40 



TROPICAL LIFE ZONE. 



The Tropical life zone has displaced the Lower Temperate 

 Ihroughoat the floor of the Great Plateau from Albuquerque, New 

 Mexico southward, from the head of the Colorado in a narrow strip 

 to the south of the Grand canon, from Springdale, Utah, along the 

 Rio Grande and westward through the low valleys of Nevada to the 

 ■ierras neai- Harv.thornn. Nevada. It also occupies the floor of the 

 two great valleys of California, and the Coast line trom near Mendocino 

 southward. It is clearly an immigrant fro.m far southward as its 

 localized flora shows. The Asltragaline flora is practically all of the 

 arid Tropical division. Its 120 species or 28 per cent of the whole 

 would seem to indicate fi period of great 'lUTerontiation but when 

 we consider that the break between the Tropical and Temperate 

 floras is an enormous one, far greater than any other divisions, we 

 see that Astragalus has not kept pace with other genera, but its 

 differentiation is insignificant considering the opportunities for change. 

 A hot climate and varying humidity are ideal for plant differentiation. 

 Had Astragalus had any wide pre-glacial distribution at the south 

 the Tropical life zone would have shown many times the species 

 that the Temperate ones show. On the other hand everything 

 shows that what Tropical species we have are derivatives from the 

 cold climates, and their offshoots. The only cosmopolitan species 

 are A. lentiginosus and Nuttallianus whose distribution is mostly 

 affected by wind movement. The majority of the species are annuals 

 and belong to the higher groups. 



At the nonth most of the species are those that belong in the 

 Ixjwer Temperate life zone but extend down into the Tropical such 

 as A. Episcopus, sabulonum, triquetrus, Preussii, tephrodes, Zionis, 

 amphioxys, Arizonicus. 



Along the central floor of the region we find A. aridus, sabulonum, 

 rarious forms of lentiginosus, Mohavonsis, Preussii, Panamintensis, 

 atratus var., coccineus, tephrodes, albens. acut5ros^tris, dispermus 

 and didymocarpus. 



The Mojave region adds A. Vaseyi, limatus. 



The Texsr, region has leptocarpus, giganteus, Wrightii, Brazoensis, 

 Lindheimeri. 



The Californian valleys have a peculiar flora due to the proximity 

 to the Ocean, mostly forms of the Inflati, though some Isolated forms 

 occur such as A. tricarinatus an inland form, Antiselli, pychnostachyus 

 of the salt marshes and Brauntoni near by, with A. Hornii. The dry 

 TRlleys have capillipes, curvipes Douglasii, leucophyllus, leucopsis, 

 macrcdon. Miguelensis, oxyphysus, Pomonensis, trichopodus, vestituts 

 of the Inflati. The Preussii have oocarpus. The Hamosi have Traskias, 

 Nevinii, didymocarpus, dispermus and nigrescens. 



Th2 Mexican region has Pasqualensis, Pueblae, quinqueflorus, Doug- 

 lasii, Jr:Iian;!3, leucopsis, Miguelensis, Magdalenae, proriferus. vestitus, 

 metanus. triflorus, circumdatus, reptans, Rosei. Guatamalonsis, Anton- 

 inus, Orizab£e, orthaiithus, Arizonicus, Coahuilas, Orcuttianus, Fran- 

 cisquetensis, leptocarpus, Nuttallianus, nothoxys, Chapalanus, Esper- 

 enzsB, orvoides, Greggii, hypoleucus, hypoxylus, Luisianus. parvus. 

 Pringlei, Purijusi. Schaffneri, Saltonis, didymocarpus, Ang(>linus and 

 fliphacus. Th life zones of half these species are not clearly made 

 out and may belong higher in the Lower Temperate or even some in 

 the Middle Temperate. The forms which occur only on the Lower 

 California coast are Douglasii, Julianus, leucopsis, Magdalenas, prori- 

 ferus, vestitus, Insularis, metanus, Hornii, cirmumdatus, Orcuttianus, 

 Francisquetensis. 



