1916 



Frye; on Gas in Nereocysfis 



87 



effect; and the plants were selected from those having their air cavities 

 sugmerged, so as to avoid changes due to air temperature. The average 

 pressures within the frondless plants was 42 mm. of mercury below that 

 of the atmosphere, and within the frondose ones 184 mm. of mercury 

 below; a difference in pressure of 142 mm. or roughly 1/5 of an atmos- 

 phere. These average lower than the general run of plants because the 

 most healthy looking specimens were selected. The results plainly indi- 

 cate that possession of fronds is a condition which must be considered in 

 connection with the lowering of the pressure within the air cavity (Plate 

 16). 



To test further the effect of the fronds, frondose kelps were fastened 

 to a raft in such a manner that they would remain submerged. They 



PLATE 16 



The variations in pressure in four plants during a period of 24 hours. 

 Also shows the difference in pressure between frondose (leafy) and frondless 

 (leafless) plants. Figures at tlie right indicate displacement of mercury col- 

 umn. Figures at top indicate hours of the day. 



were then connected with a pressure apparatus, and records taken from 

 5 P. M. to 11 A. M. at intervals of two hours to see that the plants 

 were normal. Tin- fronds were then cut off leavino; about 1 incli to 



