86 Puget Sound Marine Sta. Pub. Vol. 1, No. 10 



tube constituted a very small proportion of the total gas when the con- 

 nection was made. Nor could any difference be observed when the two 

 forms of apparatus were used fastened to a raft. The puncturing ap- 

 paratus was merely a small glass tube drawn out to a sealed point in 

 the side of which, at a distance of one or two inches from the tip, a hole 

 was blown. 



The pressure was recorded from a rowboat, in 1000 kelps under 

 various natural conditions at various times of day and night, in order 

 to get the average. Variations in atmospheric pressure were not taken 

 into account, since neither the apparatus nor the conditions of reading 

 would permit great accuracy. The pressure varied from plus 124 mm. 

 of mercury more than atmospheric pressure to 316 mm. less than at- 

 mospheric pressure, with an average of 77.3 mm. less than atmospheric 

 l^ressure. The variation, therefore, was 440 mm. of mercury, or about 

 .58 of an atmosphere. The kelps were taken regardless of size or con- 

 dition of leaf, so the average of a bed would be approximated. The 

 average of the larger, healthier plants falls nearly to 200 mm. of mer- 

 cury below atmospheric pressure. 



During the course of these readings it became apparent that (a) 

 the time of day, (b) the condition of submergence and (c) the health of 

 the kelps, were factors in determining the pressure. Afternoon readings 

 always averaged higher than forenoon readings. Submerged kelps 

 showed a lower average pressure than those whose air cavities pl'ojected 

 above the surface of the water. Healthy abundant leaves were coninci- 

 dent with low pressures. Further experiments on the effect which these 

 conditions have on the gas pressure were then undertaken. 



In explanation, it should be said that the fronds of mature Nereo- 

 cystis plants vary from approximately 25 feet to 2 feet in length and in 

 number from approximately 50 to none at all. The fronds grow mostly 

 near their bases, while at their tips they are frayed, decomposed and 

 eaten by parasites. Their length depends upon the rate of growth in 

 proportion to the number and activity of the parasites upon them. The 

 fnmds are commonly 4 or 5 inches wide and thus have something like 

 99% of the area of a large, whole plant. Their loss, therefore, makes 

 a great change in the plant. According to Peters, a snail is one of the 

 most effective causes of the loss of fronds. 



To test the effect of loss of fronds on pressure within the air cavity, 

 100 healthy-looking frondless, and 100 healthy-looking frondose plants 

 were tested for pressure at various times. At each time of testing the 

 same number of each were tested, so as to eliminate error due to diurnal 



4Peters, Rupert. A preliminary study of the causes that produce "bald- 

 headed" kelp. Kan. Univ. vSci. Bull. 9:1-10. 1913. 



