CLIMATIC CONDITIONS OF THE UNITED STATES. 283 



(than in the case of plate 46, for example), and southern Georgia and 

 Alabama, and most of Florida are here shown as in the semiarid 

 province. 



3. REMOVAL OF WATER FROM THE PLANT. 

 A. INTRODUCTORY. 

 (l) General Control of Water-loss. 



The external conditions^ that are effective in the control of water- 

 loss from ordinary land-plants are generally confined to the aerial 

 environment, for water is probably seldom lost through the subterra- 

 nean periphery of the plant-body. The water-extracting conditions 

 of the aerial environment are more directly related to climate than are 

 the water-supplying conditions of the subterranean surroundings. 

 Some of these conditions have been studied by meteorologists and 

 climatologists, and the published records of the U. S. Weather Bureau 

 will once more be drawn upon for climatological information wherever 

 possible. 



There are just two features of the aerial environment of plants that 

 are directly effective in controlling their rates of water-loss — the 

 evaporating power of the air and the intensity of absorbed radiant 

 energy. These two conditions should not be confused ; one is dependent 

 upon air-temperature, air-humidity, and velocity of air movement, 

 and the other depends upon the quality, intensity, and duration of 

 sunshine, which is not generally a function of the air conditions 

 immediately about the plant. Also, these two conditions should not 

 be confused with evaporation, which is almost always done in common 

 parlance. The rate of evaporation from a given water-surface is deter- 

 mined by various internal conditions (resident in or back of the sur- 

 face) and by these two external conditions. We shall, however, still 

 frequently employ the term ''evaporation" as practically synonymous 



^ On the internal conditions that are effective in this regard, see Li\'ingston (1906, 2). — Idem 

 (1913, 2). Bakke 1914. — Shreve, F., The vegetation of a desert mountain range as conditioned 

 by cUmatic factors, Carnegie Inst. Wash. Pub. No. 217, 1915, and the citations given in these 

 papers. The xerophytism, mesophytism, etc., of plant forms, as these have been roughly con- 

 sidered by observational and classifying ecologists, are mainly based upon the appearance or 

 structure of the aerially exposed parts and, until the recent development of the concept of transpir- 

 ing power, or resistance to transpirational water-loss (which is quite distinct from the transpira- 

 tion-rate itself), but little progress has been made toward the quantitative definition of plants in 

 this regard. If the transpiring power of plants might be as well known as the shapes of their 

 leaves and the arrangements of their floral parts, a great impetus should be given to the more 

 permanent aspect of ecological study. With a knowledge of this power for the various plant 

 forms should of course go a similar knowledge of water-absorbing power and water-conducting 

 power (see especially Livingston and Hawkins (1915), in this general connection), for the xero- 

 phytism, etc., of a plant may depend on one of these latter rather than upon transpiring power 

 alone. The measurement of these more recondite internal conditions has not as yet been seriously 

 attempted, and methods therefor are still to be devised. Ecological plant geography will 

 eventually need to define its plants physiologically as well as taxonomically, and the geographical 

 distribution of species (itself as yet attempted only in a crude way) will become of little interest 

 without some real knowledge of the physiological qualities by which these species resist or favor 

 the various influences of the en\'ironment. In the present part of our study we confine attention 

 to environmental conditions. 



