582 



climax beech-maple forest, the position already assigned to it by 

 Cowles and others in the forest succession of Indiana and Illinois." 



In the autumn of 191 1, a study of evaporation at different levels 

 above the soil surface was made. Beginning September 3, weekly 

 readings were taken with four atmometers arranged at different 

 heights in a dense growth of Phraginites coniimmis, and with three 

 atmometers added to the one already at station I, among TypJia. 

 The last readings were taken on October 22. After correction to 

 correspond with the readings of a standard atmometer cup, the data 

 were plotted graphically. Among Phragmites (Pi. LXXXVIII, Fig. 

 5) the average daily evaporation for the 7 weeks, at o cm. (the soil 

 surface), was 2.5 cc. ; at 25 cm., 4 cc. ; at 107 cm., 5.3 cc. ; at 198 cm., 

 in the uppermost atmospheric stratum among the Phragmites plants, 

 7.5 cc, or just three times as great as at the soil surface. Among 

 Typha (PI. LXXXVIII, Fig. 6)*, the average daily evaporation for 

 the 7 weeks, at o cm., was .64 cc. ; at 25 cm., 1.5 cc. ; at 107 cm., 

 2.7 cc. ; at 175 cm., in the uppermost stratum, 6.4 cc. — or just ten 

 times as great as at the soil surface. These differences in the rates 

 among Typha were strongly accentuated because the readings were 

 taken in autumn, when many of the Typha leaves had started to 

 wither and bend over, thus giving greater exposure in the upper 

 strata and greater shelter in the lower. Then, too, numerous plants 

 of Scutellaria galericiilata, Teucriuni occideyitale, Polygonum Muhl- 

 enhergii, etc., absent among Pliragmites, were present among Typha 

 and acted as a further check to evaporation in the lower strata (in 

 which, to a very great extent, they vegetated). 



The data plotted in Figures 5 and 6, Plate LXXXVIII, cor- 

 roborate very emphatically those of Yapp ('09), who found that 

 during a total of about 15 days, the evaporation rate just above {not, 

 as at Skokie Marsh, in the upper strata of) tall "sedge vegetation" 

 was over fifteen times as great as it was at 12.5 cm. above the soil 

 surface. They conform likewise with the more recent results of 

 Dachnowski ('11), who obtained during about five days, at a height 

 of 150 cm. in an American bog, an evaporation rate twice as great 

 as at a height of 7.5 cm.; also with those of Fuller ('12) who ob- 

 tained, during six months at a height of 2 m. in climax mesophytic 

 forest, an evaporation rate 2.34 times as great as at a depth of 4 m. 

 below the forest floor, in a ravine. Obviously, we must conclude, 



^Because of the faulty working of the atmometer at cm., the results for the 

 first two and the last weeks are not plotted, and the average here given (.64 cc.) 

 is for the remaining four weeks. Enough certain data were obtained however for 

 the other three weeks to show that the total average would have been even less 

 than .64 cc. 



