DEPARTMENT OF BOTANICAL RESEARCH. 53 



Mendelian recessives when crossed back with the original stock. Some 

 physiological changes react as dominants in crosses. Some of the 20 

 mutants previously observed have recurred. Accumulating evidence 

 tends to show that the mutability is not a hybridization splitting. 



Wide divergences were found in the progenies of single species of 

 Oenothera from widely separated localities and some striking aberrants 

 or mutants were found. 



The taxonomic investigation of the Cactacese has been carried to a 

 point where it has been possible to prepare the first of the volumes 

 deahng with the Pereskiese and Opuntise, in which it is proposed to 

 give a comprehensive treatment of the family. 



Some extensive regions in South America inhabited by cacti remain- 

 ing unexplored, Dr. N. L. Britton, one of the associates engaged in the 

 research, has undertaken the expense and execution of the field-work. 



The Salton Sea has come down to a stage where the level is oscillat- 

 ing, with a final reduction of 40 inches in depth annually. The total 

 solid matter in solution amounted to 1.6 per cent on June 10, 1916. 

 The increased salinity has been followed by a modification of the 

 activities of organisms supposedly active in the precipitation of 

 calcium, but that element is still being shown in a proportion not 

 consistent with mechanical concentration. 



The revegetation of an island has been followed from a stage in 

 which it was inhabited by but two individuals in 1908 to 1916, when 

 the census showed 470 individuals, representing 10 species. 



Variations in climate affecting vegetation in the preceding geological 

 periods are attributed chiefly to the frequency and distribution of 

 cyclonic storms. Solar and terrestrial data for each day of seven 

 years have been computed in this connection. 



The study of the Mohave River, the chief streamway of the Mohave 

 Desert, leads to the conclusion that this river has been of the truncated 

 type during the greater part of its history, and that its waters cut 

 across the barrier and flowed into Death Valley for a very brief period 

 only. This evidence is of great value in making out the evolutionary 

 history of the vegetation of this ancient desert. 



Precipitation being limited in desert regions, the underflow becomes 

 of the greatest miportance to vegetation. It is shown that artesian 

 conditions do not require a bowl-shaped basin, but that buried stream- 

 ways in trough-like valleys may have layers of impervious clays and 

 of sand and gravel in which water collects under such head that it 

 may rise to the surface or above it when tapped. Some generaliza- 

 tions of practical value in putting down deep wells are reached. 



Wind erosion in the Painted Desert and the torrential flow of a 

 desert river when its channels have been progressively cleared are 

 included in the surface phenomena of arid regions which are being 

 kept under systematic observation. 



