CORRELATION OF DISTRIBUTIONAL FEATURES. 563 



and the extreme Southwest to Oklahoma, Mississippi, Florida, and 

 North Carolina (see plate 31). 



The relation of Daucus to the climatic conditions of its wide range 

 requires interpretation in terms of its seasonal behavior in different 

 sections of the range. In the States east of Texas it is an early summer 

 plant, reaching maturity in July or later; in the Desert region it is an 

 early spring plant, reaching maturity in March or April ; on the Pacific 

 coast it is a late spring or early summer plant, reaching maturity from 

 May (in southern California) to July (in Washington). In order to 

 evaluate properly the conditions under which it actually lives in these 

 sections of its range, we should take into account only those climato- 

 logical values in each section that refer to the period of its activity. A 

 separate investigation of Daucus and other plants of the same facul- 

 tative seasonal habits would yield results of great value. The wide 

 amplitudes of moisture conditions shown in figure 69 would thus 

 doubtless be greatly narrowed and the temperature amplitudes would 

 be made somewhat narrower also. Whereas this plant appears at first 

 to encounter a remarkable gamut of conditions through its range, 

 nearly 4,000 miles in length, a study of the conditions in its particular 

 seasons and in the habitats which it occupies would undoubtedly show 

 that it grows only under a relatively limited set of conditions. 



The northern (and eastern) limit of Daucus follows certain of the 

 isothermal lines so closely as to indicate that its controlling conditions 

 are to be looked for in the temperature series. The danger of attempt- 

 ing a final explanation of distributional limits by correlational methods 

 alone is shown very clearly in the case of this plant. The distributional 

 limit is closely parallel to the line for a length of growing-season of 240 

 days, but the length of this season is obviously of no direct importance 



Tkmpkiiaturc 

 >«ra IN Normal rRosrtM* Suson <F. S.) 

 oT D«va. F. S. 

 OOLO OAta, F. S. 



PHTaiOLOOICAL SUMMATION. F. S. CZ 



NoKHAL Daily Mcan, colocst 14 OATa or Year CZ 

 Normal Daily Mun, Ycar I 



PnCCIRtTATION 



Normal Daily Mean. F. 8. I 

 DAYa IN LONCcaT Normal Rainy Pcrioo. F. S. ■§ 

 Dayr in lonocst Normal Dry Pcnioo, F. S. ^1 



Mcan Total, Yiar I 



Evaporation 



Daily Mcan. 1887.8, F. S. ■ 



MoiaTURC RATioa 



Normal p/e, F. S. CI 



Normal ir/E, F. S. Oi 



Normal P/E, Ycar ■ 



Humidity 



Normal Mcan, F. S. (ZZ 



Sunshinc 



Normal Daily Duration, F. S. CZ 



MOISTURC-TCMPCRATURC INDICCS 



Normal P/t x T. F. S., Pmysiological Mctmod C 



Fig. 69. Climatic extremes for Daucus pusillus. 



