THE NORTH AMERICAN DESERT FLORA. 343 



Dinan et St. Malo, p. 63 (1866) ; Arrondeau, Hist. Nat. du Morbihan, 

 Plantes Phan. p. 40 (1867); Hegelraaier, Monographic der Gattung 

 Callitriche, p. 54 (1864), and in Ascherson's Verb, des Bot. Ver. 

 Brandenb. (1867), p. 21 ; Lloyd, Flore de I'Ouest de la France, ed. 2, 

 p. 445 (1868) ; BrelHsson, Flore de la Norraandie, ed. 4, p. 283 

 (1869). Leaves all obovate or tlie lower ones linear. Bracts large, 

 persistent, lanceolate, curved, nearly as long as the mature fruit. 

 Fruit broAvn, sessile or nearly so, of a squarish obcordate outline ; 

 the lobes turgid at the side and rounded at their edge, so as to form a 

 slight ridge or blunt keel. Styles very long, about twice as long as 

 the ripe fruit, permanent, spreading or slightly reflexed, but not closely 

 adpressed to the sides of the fruit. 



In marsh-ditches, slightly brackish, bordering on Brading Harbour, 

 Isle of Wight. July, 1860. 



THE NORTH AMERICAN DESERT FLORA BETWEEN 32° 

 AND 42° NORTH LATITUDE. 



By C. C. Parry, M.D., of Washington, United States. 

 {Eead at the Meeting of the British Association at Liverpool, 1870.) 



The desert tracts in North America, as at present defined by our 

 recent geographical knowledge, comprise those interior basins, of 

 greater or less extent, shut in by mountain ranges from the influence 

 of the moist oceanic currents. These well-marked districts, while 

 presenting certain diversities of soil, corresponding to particular geo- 

 logical conditions, are everywhere characterized by an arid climate, 

 irregular and scanty rainy seasons, and wide extremes of heat and 

 cold, both diurnal and annual. The permanent watercourses of this 

 region, having their distant sources in snow-clad summits, travei^se 

 a succession of basins, presenting occasionally alluvial belts, bounded 

 by elevated and abrupt table-land, which latter is mainly composed of 

 beds of coarse gravel or drifting sand. The intervening ridges form- 

 ing tlu) basin rims are cut through by those deep chasms, known as 

 canons. 



The local drainage not connected with the main valleys terminates 

 either in salt lakes or saline flats, the intense evaporation being suf- 

 ficient to carry oil" tln^ siipcrticial supply of water, leaving their soluble 



