1913] NICHOLS—SUMMER EVAPORATION INTENSITY 145 
to be described in the present paper to determine whether the 
distribution of the climax types of vegetation could in any way be 
coordinated with differences in the evaporating power of the air 
during the growing season. As is generally recognized, one of 
the greatest dangers that beset the growing plant is loss of water 
through transpiration. In ordinary plants, as soon as the amount 
lost in this manner exceeds that absorbed by the roots, the plant 
wilts. The rate at which transpiration goes on in any particular 
plant is regulated largely by the evaporating power of the surround- 
ing air, and this in turn is dependent upon a complex of factors, 
such as humidity, temperature, direction and velocity of wind, etc. 
When exposed to uniform atmospheric conditions, however, it is 
known that the rate at which water is transpired by different kinds 
of plants varies, mesophytes transpiring more rapidly, xerophytes 
more slowly, and so on. That the great centers of plant distri- 
bution in various parts of the United States are directly related to 
well marked differences in the summer evaporation intensity has 
been ably demonstrated by Livincston,’? while the earlier work of 
TRANSEAU® indicated an analogous correspondence between the 
precipitation-evaporation ratios for the entire year and forest dis- 
tribution. It seemed possible, therefore, that in the area under con- 
sideration similar relations on a smaller scale might be detected. 
Accordingly, with this object in view, during the summer of 
1912 continuous evaporation records were taken at numerous locali- 
ties in the state by means of porous clay cup atmometers of the type 
devised by Livincston.4 For assistance in carrying out these 
experiments the writer is indebted to the various cooperators named 
below. The expense necessarily involved was in large part defrayed 
by Yale University. Altogether, 16 more or less widely separated 
stations were selected, and these were fairly uniformly distributed, 
through the three geographical regions of the state, 7 being located 
in the Western Highland, 3 in the Central Lowland, and 6 in the 
Eastern Highland. Three of the stations, one in each geographical 
region, were situated near the coast. The writer personally selected 
the sites and installed the atmometers at all the stations except 
? Plant World 14: 205-222. 1g1I. 
3 Amer. Nat. 39:875-889. 1905. 
4 Carnegie Inst. Publ. no. 50. 1906. 
