ABSORPTION OF MATERIALS IN GENERAL 99 



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.'' 



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 cutjn, 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' 

 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 

 Mangin has shown (see page 35). When stomata occur on both sides of the 

 leaf, the amount of carbon dioxide absorbed is greater on the side where these 

 openings are most abundant. In the case of Alisma plantago, the number of 

 stomata on the upper is to the number on the lower surface as 135 is to 100. 



' Blackman, F. F., 189s, No. II. [See note 2, p. 36. ] 



'' Molar movement of gases can occur (Jnly through intercellular 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 is possible if the gas is soluble in the membrane. 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. 



