1919 Gail; on Fucus and Hydrogen Ion 301 



only conditions of the tidepools measured by the writer that were dif- 

 ferent from those of the surrounding Fucus beds were temperature and 

 the pH values of the seawater in the tidepools. In the light of this 

 investigation, these two factors are believed to prevent the growth of 

 Fucus in tidepools. 



Depths at which Fucus grows below surface of water 



It was shown by experiment (Gail, 1918) that the oospores, spore- 

 lings, young and mature plants of Fucus died, or that decomposition took 

 place, when suspended in the seawater of the Sound at a depth greater 

 than 1 meter. The average pH value of the seawater taken the following 

 summer at the same location and at a depth of nearly 1 meter was 8.0. 

 This is well within the limits for a good growth of Fucus. The pH 

 value of the seawater, however, does decrease with the depth. This 

 would also indicate a low oxygen content. At the close of the season of 

 1918, light was believed to be a controlling factor, and is still so regarded. 

 This is in accord with Davis (1911) who says "The depth to which cer- 

 tain algae may descend depends upon the penetration of light." The 

 writer now considers that the pH value and the low oxygen content of 

 the seawater at the greater depths may also be important factors. Lack- 

 ing facilities for measuring the pH value and the oxygen content at any 

 great depth, no accurate measurements could be made. 



