32 



naul temperature, that of the Lower Temperate about 49 degrees, but 

 these will vary with the humidity. The limits given are for the arid 

 west. 



The two grand divisions of climate are Tropical and Temperate, 

 the latter being divided into Lower, Middle and Upper. The Tropi- 

 cal is also divisible into Arid and Humid, but the arid is the only 

 climate in our region, as affecting Astragali. 



As to climatic distribution of Astragalus 97 si)ecies are Tropical, 

 and of those 75 grow exclusively in this zone, 12 only extending into 

 the Lower Temperate, and these mostly annuals. In the Lower Tem- 

 perate are 122 species of which 61 are exclusive. In the Middle Tem- 

 perate there are 125 species, of which 43 are exclusive. The Upper 

 Temperate has 22 species, of which 5 only are exclusive. 



UPPER TEMPERATE LIFE ZONE. 



In Contributions to Western Botany No. \?,, page 46ff. 1 discussed 

 the distribution of Great Plateau species of plants in Glacial times. 

 Briefly stated, the vegetation in the Glacial period was forced far 

 south by the advance of the ice sheet. (The Glacial Period was 

 preceeded by a Teritary tropical climate). There is little likeli- 

 hood that any part of the United States had a climate warmer than 

 the Middle Temperate of today, and then only along the tip of Flori- 

 da, while most of the flora was Arctic, with a narrow strip of Middle 

 Temperate flora in southern Arizona and which reached great pro- 

 portions in Mexico, and another along the California coast. It 

 was a period of high humidity and low temperate, which is hostile 

 to specific differentiation necessarily so from the vast ice sheet of 

 the Rocky Mountains to the Atlantic, and from the great lake of the 

 Columbia Basin (See Contributions No. 14), and Lakes Lahontan of 

 Nevada and Bonneville of Utah. The whole Columbia Basin was then 

 a sheet of water, and there were about 18,000 s(juare miles of water in 

 Utah, and nearly as much in Nevada, while the mountains were almost 

 wholly covered by a sheet of perpetual snow and ice. It can be readily 

 seen that very few species of our present day Astragali could have 

 existed outside of limited areas in central Mexico, if such areas existed 

 at all which is very doubtful. For such a profound humidity through- 

 out the United States must have transformed the entire arid region 

 of Mexico into fertile prairies or forests, which wouUl have precluded 

 the existence of 90 per cent of the species of Astragalus. Had the 

 genus any considerable development at that time, particularly in the 

 line of arid plants, Mexico would now show the greatest number of 

 species and the most differentiation, while in fact its species are 

 relatively few and confined mostly to the Micranthi and allied groups 

 though its diversity of climates, humidity and soil condition is greater 

 than in the United States. This would indicate a differentiation 

 after the close of the Glacial period and not before. Whether thes9 

 early Astragali passed the generic limits into Colutea and Crotalaria 

 etc. is not capable of proof, but is unlikely, since the gap is too great. 



Following the uniform rule the Arctic meadows that laid next 

 the ice sheet and extended from Ocean to Ocean must have formed 

 a broad area through the southern States ot Missouri and Texas and 

 over the low Plains of Arizona, New Mexico and southern Nevada 

 and around the Sierras into the two big valleys of western Califor- 

 nia, and into the Columbia Basin around Lake Columbia which 

 then filled most of the Basin. This was an ideal climate for the 



