CORRELATION OF DISTRIBUTIONAL FEATURES. 565 



The area of Oxyhaphus is somewhat similar to that of Silphium 

 laciniatum with respect to eastern and western limits, but the latter 

 plant is definitely limited at the north, failing to reach the Canadian 

 boundary, while Oxyhaphus is limited at the south, failing to reach the 

 Gulf coast. The eastern and western limits of Oxyhaphus, like those 

 of Silphium, appear to be set by definite constellations of moisture 

 conditions, while the southern Umit must apparently be sought in the 

 temperature conditions. 



The western edge of the distribution of Oxyhaphus is set by the lines 

 for dry periods of 75 to 125 days, according to the latitude, and nowhere 

 does this plant enter regions with a moisture ratio of less than 0.20. 

 Its eastern limit is like that of SoUdago missouriensis (see plate 25), in 

 being located along the western boundary of the Deciduous Forest 

 more nearly than along any of the climatic lines. The actual ecological 

 conditions for these plants of the open prairies are more radically 

 changed on passing from the Grassland Deciduous-Forest region into 

 the Deciduous Forest than the data of ordinary climatological stations 

 are capable of showing. A normal mean relative humidity above 70 

 per cent and rainy periods of more than 100 to 125 days doubtless serve 

 as limiting intensities of important conditions for these plants in a 

 much more precise manner than is indicated by the positions of the 

 isoclimatic lines as shown on our charts. 



At its southern edge Oxyhaphus does not enter the regions with more 

 than 200 to 240 days in the frostless season, does not encounter more 

 than 180 hot days, and grows at no place with a physiological summa- 

 tion of more than 17,500. 



Trautvetteria grandis (fig. 71) and Trautvetteria carolinensis (fig. 72). — 

 These closely related species are the only North American representa- 



TtMWIATUIIC 



0*vs IM Noi«M*L FnotTtua Season (F. SJ 



Hot Dkv*. F. 8. 



Cold Days. F. S. 



Phtsiolooical Summation. F. S. 



NoHMAL Dailv Mcan, couitST 14 OATS or Vcau I 



Normal Daily Mean, Vcak I 



PnCCimTATIOH 



Normal Daily Mean. F. S. I 



Days ih lonocst Normal Rainy Pcrioo, F. S. ■■ 



Days in lonocst Normal Dry Pcrioo, F. S. I 



Mean Total. Year I 



Evaporation 



Daily Mean. ie87-S, F. S. ^H 



Moisture Ratios 

 Normal p/e. F. S. CIM 



Normal ir/E, F. S. CH 



Normal P/E, Year > Oi 



Humibity 

 Normal Mean. F. S. I 



Sunshine 

 Normal Daily Duration, F. S. 



Moisture-Temperature Indices 



Normal P/E x T. F. S , Physiological Method d 



Fig. 71. Climatic extremes for Trautvetteria grandis. 



