NO. 2 GENTRY I LAND PLANTS 89 



California Gulf Region is most responsible among the climatic factors 

 for the tropical elements in the southern flora. It fosters the existence 

 of many plants requiring or responding only to summer rains, when 

 higher temperatures, longer daylight periods, periods of shortening day- 

 light (fall), higher humidities, etc., prevail. The lack of synchrony of 

 these factors in the northern area excludes many subtropical plants. 



TABLE 5 



p, ^ Length Average Average Average 



Station r.^* of record summer winter annual 



(years) rainfall rainfall rainfall 



Brawley, California -100 29 0.63 2.01 2.64 



Mexican, Baja California 3 15 1.0 2.0 3.0 



Muleje, Baja California 110 15 2.7 1.3 4.0 



La Paz, Baja California 60 15 4.4 2.7 7.0 



Lechuguilla, Arizona 700 5 1.7 2.6 4.3 



Tule Tank, Arizona 1100 9 2.2 1.9 4.1 



Cirio Point (Sierra Coloral), 



Sonora 180 10 2.1 1.5 3.6 



Libertad, Sonora 100 10 2.1 1.8 3.7 



Hermosillo, Sonora 700 15 10.1 2.1 12.2 



Guaymas, Sonora 13 15 7.6 2.5 10.1 



Alamos, Sonora 1200 15 22.1 6.2 28.3 



Topolobampo, Sinaloa 10 15 11.0 3.2 14.2 



Table 5: Average rainfall in inches for some representative stations in the Cali- 

 fornia Gulf Region. Data extracted from Turnage and Mallery (1941) and Atlas 

 Climatologico de Mexico (1939). 



COLLECTION LOCALITIES 



The Cape District since its beginnings in the Cretaceous (cf. Schu- 

 chert 1935:132-133), apparently has had a relatively long and stable 

 geologic history as an island. Due to its age, its high elevations, and 

 its tropical and wetter climate, it has the best developed vegetation and 

 the richest flora of any other part of the gulf region of comparable size. 

 The complex physiography and variety of rock also contribute to its 

 floral richness by providing a wide range of habitats. Present are val- 

 leys, hillsides, cliffs, mountain tops, sandy beaches, detrital slopes, springs, 

 short intermittent streams, and on top of Sierra Laguna a small meadow- 

 like basin holds shallow water during the summer rains. The arroyo 

 beds, although with steep gradients, carry deep lenses of granitic sands 

 and support a rich assemblage of mesophytic trees and shrubs. 



The principal vegetation formations are: Desert Shrub, Thorn 

 Forest, Short-tree Forest (a subtropical, mixed, drought-deciduous for- 



