98 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



Spina, Lysiloma divaricatu?n, Melochia tomentosa, Bursera laxifiora, 

 Pithecolobium sonorae, Zizyphus sonoriensis^ and Hinionia pterosperma, 

 which here are either reduced in stature and incidence, or confined to 

 the more favorable situations of soil moisture, as along valley drainways. 

 Certain shrubs achieve local dominance in the Guaymas area that are 

 quite secondary in vegetational weight elsewhere. Among these are 

 Cordia parvifolia and Lippia Palmeri. With low to medium shrub stat- 

 ure they act a strong part locally in the dispersed shrub formation. 



There is present also an arborescent element typical of the Sonoran 

 Desert, including species of Prosopis and CercidiuTu, which have normal 

 growth in the valleys but are stunted on the dry rocky slopes. Pachy- 

 cereus Prtnglei is abundant upon the rocky slopes and like Ficus Palmeri 

 presents a special problem in distribution. These two plants are found 

 widely over the peninsula and on the adjacent islands. Why is their 

 incidence on the Sonoran coast so restricted? 



The Guaymas flora was referred by Axelrod (1939) to the Sierra 

 Madre element of the American flora. Chaney's (1944) inclusion of it 

 in the ''Southwest American Element" is more appropriate, since the 

 relation of the Guaymas flora to that of the Sierra Madre is remote and 

 at best but general. It is, however, an integral part of the great floral 

 complex that has a long but obscure Tertiary history and a large spacial 

 occupancy in the arid and semiarid southwest of North America. As sug- 

 gested above, the Guaj^mas flora has as yet an indeterminate uniqueness 

 in so far as insular isolation may have effected local evolution. Another 

 striking feature of the flora is the number of apparent natural erratics. 

 Some of the species whose occurrences at Gua^^mas and vicinity appear 

 extralimital are Ficus Palmeri, Pachycereus P ringlet, Hermannia pauci- 

 fiora, Lysiloma Candida, Indigofera mucronata, Acacia cymbispina, Vin- 

 cetoxicum petiolare, Colubrina glabra, Boerhaavia Xantii, Lobelia splen- 

 dens, Bouchea dissecta, and Vitex mollis. There is, of course, always 

 the possibility that early man made such local displacements, but in any 

 case they now belong to the native flora. 



Guaymas and San Carlos Bays are well known collection localities. 

 Edward Palmer in 1877 was the first to do detailed botanizing in their 

 vicinities. According to Watson's report (1888:36-87), the only paper 

 that has itemized the Guaymas flora. Palmer collected 299 species of 

 flowering plants during the summer months from mid-June to mid- 

 November. Dawson's collections (43 numbers) from there and Rempel's 

 8 numbers from neighboring Ensenada de San Francisco represent the 

 land plant samplings of the Velero III, catalogued below. 



