DE:PARTME;nT of botanical RlCSEARCH. 59 



logical requirements. The studies of the aquatic Roripa americana, originally 

 from the muddy bottom of Lake Champlain, begun in 1902, and since grown 

 in the system of plantations connected with the Desert Laboratory, may be 

 expected to be brought to a definite advanced stage within the next year. 



DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 



The investigations upon the distribution of desert plants carried on by Prof. 

 V. M. Spalding, assisted by Mrs. E. S. Spalding during the past four years, 

 have reached a stage in which some decisive generalizations may be made, and 

 a manuscript setting forth the results in question has been transmitted for 

 publication. Professor Spalding and his cooperators have made a close analy- 

 sis of the geological history, nature of the soil-formations, relation of physical 

 factors to local distribution, seasonal march, and determining factors in 

 grouping or association of plants, the whole probably constituting the most 

 thorough, comprehensive, and exact study of any area yet made, with respect 

 to these features. 



Among the principal conclusions warranted by this work the following may 

 be cited : 



(i) Within the limits of the Desert Laboratory domain and that part of 

 the Santa Cruz Valley immediately adjacent, chosen as a representative area 

 for the study of desert plants in southern Arizona, twelve plant associations 

 have been recognized and defined. These fall naturally into four groups cor- 

 responding with the main topographical features of the area under observa- 

 tion. The local distribution of certain species of these associations has been 

 carefully mapped with special reference to topographical and soil relations, by 

 this means confirming conclusions based on previous observations. 



(2) Observations within this limited area, supplemented by comparative 

 studies in the Gila Valley and elsewhere, have led to the conclusion that soil 

 properties and aspect are of paramount importance in determining the local 

 distribution of desert plants. Evidence has been gained that soil-water exer- 

 cises a controlling influence, but that, with certain species at least, aeration 

 and percentage of alkali salts are also efficient factors. 



(3) The importance of aspect in determining distribution in closely adja- 

 cent areas has been established, the selective element being the range in tem- 

 perature. 



(4) Inherited habits and structures, together with the capacity for adjust- 

 ment or accommodation, are the most important features of the plant involved. 



(5) The inter-relations of desert plants are not of a simple nature, but it 

 seems evident that the competitive struggle among them is as intense as in 

 other areas. 



(6) No special agencies or methods of dissemination of seeds or propaga- 

 tive bodies were discovered. 



