283 
Marsh , and its Relations to Evaporation , dxV. 
merely one of evaporation. The chief external factors which exert a direct 
influence on transpiration are therefore those which affect the evaporation of 
water in air. These are the relative humidity of the air, temperature, air- 
movements and air pressure. 1 Temperature and wind act more especially 
by inducing changes, general or local, in the relative humidity. This is of 
course apart from the action of light, &c., on the movements of stomata. 
The action of these factors may be determined either separately or 
in combination. Clements, 2 Hesselman, 3 and others, in studying plant 
habitats, have investigated them separately. But so far as transpiration 
problems in the field are concerned, the combined effect of these factors will 
present to the mind a much clearer picture than a consideration of each 
factor taken separately. This combined effect may be determined by 
measuring the evaporating power of the air, which will afford a good indica- 
tion of the extent to which it promotes transpiration. 
Hitherto evaporation determinations have been little used in field 
studies of vegetation. Schimper 4 suggested their use, Clements 5 mentioned 
atmometers, and Blackman and Tansley 6 pointed out their superiority to 
a separate measurement of the individual factors affecting transpiration. 
Leist, 7 in investigating alpine plants, calculated the evaporation at different 
altitudes, by combining the various meteorological factors according to the 
formula of Hugo Meyer. 8 But the only published records I have met with 
of actual evaporation experiments in connexion with ecological studies are 
those of Livingston and Transeau. 
Livingston 9 published in 1906 a very interesting paper on the results of 
evaporation experiments he had carried out at the Arizona desert laboratory. 
He determined the evaporating power of the desert air under various con- 
ditions, and also compared this with the actual transpiration from potted 
plants. Reference will be made later to the evaporimeter used by this 
author. 
Transeau 10 has recently determined, in a preliminary way, the relative 
evaporation in various plant associations. The differences obtained were con- 
siderable ; the greatest evaporation, i. e. that from a salt marsh, being about 
twelve times that recorded in the most humid association, a swamp forest. 
Evaporation has been studied chiefly from the meteorological point of 
view. A few meteorologists, however, have worked at it in relation to 
vegetation. Thus Miller, 11 and later Wollny, 12 have shown that the covering 
of the soil by living plants considerably increases the amount of evaporation, 
I Cf. Livingston (’06), p. 24. 2 Clements (’05), Ch. II. 3 Hesselman (’04), p. 347. 
i Schimper (’03), p. 176. 5 Clements (’05), p. 46. 
6 Blackman and Tansley (’05), p. 239. 7 Leist (’90), p. 195. 
8 Meyer (’85), p. 154. 9 Livingston (’06). 10 Transeau (’08). 
II Miller described a new evaporimeter for soils, in Symons’s British Rainfall (’72), p. 206. In 
Miller and Skertchly (’78), p. 270, he records his chief results. 
12 Wollny (’96), p. 363. 
