140 PLANT SOCIOLOGY 



Evaporating cups standardized by comparison with a normal cup 

 may be purchased. ^ Black cups are also available (radioatmometers). 

 These have a higher rate of evaporation in the sunlight on account 

 of the absorption of heat rays. 



Convenient mountings have been devised by Johnston and Living- 

 ston (1916), Wilson (1928), and Chalkey and Livingston (1929) for 

 making readings from the Livingston atmometers at intervals of one 

 minute or less. 



One of the best instruments for obtaining almost instantaneous 

 readings in different plant communities is the Piche evaporimeter, 

 devised in 1872 and much used by meteorologists. It is a simple, 

 easily manipulated instrument and suitable for studies of limited 

 duration. It consists of a graduated glass tube to the lower end of 



which is affixed a disk of filter 

 paper that may be colored green, 

 more closely to resemble a leaf. 

 As the disk is kept constantly 

 wet from the water in the tube, 

 the loss by evaporation may be 

 ^■^■" 12 1 I i ^^:"" read from the graduations at inter- 

 ^i^-Jli^^:^ SThT'^pL'T : vab of half an hour or more, 



Brachypodium ramosum garigue associa- and the results expressed in Cubic 

 tion (A) and under the Quercus ilex scrub „p„^i™p4.p„„ „p^ u^..j. T+ permits 

 (B) near Montpellier, France, November Centimeters per nour. It pel mil b 



21. Light clouds; temperature 16° to simultaneous readings in adja- 

 ^^°^' cent communities and easily dem- 



onstrates the great differences between the aerial conditions of 

 associations in the open and those in the shade (Fig. 77). 



Evaporating Power of the Air as a Habitat Factor. — The measure- 

 ments of Wetter (1918), Liidi (1925), Muller (1924), and Walter (1928) 

 in Switzerland and Germany show clearly the importance of the 

 evaporation factor for the ecological differentiation of plant communi- 

 ties. But they were on too small a scale to show the precise relations 

 of evaporation to the individual communities. The investigations of 

 Yapp (1909) in England and of Fuller (1914,) Weaver (1914), Transeau 

 (1910), and Gates (1917) in America have been more extensive and 

 more conclusive. 



Fuller (1914) studied evaporation and soil moisture in relation to 

 vegetational succession near Lake Michigan throughout three growing 

 seasons. The evaporation curves of the different years show con- 

 siderable differences. The maximum in midsummer (July-August) 

 was preceded by a smaller maximum in May. The maxima and 

 ^^1 Atmometer Apparatus Co., Baltimore, Maryland; 



