BY A. H. S. LUCAS. 629 



The analyses give no decisive evidence as to which of tlie two 

 possible sources of supply is the actual one. The gases as found 

 may be residues of either ordinary or water-dissolved air. The 

 plants, while in the main submerged, are at low water as large 

 waves retire, exposed to the air. Ilormosira may often be left 

 quite exposed at low tide. The vesicle is always closed; theie 

 are no passages or pores in the Avails. The structure of the walls 

 is as continuous as that of the rest of the frond. The vesicle 

 originates in a solid growth from the frond, the growing cells 

 gradually separating from the centre and leaving a central 

 cavity. Hence diffusion from the air seems to be excluded, for 

 the gas must be formed in the cavity, pari passu, with its growth, 

 otherwise the vesicle would collapse. Hence we seem to be driven 

 to osmosis, or osmosis with selective absorption, as the process by 

 which the gas passes, dissolved in water, through the cells of the 

 plant until the residue is set free in the cavity. Alga- obtain all 

 their nourishment, including the oxygen necessary for respira- 

 tion, from the surrounding sea-water, and there must exist a 

 circulation from cell to cell, which I have termed, perhaps rather 

 crudely, a selective osmosis. It is in this way that the plants 

 obtain their salts, with a marked preference for potassium sul- 

 phate, and their oxygen, and it seems perfectly natural that just 

 as common salt is very generally rejected, not absorbed, so the 

 useless nitrogen, together with the oxygen not required, may be 

 eliminated, and set free in the floats in order to serve a mechanical 

 purpose. All the surface of the plant can be employed in the 

 initial absorption, and in many individuals the great number of 

 the floats seems to require some such general agency. I consider 

 then, that the source of the gases is the gas dissolved in the sur- 

 rounding sea-water. 



The Gases of the Inflated Capsules of two Land-plants. 



It was suggested to me by Mr. T. Steel, that I should examine, 

 as a parallel investigation, the gaseous contents of the Balloon 

 Vine, Cardiosperimim Halicacabum L., plants of which were 

 growing in his garden. He kindly supplied me with sutticient of 



