116 DIFFUSION AND OSMOTIC PRESSURE 



(and of the other two gases also, though they are not used 

 in metabolism) throughout the water in the substratum and 

 that contained within the plant. And, since these two 

 masses of water are continuous, simple diffusion will account 

 for the exchanges which take place between the dissolved 

 gases of the soil and those of the roots of the land plant. 

 The same sort of diffusion takes place between the internal 

 solution of the water plant and the surrounding water. Not 

 only may absorption thus take place, but also the giving off 

 of gaseous waste products. In the case of aquatic sapro- 

 phytes and parasites, oxygen is absorbed and carbon dioxid 

 given off. In that of aquatic green plants this process occurs 

 in darkness, but in light, carbon dioxid is absorbed, while 

 oxygen is given off. The roots of land plants are always 

 absorbing oxygen and eliminating carbon dioxid. 



But by far the greater portion of the gaseous exchange 

 in land plants and semi-aquatics takes place, not through 

 the soil water, but through the wet membranes which are in 

 contact with the air. This is especially so in green plants, 

 whose leaves are peculiarly constructed so as to expose moist 

 cellulose membranes in air chambers which are in connec- 

 tion with the outer air through stomata. 1 By a number of 

 researches 2 it has been shown that dry walls are but slightly, 

 if at all, permeable to gases and that the more moist they 

 are the more readily are they permeable. 



1 F. F. BLACKMAN, " Experimental Researches on Vegetable Assimilation and 

 Respiration": II, "On the Paths of Gaseous Exchange between Aerial Leaves and 

 the Atmosphere," Phil. Trans. Roy. Soc., London, B., Vol. CLXXXVIU895), pp. 503-62; 

 H. T. BEOWNE and F. ESCOMBE, " Static Diffusion of Gases and Liquids in Relation 

 to the Assimilation of Carbon and Translocation in Plants," ibid., Vol. CXCIII 

 (1900), pp. 223-92. 



2 N. J. C. MijLLEB, " Untersuchungen fiber d. Diffusion atmospharischer Gase in 

 derPflanzeundder Gasausscheidung unter verschiedenen Beleuchtungsbedingungen," 

 Jahrb.f. wiss. Sot., Vol. VII (1869), pp. 143-92; E. LIETZMANN, " Ueber die Permeabili- 

 tat vegetabilischer Zellmembranen in Bezug auf atmospharische Luft," Flora, Vol. 

 LXX (1887), pp. 339-86; J. WIESNER and H. MOLISCH, "Untersuchungen fiber die 

 Gasbewegung in der Pflanze," Sitzungsber. d. kais. Akad. d. Wiss. zu Wien math.- 

 nat. hist. Klasse, Vol. XCVIII (1889), Abth. 1; P. CLAUSEN, " Ueber die Durch- 

 lassigkeit der Tracheidenwftnde flir atmospharische Luft," Flora, Vol. LXXXVIII 

 (1901), pp. 422-69. 



