanocarpa overlapping the limits of the other two. 



Less definite though still suggestive evidence of former con- 

 tinuity across the desert is presented by a number of paired 

 genera, such as Castanea-Castanopsis, Catalpa-Chilopsis, Persea- 

 Umbellularia, Sabal-Washingtonia, Taxodium-Sequoia, and Aes- 

 culus-Ungnadia, as already mentioned. In each pair the eastern 

 genus finds its western limits in Texas, except Catalpa, which 

 stops in Mississippi; while the western genera are confined to 

 California or the Pacific Coast, except Chilopsis, which reaches 

 the lower Rio Grande, and Ungnadia, which ranges from east- 

 ern Texas to New Mexico and Mexico. 



SHRUBS 



Several phylads of this group cross the continent almost without 

 interruption, though it is a pertinent question whether each of 

 these is not a single species. Sambucus canadensis ranges from 

 New Brunswick to Colorado and Texas; it gives way in New 

 Mexico to mexicana, which reaches California and there meets 

 the closely related glauca. Cercis canadensis-reniformis-occiden- 

 talis is unique in that all three members occur in Texas, reni- 

 formis being the intermediate both morphologically and geo- 

 graphically. Vitis exhibits a phylad of even greater continuous 

 area : girdiana extends from coastal southern California into the 

 deserts, californica grows throughout the state, arizonica occurs 

 from Arizona to west Texas, and aestivalis from Texas along the 

 Gulf and Adantic Coasts. Staphylea and Styrax are likewise 

 transcontinental, but with a wide gap in the mountain region. 



