ABSORPTION AND MOVEMENT OF WATER 143 



in other leaves. * The large size is due to the need of get- 

 ting rid of water-vapor as rapidly as possible, and of ob- 

 taining sufficient quantities of the carbon-dioxide contained 

 in such minute proportions in the air. There must, there- 

 fore, be a rapid passage of comparatively large volumes of 

 air past these mesophyll cells. On the other hand, the in- 

 tercellular spaces in the deeper tissues, where the cells de- 

 mand mainly, if not exclusively, the much more abundant 

 oxygen, are relatively very small. Merismatic tissues for 

 example, cambium enclose no intercellular spaces, and 

 though aerated only osmotically, they obtain oxygen in suf- 

 cient quantities from the adjacent tissues which do enclose 

 air passages. Hollow stems such as those of the grasses 

 and dead cells are filled with air, but these are not to be 

 regarded as forming part of the aerating system. On the 

 contrary, they merely contribute, like the air chambers, blad- 

 ders, etc., of water-plants, to the lightness of the organism. 

 The cells enclosing the air spaces have walls which are 

 freely permeable to water-vapor and to gases. Because 

 these cell- walls, like all others in the plant except those 

 forming the outermost bark-layers, are saturated with 

 water, the. passage of oxygen, carbon-dioxide, and nitrogen 

 (if this is taken up at all) through them must be in solu- 

 tion, by osmosis, just as salts enter the cells. The cell-walls 

 bordering upon the unconfined air, however, are so modified 

 chemically by cutinization, suberization, and various im- 

 pregnations (e. g. with SiO 2 ), and by being overlaid with 

 wax, that they are far less permeable to gases and still less 

 to water-vapor. The slow exchange of gases through epi- 

 dermis with closed stomata or with none has been re- 

 peatedly demonstrated, f most recently and certainly most 

 simply, however, by the use of dry filter paper impregnated 

 with cobalt chloride and by the iodine test for starch. t 

 It is well known that cobalt paper, blue, when dry, will 



* Stahl, E. Einige Versuche iiber Transpiration und Assimilation. Botan- 

 ische Zeitung, 1894. 



tSee discussions of this in Pfeffer, Pflanzenphysiologie, I., 21, 29, 30. 

 Engl. transl., vol. I., 21, 29, 30. 



t Stahl, E. L. c. 



