June 3, 1910] 



SCIENCE 



863 



/. 



Distribution and Movements of Desert Plants. 

 By VoLNEY M. Spalding. Carnegie Insti- 

 tution of Washington, Publication No. 113, 

 issued October 23, 1909. 

 Those who have for some years expected the 

 publication of Professor Spalding's arduous 

 and prolonged studies of the desert vegetation 

 of the southwest, but more particularly in the 

 vicinity of the Desert Botanical Laboratory, 

 welcome it in a peculiar sense of gratifica- 

 tion. The work, entitled as indicated above, 

 embraces, to be sure, a wider range of ob- 

 servation than that within the purview of 

 the leading author. The following are the 

 themes discussed: Plant Association and 

 Habitats; Local Distribution of Species, in 

 which Cannon's studies on root distribution 

 are made use of; The Lichens, by Professor 

 Bruce Fink; Environmental and Historical 

 Factors, including the geology and soils of the 

 vicinity of the Laboratory Domain, by Pro- 

 fessor C. F. Tolman and Professor B. E. Liv- 

 ingston, respectively; The Vegetative Groups, 

 by Professor J. J. Thornber; The Origin of 

 Desert Flora, by Dr. D. T. MacDougal; fol- 

 lowed by a general discussion. This serious 

 attempt to correlate the results of specialists 

 in a vegetational study has everything to com- 

 mend it, and the results which have emerged 

 fully justify the expectation that this method 

 of procedure will, for the future, serve an in- 

 creasingly important role. 



Aside from the hydrophytes, of minor inter- 

 est in the work before us, the range of biolog- 

 ical types found in the Tucson region in- 

 cludes two ecological groups, the xerophy1:es, 

 generally distributed on the slopes and 

 " mesas " so called, and the mesophytes, which 

 are found especially near the watercourses 

 and, as the result of irrigation, in the flood 

 plains. This distinction in habitat is, how- 

 ever, operative only in general. The shade 

 afforded by other plants and the nooks of 

 sheltering rocks extend, very locally, into the 

 drought period, the mesophytic conditions es- 

 tablished by a rainy season. It thus comes 

 about that antithetically pronounced meso- 

 phytes and xerophytes frequently stand close 

 together in contingent habitats. It is to be 



noted, however, that the mesophytic conditions 

 are relative and may not be compared with 

 their analogues in the eastern or northern 

 United States. 



The winter and summer rains produce two 

 mesophytic seasons of varying length, accord- 

 ing to the character of the precipitation. 

 These are times of rich vegetation of annuals, 

 which, however, are not common to the two 

 seasons. Thornber, by experiment, has shown 

 that the temperature relations exhibited by 

 the seeds of these annuals are prepotent in fix- 

 ing their times of germination. 



It is noted that the cryptogamic elements of 

 the vegetation are relatively unimportant. 

 The reviewer has had occasion to remark the 

 very striking difference in this regard be- 

 tween the desert about Tucson, and that of 

 north Zacatecas, where the land cryptogams, 

 including algffi, lichens, bryophytes and pteri- 

 dophytes are much more in evidence. This 

 difference may be charged to a lower rate of 

 evaporation in Zacatecas, as also may the gen- 

 eral as well as local differences in the occur- 

 rence of phanerogamic as well as cryptogamic 

 parasites. These, in the Tucson desert, are 

 very inconspicuous; the cases noted by Spal- 

 ding are Phoradendron on the mesquite and a 

 root parasite Orthocarpus, studied by Cannon. 

 The mesquite is recognized as the dominant 

 element in the mesquite forest association of 

 the flood plain. While adapted to low degrees 

 of atmospheric humidity, its demands for soil 

 water are relatively high. Its maximum de- 

 velopment is therefore in the flood plain, in 

 which situation its roots are in correspondence 

 with " a sufficient water supply." Its success 

 in maintaining its foothold is attributed to the 

 effective root system " always within reach of 

 a permanent, deep water-supply." The re- 

 viewer takes this not of necessity to mean a 

 water table. At any rate, it is certainly known 

 that vast mesquite areas are to be found where 

 no water table has been discoverable within 

 several hundreds of feet. The high capillarity 

 of the very fine, compact, very deep soil of the 

 flood plain is sufiicient to explain the presence 

 of the mesquite. 



The mesquite occurs also along washes, but 



