GEOGRAPHICAL RELATIONSHIPS OF THE FLORA. 4-47 



with which submerged aquatics wilt when removed from the water. 

 The floating leaves are better protected, possessing stomata (on the 

 upper surface only); a thicker-walled epidermis; often true palisade; 

 and sometimes a dense covering of papillae (e. g., Nelumbo), which 

 causes water to roll off the surface without wetting it. In aquatics 

 of this class a transpiration current is maintained from the roots to the 

 leaf surfaces. Large floating leaves are, moreover, provided with more 

 or less mechanical strengthening tissue and with large intercel hilar 

 spaces near the lower surface which serve as swim-bladders, cooperat- 

 ing with the water-shedding papillae in keeping the leaf afloat upon 

 the surface and the layer of palisade always horizontal and opposed 

 to the light rays. 



The flowers of embryophytic water plants in most cases either are 

 emersed or float upon the surface. 



PHYTOGEOGRAPHICAL AFFINITIES OF THE FLORA. 



The factor in the physical environment of plants and animals which 

 exerts the largest control over their geographical distribution is tem- 

 perature. It is the sum total of effective temperatures (above <j° C.) 

 received during the period of greatest vital activity, the "growing 

 season," that seems to determine the polar or the upward limit beyond 

 which a given organism can not successfully maintain itself against the 

 stress of its physical or biological environment. Hence it is the sum 

 of effective temperatures which fixes the limits, polar in point of lati- 

 tude, upward in point of altitude, of the great life zones. These zones 

 in North America, as now often recognized, 1 are as follows: 



1. Boreal Region. 



(a) Arctic- Alpine zone. 

 yb) Hudsoman zone. 



(c) Canadian zone. 



2. Austral Region. 



(d) Transition zone. 



(e) Upper Austral zone \ Carolinian area. 



I Upper Sonoran area. 



(/) Lower Austral zone \ Austroriparian area. 



' Lower Sonoran area. 



3. Tropical Region. 



Another factor which in great measure controls the distribution of 

 life on the surface of the globe is water. The quantity of atmospheric 

 humidity and of rainfall is next in importance to that of heat in deter- 

 mining the distribution of plants, and, less directly, of animals. We 

 have, therefore, a division of the Lower Austral life zone into two 

 areas — an eastern or humid, the Austroriparian area; and a western 

 or dry, the Lower Sonoran area. The actual difference in quantity 

 of rainfall which fixes the dividing line between these two areas has 

 not yet been ascertained. They are even more strikingly different 



1 Merriam, C. H., Yearl ook U. S. Dept. Agr. for 181)4. pp. 203 to 214 (189o) ; and 

 Bull. Div. Biolog. Hurv.. U. S. Dept. Agr., No. 10, pp. 18 to 53 (1898). 



