96 AMERICAN MIDLAND NATURALIST 



water, as at Argyle Bay, and frequent exposure to the air for a 

 short time is advantageous. Exposure to light seems to be of 

 secondary importance. 



Its presence in tide-pools may be due, in part to the abundance 

 of animal life there, such as small crabs, snails, etc., which would 

 furnish abundant Coj and preserve the well-known balance of 

 plant and animal life. According to this view, however, if true, 

 Prionitis should be able to grow in the center of pools as well as 

 on the margins; but there it is not found. Possibly the gases 

 accumulate on the margin. This point was not investigated for 

 want of apparatus. This would be interesting and perhaps a 

 profitable point for future investigation. 



It was suggested at the outset that the difference in dis- 

 section, greatest at the top or upper tide-line and above, might 

 be an adaptation to higher temperatures which would diminish 

 the amount of gas and hence call for more exposure surface, which 

 could be accomplished by more minute dissection of the fronds. 

 This, however, does not seem to be true. In fact the opposite 

 tendency is shown by Theoderesco (7) who, using certain liver- 

 worts for his investigations, found that the fronds of the thallus 

 became shortened and more dissected when the concentration of 

 the CO2 was increased. 



This question was later taken up by the writer (2) in a some- 

 what extensive study on freshwater algae in which it appears 

 that the plant body becomes more compact when the CO2 accu- 

 mulates and that the alga is much concerned with its oxygen 

 relation as well as with that of CO2. 



In this connection I would cite the luxuriant growth of Ulva 

 and Alaria at Olga and at Cattle Point, in the quiet shallow bays 

 where the water becomes quite warm, 64°F. or more. These, with 

 their broad thin leaves, are not adapted to rough water nor even 

 to a strong current, but to quiet shallow bays with a pebbly 

 bottom. 



The finer dissection of Prionitis at the higher altitudes seems 

 to be an adaptation to meet the force of the surf and strong currents, 

 while the broad, simple fronds of the deeper levels are adapted to 

 more quiet waters, and are especially prominent at Argyle Bay, 

 where there is no surf but only a strong uniformly flowing current 

 which rapidly renews the supply of CO2 without the danger of 

 dislodgement or tearing. It is worthy of note that the light ex- 



