32 ' ALFRED DACHNOWSKI 



sphagnum bog is able to shrink or float. Variations in the posi- 

 tion of the water table do not influence the association so greatly 

 as is often stated, nor do they offer an explanation of the xero- 

 phytic structure of the plants. On Cranberry Island (Bot. Gaz., 

 52) bog xerophytes succeed hydrophytes upon a habitat with a 

 prevailingly high water level. Variations in the water-table due 

 to various causes range at times between 4 and 7 feet (1.2 mm. 

 to 2.1 mm.), but they do not affect the association since the mat 

 is floating. 



The evaporating power of the air is not such as to approach 

 the limit of favorable conditions. The average rate of evapora- 

 tion is about 10.9 cc. daily as compared with 8.1 cc. for a forested 

 (Maple-Alder) zone, and 15.8 cc. for an open lawn on the uni- 

 versity campus. The data plotted as graphs exhibit a great 

 similarity in the general course, and show coincident and pro- 

 portionate minima (7.6 cc, 3.9 cc, 12.6 cc. respectively for each 

 station as mentioned) in early June, and maxima (17 cc, 11 cc, 

 21.5 cc) in late July. The data indicate that the rate of evapora- 

 tion in the bog meadow is not a sufficient cause for the xerophytic 

 character of the vegetation; nor are the differences in the rates 

 efficient factors in causing successions. 



In this association the growing season is determined not by the 

 length of period of suitable soil temperature but by the length of 

 the period when the available water is physiologically sufficient 

 to maintain growth. Ability to resist toxicity is essential to 

 the successful growth of the plants in this environment. Winter 

 temperatures below 32° F. at the one-foot level have not been 

 found to occur in a bog meadow of Central Ohio. 



Acidity in the soil varies from less than 0.00075 to 0.004 but 

 can not be associated with the injurious substances accumulating 

 through bacterial action and otherwise in the peaty substratum 

 (I. c.) 



2. Carex-Juncus association. The second type of bog meadow 

 is produced principally by sedges (see Carex-Juncus association 

 p. 29). The association includes many of the species of the 

 cranberry-sphagnum bog meadow but in numbers rarely above 

 a third of that of the sedges. The sedge bog meadow is character- 



