PASSAGE OF GASES THROUGH STOMATA. 303 



808. Experiments by a wholly different method were con- 

 ducted by Boussingault,^ upon leaves of Oleander. By a leaf 

 having an upper surface of 37.2 square centimeters free from 

 stomata, and completely closed on the under side by tallow, 

 17.5 cubic centimeters of carbonic acid were absorbed in a given 

 time. 



In another series of experiments Boussingault fastened the 

 under surfaces of two leaves closely together by means of paste, 

 so that only the upper surfaces (free from stomata) were exposed 

 to the air ; with these leaves nearly the same results were ob- 

 tained as in the first series. 



809. Passage of gases through stomata. Stomata (see Figs. 

 52 and 54) are practically minute apertures in thin plates, and 

 under ordinarj' circumstances there is no obstruction to the 

 readj' passage of gases through them from the surroundings into 

 the interior of the plant. The changing pressure caused by agi- 

 tation of the foliage exerts, as it does in aqueous transpiration, 

 an important influence in facilitating this passage. 



810. Merget '' holds that it is chiefly through stomata that 

 the interchange of gases with the outer air takes place in the 

 plant ; but, on tlie other hand, it is claimed by Barthelemy ^ that 

 they play only a very subordinate part. There can be little 

 doubt that the earlier view advanced and illustrated by Du- 

 trochet,* and further by Garreau,^ is substantially correct ; namely, 

 that gases enter and escape from the plant freelj' both by diff'u- 

 sion through the cuticularized cell-walls of the epidermis and 

 by passage through the stomata. 



811. Atmospheric air is chiefly amixture of two gases, oxygen 

 and nitrogen. The proportions in which these substances and 

 others, occurring in much smaller amounts, are found in drj' air 

 are usually stated as follows : — 



Proportions by volume. Proportions by weight. 



2Sritrogen . 79.01984 76.8399 



Oxygen 20.94000 23.1000 



Carbonic acid 04000 .0600 



Ammonia 00016 .0001 



100. 100. 



1 Agronomie, iv., 1868, p. 374. 



2 Comptes Rendus, Ixxxiv., 1877, p. 376. 



Ann. des Sc. nat. hot., ser. 5, tome xix., 1874, p. 131. 

 - Ann. des Sc. nat., tome xxv., 1832, p. 242. 

 ^ Ann. des Sc. nat. bot., ser. 3, tomes xv., xvi. 



