TRANSACTIONS OF SECTION K. 671 
of latitude corresponds to 450 feet in altitude. The author would not neglect 
the importance of topographic features in determining a flora, but would 
emphasise the point that, taken in the large, it is temperature rather than 
topography, soil, or rainfall which permits or restricts the extension of 
plants over great areas. The climate of any part of the Rocky Mountains 
can be easily judged from an examination of the flora. 
4. The Porous Cup Atmometer as an Instrument for Ecological Research. 
By Professor Burton Epwarp Livinaston. 
Introduction.—Evaporation approaches being a summation of the 
various climatological factors which influence plant behaviour. It is deter- 
mined primarily by temperature, humidity, and wind velocity, and is 
usually deeply affected by variations in rainfall and sunshine. A pro- 
perly constructed atmometer will thus automatically sum the various 
meteorological elements as they influence the plant, and may be said 
approximately to integrate the march of plant environment above the soil 
surface. 
From the curve of evaporation for the growing season in any region, 
together with the rainfall and certain physical data in regard to the soil, 
the general nature of the vegetation, in an ecological sense, may be quite 
closely deduced. 
In the progress which we are now witnessing toward an answer to the 
question,—Under what environmental conditions will the different vege- 
tational formations and societies be produced ? the measurement of evapora- 
tion bids fair to take so important a position that I have thought it might 
not be untimely to call attention to some of the advantages and difficulties 
to be met with in the operation of the porous-clay form of atmometer. 
Advantages of this Atmometer.—The instrument (first brought to the 
attention of plant physiologists in Publication No. 50 of the Carnegie 
Institution) consists essentially of a hollow cup or cylinder of porous clay, 
so mounted that evaporation of water from its external surface draws more 
water from the filled interior, which, in turn, is kept filled from a reservoir 
at a lower level through the principle of the water barometer. The reservoir 
may be a burette or any suitable vessel, and the amount of water evaporated 
may be determined from time to time. A recording device has been con- 
structed, which registers on a paper strip the time required to evaporate 
a unit of water volume, this unit being any convenient one, from 0°2 to 
03 c.c. to several cubic centimetres. 
The main advantages of this form of instrument over the open dish of 
water are as follows :— 
1. The surface of the cup is better exposed to wind action. 
2. The evaporating surface is not continually varied by the wind, but 
remains constant. 
3. The relatively large evaporating surface and the relatively small 
volume of the water contained make the lag due to temperature changes 
much smaller than it is in the case of the ordinary evaporator pan. 
4. This instrument cannot lose water in any way excepting through 
_ evaporation ; animals do not drink from it, and wind does not spill water 
over the edge. 
5. The porous cup does not trap insects, and thus diminish the evaporat- 
ing surface. 
6. The negative evaporation due to rain is very slight, and may be 
corrected for if necessary. 
7. The form of the cup admits of its simulating to a high degree the 
evaporation conditions presented by the aerial portions of a plant. 
