132 PLANT SOCIOLOGY 



the soil to prevent the growth of fungi and bacteria. The bottles are 

 then placed in a compartment maintained at a constant temperature 

 (ice box or tireless cooker) for 24 hr., when the paper is weighed again. 

 If necessary, the experiment may be repeated with a sugar solution of 

 another concentration. 



Wilting.— Maximov (1929, 1931) regards the behavior of the plant 

 during wilting as a good indicator of its drought resistance. He defines 

 xerophytes as "plants of dry (or physiologically dry) habitats which are 

 able to decrease the transpiration rate to a minimum under conditions 

 of minimum water supply." The wilting of leaves is accompanied 

 by a great reduction in the rate of water loss due to the closing of the 

 stomata. Wilted corn plants lose by transpiration less than one- 

 tenth the water lost by unwilted specimens, and in highly cutinized 

 leaves the reduction of water loss due to wilting is even greater. Thus 

 in the wilted condition the last water reserves of the plant are retained 

 for a long time. 



Humidity of Air and Plant Communities.— The autecological side of 

 the problem of drought resistance offers, as we have just seen, new and 

 unsuspected difficulties. Much more is this true of the synecological 

 side. The aspect of a plant community gives no reliable indication of 

 its water economy. In many cases its growth forms have doubtless 

 come down from earlier geological periods and are genotypically 

 fixed. Only thus can we explain the xeromorphic structure of 

 ericaceous heaths and the Tojal {Ulex scrub) of the permanently 

 moist coastal regions of western Europe. The drought-sensitive 

 Empetrum heaths of the mountains of central Europe, protected by 

 snow cover, and the ericaceous scrub of high moors probably acquired 

 their xeromorphic character under very different conditions in 

 Tertiary times. 



Measurement of Humidity.— The climatologist measures the 

 humidity of the air in grams, or in millimeters, of vapor pressure. 

 Besides this, the relative humidity is also determined, that is, the 

 relation of the actual vapor pressure to that of total saturation at the 

 existing temperature. 



The plant sociologist, however, lays greater stress upon the 

 measurement of two other functions of the humidity of the air, namely, 

 saturation deficit and evaporation. 



Relative Humidity.— ReMive humidity is measured by the well- 

 known commercial hygrometer or psychrometer and is expressed in 

 percentage of the possible humidity (saturation point). 



Relative humidity, like vapor pressure, decreases toward the 

 interior of great land masses. 



