ABSORPTION OF MATERIALS IN GENERAL 107 



i. Gases move through cell walls only in solution in the water imbibed in the 

 wall; when intercellular spaces are present, they of course facilitate the move- 

 ment through the tissue. 



2. Gases pass through cell walls the more easily, the more thoroughly the 

 latter are impregnated with water. Diffusion is most rapid through cell walls 

 of algae and, in general, through those of submerged plant parts. 



3. Cell walls that are neither lignified nor suberized do not permit the 

 passage of some gases when the walls are dry, but carbon dioxide and oxygen 

 pass through practically dry walls if the latter are lignified or suberized. d 



These experiments suggest an important ecological consideration as regards 

 suberization and cutinization in plant tissues. If the entire surface of the plant 

 were covered by a dry membrane of pure cellulose, then the interior cells would 

 be suffocated, but the presence of cork and cutin, in the absence of lenticels and 

 while the stomata are closed, protects plants from desiccation without at the 

 same time preventing gaseous exchange. 



4. Carbon dioxide passes out of plant cells more rapidly into air than into 

 water. 



Since Wiesner's experiments indicate that gases may pass through the cuticle, 

 the question arises, to what extent do open stomata increase the rate of gaseous 

 exchange through the epidermis!* To answer this question F. F. Blackman 1 

 constructed a special apparatus described below (Fig. 66). Two brass rings, 

 each prolonged into two tubes at opposite points and each with a glass plate 

 attached to one side, were used as gas chambers, each chamber being about 

 5 mm. deep and 36 mm. broad. A leaf was clamped between two chambers of 

 this kind and the joints were sealed with wax. Oblong chambers were used for 

 narrow leaves (Fig. 66, A). Gas of known composition was passed simulta- 

 neously, but separately, through both chambers and then analyzed. Experi- 

 ments with leaves having stomata only on the lower surface showed that the 

 respiratory gas exchange occurred almost entirely through these openings. 

 For example, a leaf of Nerium oleander gave out 0.002 g. of CO2 from its upper 

 surface while 0.065 g- escaped from the lower; thus the two sides gave off this 

 gas in the ratio of 3 to 100. 



Further experiments upon the assimilation of carbon dioxide in light showed 

 that leaves absorb this gas from the air almost exclusively through the stomata. 

 Leaf surfaces without stomata practically fail to absorb carbon dioxide. When 

 the lower surface alone is provided with stomata, coating this surface with 

 petrolatum greatly decreases gaseous exchange without wholly stopping it, as 



'Blackman, F. F., 1895, No. II. [See note 5, p. 36.] 



d Molar movement of gases can occur only through interceullular spaces and relatively large 

 openings in plant membranes (stomatal openings, etc.), and gas diffusion can occur through 

 such openings and through dry membranes with relatively large pores (porous porcelain, etc.). 

 The diffusion of dissolved gases through a membrane is possible if the gas is soluble in the mem- 

 brane. When the latter contains water this kind of diffusion can occur, for the gas dissolves in 

 the water. When the membrane contains little or no water, but contains suberin, etc., the 

 action is similar to that of a wet membrane, if the gas dissolves in the wax-like material as it 

 does in water. — Ed. 



