154 PLANT PHYSIOLOGY 



the surrounding air. In certain species, gas of decidedly 

 and permanently different composition from that of the sur- 

 rounding air accumulates in chambers of considerable size. 

 As reported in a preliminary paper by Wille, * the bladders 

 of the Fucacese contain absolutely no carbon-dioxide and a 

 varying percentage of oxygen, thus — 



Bladders wholly immersed in water contain 35-37^ oxygen 

 " lying for 10 hours in air " 20^? " 



darkened for 12 hours " 2.1% " 



These figures, in connection with what has just been said 

 about the different diffusibilities of oxygen and carbon- 

 dioxide, are significant. The buoyancy of the plant depends 

 upon its photosynthetic activity and upon the consequent 

 accumulation of oxygen which is collected in specially differ- 

 entiated reservoirs. Nitrogen necessarily makes up the 

 greater part of the total volume of gas, but this inert gas 

 varies in proportional amount only because of the produc- 

 tion of oxygen in photosynthesis and of the consumption of 

 oxygen in respiration during the hours of darkness. So in 

 all plants the proportion of oxygen in the intercellular 

 spaces decreases in darkness, and increases, especially in the 

 intercellular spaces of photosyntheticaUy active tissues, in 

 the light. The impermeability of the cutinized membranes 

 of the epidermal cells of the Fucacese permits the develop- 

 ment of high gas-pressure by means of the abundant pro- 

 duction and tardy diffusion of oxygen. Consequently, in 

 spite of their large intercellular spaces, these plants do not 

 collapse even in very deep water, f Their buoyancy is thus 

 maintained largely by gas-pressure, and though the form of 

 the cells is maintained by their own turgescence, the form of 



* Wille, N. Abstract, in Just's Jahresbericht der Botanik, vol. XVII., 

 p. 226, 1889, of a " vorlaufige MittheUung," in Norwegian. Also Gasarten 

 in den Blasen der Fucaceen. Chem. Centralbl., 1890. 



f Berthold, G. Uber die Vertheilung der Arten im Golf von Neapel. 

 Mittheil. a. d. Zoolog. Station in Neapel, Bd. in., 1882. In this connec- 

 tion it should be mentioned that all seaweeds living between the tide- 

 marks are subjected twice daily to very different pressures. Where the 

 range of the tide is great, as in the Bay of Fundy on the north Atlantic 

 coast, the algse living near the low-tide mark must adapt themselves to 

 the great differences in pressure. 



