230 W. C. ALLEE. 



This association is in the region of tall eel grass which at 

 high tide floats erect, allowing light to penetrate to the bottom. 

 As the tide recedes, the grass falls over and finally at low tide 

 forms dense mats which effectively shade the bottom except in 

 queer open spaces where for some reason the eel grass fails 

 to grow. 



The eel grass supports a considerable fauna and flora. Davis 

 (191 1 ) lists 42 species of plants known to occur as epiphytes 

 upon it in the more open situations. The number of animals 

 found in the type of region under discussion is restricted. List 8 

 shows those found in Northwest and Blind Gutters in this 

 association. The animals listed were collected in about three 

 times the collecting time that gave the Cumingia list. 



Above the mat of living eel grass at low tide, the temperature 

 runs up as high as 32 degrees C. on a hot afternoon. Just below 

 the matted eel grass, six inches from the water surface the 

 temperature was 27, while at the bottom the thermometer 

 registered 24 degrees. Thus there is a temperature gradient here 

 of eight degrees in 30 inches. 



Similar gradients of other environmental factors are present. 

 Thus the oxygen content of the water at the bottom just above 

 the Thy one and 5. acutus, may fall as low as to give a mere trace 

 with the Winkler method, when 24 inches higher, just below 

 the eel grass mat, it was found to be 10.22 c.c. per liter and six 

 inches higher, at the surface, 12.97 c.c. per liter. This makes a 

 total gradient of 12.97 c.c. per liter in 30 inches. The pH 

 gradient was found to be wholly similar: 7.3 at the bottom, 

 8.5, below and 9.0 above the eel grass matting. 1 These results 

 are the natural concomitants of rapid photosynthesis at the 

 surface of quiet water overlying thick muck. 



The greater density was found at the bottom, but when the 

 density was corrected for temperature the determinations showed 

 a higher percentage of salinity at the surface. These and other 

 details are shown in Table X. Particular attention is called to 

 the wide range of variation in the different factors analyzed for 

 this association. In the table, collections 99 and 102 are from 



1 Under laboratory conditions Atkins ('22) reports that sea water may come to 

 have a pH of 9.77 when jars containing Ulva are exposed to sunlight. 



