Phytogeographic regions of North America. 343 
The Upper Austral species require a total quantity of heat of at least 6.400° C. (11.500° F.), 
but apparently cannot endure a summer temperature the mean of which for the six hottest con- 
secutive weeks exceeds es C. (78.8° F.). The northern boundary of the Upper ee RT 
RR is marked by the isotherm showing a sum of normal positive temperatures of 6.400° C, 
while its Meet boundary agrees very closely with the isotherm of 26°C, ne 3° F.) 
for the six : hottest consecutive weeks. 
MERRIAM !) recognizes two principal ern of this zone, viz., an eastern or Carolinian 
area and a western or uppe r Sonoran area. 'The lower austral species require a total quantity of 
heat of at least 10.000° C. (18.000° F.). I northern boundary of this zone, therefore, is 
marked by the isotherm showing a sum of normal positive temperature of 10.000° C. (18.000° F.). 
This zone comprises an eastern or Austroriparian area and a western or lower Sonoran area. 
cal species require a total quantity of heat of at least 14.400°C. (26.000° F.), and, 
since & ‚Wöplis life region is a broad equatorial belt, it is probable that both its northern and 
southern boundaries are marked by the isotherm showing a sum of normal positive temperatures 
of 14.400° C. (26.000° F.). 
CLEMENTS ?) proposes a classification of the phytogeographic divisions of 
North America and emphasizes the importance of using the Latin equivalents 
(f. ex. zona polari-nivalis, provincia alaskana, prov. cordillerana etc.) in order 
to avoid mistakes and to preserve a uniformity of treatment in phyto-geographic 
treatises. The following tabulation, omitting the latin names, represents his 
views on the subject. 
Polar-nival Zone. 
Arctic-alpine Zone. Arctic Prov., Alpine Province. 
Boreal-subalpine Zone. 
Alaska Prov., Mountain Prov., Ontario Province. 
Temperate Zone. 
Atlantic Prov., Appalachian Prov., Nebraskan Province. 
Prairie Region, with Elkhorn, Platte, Nemahadistrict. 
Sandhill Region, with Re ra, Loup, RE 
Utah Province, with Nevada and Mohave Reg 
Litoral Province, with sg and California‘ Fe 
Pacific Province. 
Subtropical Zone 
Florida Prov., Mexico Province. 
Tropical Zone. 
Antilles Prov., Andean Province. 
The most complete and satisfactory classification of the phyto-geographic 
regions of North America is one published by ENGLER °) in 1902 in a brochure 
which while primarily intended as a guide to the American plants in the new 
royal botanic garden at Dahlem-Steglitz near Berlin, is much more, because 
it gives an account of the characteristic plants and peculiar vegetation of the 
regions given below in the classification printed in the original. 
1) MERRIAM, C. HART: Laws of temperature Control. Dec. 1894. Also: Life Zones and crop 
Zones of the United States. 1898. (Bibliography, p. 49.) 
2) CLEMENTS, FREDERIC E.: A system of Nomenclature for Phytogeögraphy 1902. (Bibl., p. 46.) 
3) See Bibliography, p. 47- 
