S 8 BOTANICAL GAZETTE [july 



and the state of resting vegetation, and no precipitation occurred dur- 

 ing the month the expedition was actually in field. Attention is to 

 be called to the table of relative humidity, in which it is to be seen 

 that the minima arc very low, yielding averages from 17 to 30 per 

 cent. So far as a general inspection could be relied upon, it did not 

 appear that precipitation had occurred at San Felipe Bay within three 

 months, and it might well have been three times that period since 

 any had been received. 



Dr. Edward Palmer visited the Raza Islands in the lower part of 

 the Gulf, 2 2 5 km northwest from Guaymas, in February 1890, and 

 notes that no rainfall had been received there for more than a 

 vear. 13 Nothing can be hazarded as to the extent of the region 

 with this extreme limit of aridity on the Sonoran side of the Gulf, 

 except that it does not include the mesa at an elevation of 300™ at 

 Torres, and it does not appear to include the western slope of the 

 central range in Baja California, although no definite information is 

 available. So far as known at the present time, therefore, this region 

 of extreme and constant drought, constituting the most pronounced 

 type of desert in North America, lies on the eastward or lee side of 

 the San Pedro Martir range of Baja California, and includes areas on 

 the Sonoran mainland, the whole being a southern extension of the 

 Colorado desert. It is evident, however, that a further investigation 

 of the region is necessary to determine the exact meteorological status 

 of this area, as well as the general character, derivation and rela- 

 tionships of its flora. The extreme type of strict desert offered by 

 the area in question points to the possibility of finding here the readi- 

 est solution of some of the more important problems presented by 

 desert vegetation. 



RELATIVE HUMIDITY IN DELTA AND DESERT. 



The relatively brief time during which the expedition was in the 

 field made it impossible to secure records of value as to precipitation, 

 although it has been noted above that no rainfall occurred, except 

 perhaps on the higher peaks of the Cocopa Mountains and of the 

 main range in Baja California. A Lamprecht's hair hygrometer 

 was carried, however, and observations were taken daily, the instru- 



■3 Contrib. U. S. Nat. Herb. 1:79. 1890. 



