40 
TROPICAL LIFE ZONE. 
The Tropical life zone has displaced the Lower Temperate 
throughout 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 
Río Grande and westward through the low valleys of Nevada to the 
Sierras near Harwthorne, Nevada. It also occupies the floor of the 
two great valleys of California, and the Coast line from near Mendocino 
southward. It is clearly an immigrant from far southward as its 
localized flora shows. The Astragaline flora is practically all of the 
arid Tropical division. Its 120 species or 28 per cent of the whole 
wouid seem to indicate a period of great differentiation 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 north most of the species are those that belong in the 
Lower 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, 
various forms of lentiginosus, Mohavensis, Preussii, Panamintensis, 
atratus var., coccineus, tephrodes, albens, acutirostris, dispermus 
and didymocarpus. 
The Mojave region adds A. Vaseyi, limatus. 
The Texan 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 
valleys have capillipes, curvipes Douglasii, leucophyllus, leucopsis, 
maorodon, Miguelensis, oxyphysus, Pomonensis, trichopodus, vestituts 
of the Inflati. The Preussii have oocarpus. The Hamosi have Traskiz, 
Nevinii, didymocarpus, dispermus and nigrescens, 
The Mexican region has Pasqualensis, Pueblze, quinqueflorus, Doug- 
‚lasii, Julianus, leucopsis, Miguelensis, Magdalene, proriferus, vestitus, 
metanüs, triflorus, circumdatus, reptans, Rosei, Guatamalensis, Anton. 
inus, Orizabæ, orthanthus, Arizonicus, Coahuilæ, Orcuttianus, Fran- 
cisquetensis, leptocarpus, Nuttallianus, nothoxys, Chapalanus, Esper- 
anzæ, ervoides, Greggii, hypoleucus, hypoxylus, Luisianus, parvus, 
-Pringlei, Purpusi, Schaffneri, Saltonis, didymocarpus, Angelinus and 
‘diphacus. 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, Magdalene, prori- 
ferus, vestitus, insularis, metanus, Hornii, cirmumdatus, Orcuttianus, 
