580 



Certain Ecoi.ogicaiv Factors 



Livingston, in his well-known studies of transpiration, found that, 

 in a general way, the measure of transpiration in plants was fairly 

 indicative of their respective environmental conditions. The tran- 

 spiration rate for most plants being roughly proportionate to the 

 rate of evaporation of water from a partially open receptacle, he 

 introduced the porous-cup atmometer for measuring the evaporation 

 rate of water. Four of these atmometers* were set out May 21, 

 191 1, at different stations indicated on the map: an instrument at 

 station i, near the edge of Skokie Stream; one at station 2, in the 

 outer part of the reed swamp; one at station 3, in the outer part of 

 the swamp meadow; and one at station 4, in a stretch of forest east 

 of the marsh. Instrument No. i was in the center of a dense growth 

 of Typlia latifolia. As the summer advanced, plants of Scutellaria 

 galcriculata and Teucrium occidcntalc grew up in the shelter of 

 Typha. No. 2 was surrounded by Iris versicolor, Siiun cicutacfolimn, 

 and a few plants of Typha. No. 3 was in a dense growth of Cala- 

 iiiagrostis canadensis, and No. 4 in a small area of pastured forest, 

 composed chiefly of Oucrcns hicolor and Fraxinus ainericana, but 

 with a moderate proport'on of F. nigra. The unglazed part of each 

 porous-cup extended from about 22 cm. to about 28 cm. above the 

 ground, giving a mean height of 25 cm. Readings were taken weekly, 

 up to and including October 15, 191 1. After correction according to 

 the method outlined by Livingston, they were plotted graphically,f 

 appearing as shown in Plate LXXXVIII, Fig. 4. The ordinates 

 represent the number of cubic centimeters of water lost per day by a 

 standard atmometer, while the abscissae represent the intervals be- 

 tween the weekly readings. 



A study of this figure (4, PI. LXXXVIII) shows the periods of 

 maxinuim and minimum evaporation to have been fairly harmonious 

 at the four stations. And, again, the evaporation rate for the center 

 of the reed swamp (Fig. 4, a), where hydrophytism is greatest, was 

 usually lowest; in the swamp meadow (Fig. 4, c), it was somewhat 

 higher; in the outer part of the reed swamp (Fig. 4, b), still higher-, 

 and in the O.uercus bicolor-Fraxinus ainericana or swamp white oak- 

 white ash forest (Fig. 4, d), it was highest of all. These differences 

 become perhaps even more evident if we compare the following aver- 



*None of the atmometers used were provided with a rain-exchiding device, 

 such as is recommended by Livingston ('14). 



tA summarized account of these resuhs first appeared in the Botanical Ga.cefte 

 (Sherff, '12), and later a more complete account, substantiallly as presented here, 

 was published in the Plant World (Sherff, '13). 



