88 Piiget Sound Marine Sta. Pub. Vol. 1, No. 10 



stump, after which observations were continued for 19 hours and showed 

 a gradual increase in pressure beginning within six hours. This sug- 

 gests that the fronds use the gases within the air cavities. 



At the same time 100 frondose plants were experimented upon to 

 test submergence as a factor affecting the pressure within the air cavi- 

 ties. The most healthy looking and vigorous plants were selected. These 

 were tested for pressure and gave the following averages: 



Submerged, 184 mm. of mercury below that of the atmosphere. 

 Floating, 166 mm. of mercury below that of the atmosphere. 

 Difference, 18 mm. of mercury, or about .023 of an atmosphere. 



Evidently submergence is a factor, but not so potent as the pos- 

 session of fronds. From the fact that the water at the surface is slightly 

 warmer it is presumably the difference of temperature which causes the 

 change in pressure. 



To test diurnal variations, plants were fastened to a raft so they 

 remained submerged and in good condition, and their pressure recorded 

 at intervals of a few hours during day and night. The results show 

 that the time of lowest pressure is about 7 A. M. and that of highest 

 pressure about 3-4 P. M. (Plate 16). The diurnal variation of the plants 

 thus experimented upon averages 16 mm. of mercury. This variation in 

 pressure is probably due to changes in temperature, for the plants were 

 submerged in sea water whose variations in temperature coincided almost 

 always with the variations in pressure within a given air cavity. 



SUMMARY 



1. The gases within the air cavity average about 77 mm. of mer- 

 cury below air pressure. 



2. The loss of fronds causes a rise of pressure within the air cav- 

 ity until it approaches or even exceeds that of the air. 



3. There is also a fall of pressure at night, and a lower pressure 

 in submerged plants. These reductions are likely due to changes in 

 temperature. 



