CORRELATION OF DISTRIBUTIONAL FEATURES. 559 



With respect to the westward range of Hilaria, it appears that the 

 same limiting conditions are operative that have been mentioned in 

 connection with Bouteloua oligostachya. Extremely long normal dry- 

 periods, in excess of the 250-day maximum for Hilaria, are inimical to 

 it as to all other perennial grasses. 



Tcmkhatuiic 

 D*r« IN NeiiMtL Fnostlcm Simon <F. SJ 

 Hot Divs, F. S. 

 Colo Dats, F. S. 

 Pmt*iolooic*l Summation, F. S. 

 NoDMkL Daily M«h, coldut 14 oat» or YtAN 

 Nonmal Oailt Mcah. Vcar 



Pnccihtation 

 Normal Daily Mcan. F. S. 

 Days in LONatST Normal Rainy Pckiod, F. S. 



DAY! IN longest NORMAL DRY PcfllOO, F. S. 



' MUN Total. Ycar 



EVARORATION 



^Daily Mcan, 1887.8, f. S. C 



MoicTURE Ratios 

 Normal P/E, F. S. 

 Normal ir/E, F. S. 

 Normal P/E, Yeah 



Humidity 

 Normal Mean, F, S. ^ 



Sunshine 

 Normal Daily Duration. F. S. U 



Moistvre.Temrcraturi Inoiccs 

 Normal P/t i T, F. S., Physiological Methob I 



Fig. 65. Climatic extremes for Hilaria jamesii. 



Sparganium americanum (fig. 66). — Sparganium is widely distributed 

 throughout the eastern United States, except in peninsular Florida, 

 and from its westward extension in Canada it reappears within our 

 limits in Washington. A considerable number of northern plants 

 exhibit distributions of this, type, Dulichium arundinaceum being 

 another example (see plate 27), with a range closely Uke that of Spar- 

 ganium. The range of another palustrine plant, Sium cicutcefolium, 

 is of an analogous character (see plate 28), its limits being far enough 

 south, however, for it to present a continuous area within the United 

 States. The distribution of each of these palustrine plants is such that 

 they are absent from the arid and semiarid regions, where they might 

 find locahties with suitable soil-moisture conditions, although much 

 more widely separated than in the moist regions. The western edges 

 of the ranges of Sparganium and Dulichium in the Eastern States run 

 parallel to the isoclimatic lines for moisture conditions. Although 

 these are plants of wet habitats, there is here a suggestion of their 

 inabiUty to extend into the regions with very high evaporation. In 

 other words, the conditions expressed by the moisture ratio are of 

 importance to them even when the numerator of the ratio is constant 

 and of high value. Their ability to withstand lower moisture ratios in 

 eastern Washington than they endure along their western edge in the 

 Central States is doubtless due to the interaction of temperature con- 

 ditions. 



