28 THE PLANT WORLD. 



which the bag is removed for the purpose of castration and 

 pollination. The audacity of some of the short-tongued bees 

 has necessitated the sacrifice of a number of flowers after 

 the greatest care had been used in controlling their pollina- 

 tion. Experience has taught in all those cases In which the 

 entrance of foreign pollen would make Itself plainly apparent 

 in the result, that the precautions taken gives essentially per- 

 fect control of poiiination. 



{To be continued.) 



PROBLEMS OF THE DESERT.* 

 By Dr. D. T. MacDougal. 



If an adequate pluvlographic map of the world were 

 compiled it would reveal the fact that a considerable area in 

 each of the great land-divisions receive a much smaller 

 amount of rainfall than the possible evaporation, and Is there- 

 fore inevitably arid, the low precipitation in the main being 

 due to orographic features, intervention of wind barriers, 

 and lack of conditions necessary for condensation and other 

 relations to the prevailing winds. 



These regions, amounting together to the area of an 

 American continent, are distributed from the equator to high 

 temperate latitudes, and are for the most part central or 

 Interior in their localization, although the stated conditions 

 obtain to a marked degree in some coastal tracts, especially In 

 western America. 



The essential and resultant features of deserts include 

 undeveloped drainage, activity of wind erosion, great diurnal 

 variation in temperatures, low relative humidity, and nearly 

 humus-free soils, with comparatively small vertical Increase 

 In the proportion of soil-moisture. Landscapes of this char- 

 acter are readily recognizable by the xerophytic aspect of the 

 vegetation Inhabiting them. 



* Given before the Washington Hcademy of Sciences, Jan. 7, 1908. 



