302 



ASSIMILATION. 



805. The movements of gases within the plant are of two 

 kinds, (1) molecular (see note on the previous page), and (2) 

 "the movement of the whole mass depending exclusively- on 

 expansive force." These are generally conjoined in the passage 

 of gases through the plant. 



806. Passage of gases through epidermis free from stoinata. 

 The assimilating apparatus in ordinary land plants consists of 

 parenchj'ma cells frequently so loosely conjoined as to liave very 

 conspicuous intercellular passages, which communicate with sto- 

 mata either directly or indirectly. All of these parenchyma cells, 

 have walls of cellulose generally without any impregnation of 

 foreign matter. But the peripheral cells which bound the whole 

 as epidermis proper are cutinized on their exteinal aspect, and 

 must possess relations to gases different from those presented by 

 common parenchyma with uninfiltrated walls. 



807. Through ordinary cell-walls, that is, those which are com- 

 posed of nearly pure cellulose, water passes and gases diffuse 

 with facility. But as cutinized cell-walls, like those of the epi- 

 dermis of leaves, are nearl3' impervious to water and to aqueous 

 vapor, it would at first sight appear unlikely that gases could 

 make their way through them ; such, however, is not the case. 

 Experiments upon epidermal tissues free from stomata show 

 that under ordinary circumstances gases can diffuse through 

 cutinized walls. 



Thus N. J. C. Miiller* used the epidermis of the leaves of 

 Heemanthus puniceus in three series of experiments upon the 

 diffusion of different gases. The membrane employed was, in 

 the first series, two films of epidermis with a layer of water be- 

 tween them ; in the second, two moist films without any layer 

 of water ; in the third, two films joined together and then care- 

 fully dried in an exsiccator at 40° C. The method used by 

 Miiller is open in some of its details to criticism, but in a general 

 way the results are instructive. The following are the mean 

 ratios indicating the rate of diff'usion obtained : — 



molecules also takes place without any change in volume. With gases opposed 

 of unequal density and molecular velocity, the permeation ceases, of course, to 

 be equal in both directions " (Philosophical Transactions, 1863). 

 1 Pringsheim's Jahrb., 1869, p. 169. 



