CORRELATION OF DISTRIBUTIONAL FEATURES. 569 



Wide amplitudes are exhibited by both of these plants for a number 

 of the temperature conditions. Cornus shows a narrow amplitude for 

 the number of hot days and the physiological temperature sunomation, 

 and Spermolepis shows narrow ones for the number of cold days and 

 for the annual daily mean temperature. The region with less than 60 

 hot days coincides roughly with the area of Cornus, except in the Great 

 Basin region, where the plant is absent. The isotherm for a physio- 

 logical sunomation of 5,000 corresponds in a striking way with the 

 limit of Cornus, from the Pacific to the Atlantic. Spermolepis nowhere 

 encounters a daily mean temperature of less than 55°, and barely 

 enters the region with cold days. These temperature conditions are 

 manifestly the strongest determinants operating to limit the ranges of 

 these two plants. 



Both of the plants under comparison exhibit wide ranges for all of 

 the moisture conditions, and in most cases their amplitudes overlap 

 to a considerable extent, or even show closely similar extremes. The 

 moisture ratios for Spermolepis show much narrower amplitudes, and 

 much lower maxima, than those for Cornus. The sunshine conditions 

 are more nearly wholly dissimilar than any other condition, even than 

 those of the temperature series. 



These plants are of particular interest as exhibiting the influence of 

 temperature conditions in controlling the distribution of individual 

 species. Such plants as these and Arenaria lateriflora, Parietaria 

 pennsylvanica, and others of transcontinental distribution are able to 

 range through widely diversified conditions of precipitation, evapora- 

 tion, humidity, and moisture ratio at the same time that they are 

 strongly controlled by temperature conditions. 



TfHmATUMC 



Days in Normal FnosTiist ScAtox (F. S.) 



Hot Days. F. S. 



CotB Days, F. S. 



PHYatoLOGicAt. Summation. F. S. | ""'"''^^^^^y^^fa 



Normal Daily Mian, coldest 14 days or Year 



Normal Daily Mean. Year 



Pbecipitation 

 Normal Daily Mean, F. S. [ ^^^^^^j^^^^^y^ j^| j^^^^'^^^^^^^^^^^^y^'^^^m^j^jm^^^ m , i . ; ; . i r7^ 



t Days in longest Normal Rainy Period. F. S. M^^^^^^^^^^a^m^ii^^^^^_^^^^^^,— . ■ ■ ^ 



Days in longest Nsrmal Dry Period, F. S. ^^g^^^^^BSHnniSB^H^^^^^^^B^^^S^EHBHiSii^SMi^HainS 



MCAN Total, Year | mm^mmiiiUimmimnilUiMinmammm^^^^^m-r- 1 



Evaporation 



Daily Mean. 1887-8, F. S. f.w:i.wrui: j.Lijj^^^^,i^^,^^^,i^^^^jj^^i;^f^iiijjjjjj^jjj^,;_,^j^^ 1 



' Moisture Ratios 

 Normal P/e, F. S. 

 Normal ir/e, F. S. 

 Normal P/E, Year 



Humidity 

 Normal Mean. F. S. I .„.„f,ff jj;^ ' ...;j,f > ■;,■, rf. „,■,.,, ,.■,„„,, ,j„.„ q^j^r7r7,r- 



SUNSHINE 



(Normal Daily Duration, F. S. ^iiniiinnmninniinin'i hi iiiiiii ..^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 



Moisture-Temperature Indices 

 Normal P/t x T. F. S.. Physiological Mcthod ^^gjg 



Fig. 74. Climatic extremes for Cornus canadensis (shaded) and Spermolepis echinatus (black). 



