1 86 THE MECHANISM OF GASEOUS EXCHANGE 



Normal respiration continues in many leaves if the surface bearing 

 stomata is smeared with vasdline, but in other cases oxygen diffuses in- 

 wardly in insufficient quantity to allow full respiration to continue under 

 such circumstances. The same will usually be the case when tissues are 

 completely surrounded by relatively impermeable cork layers l . 



It is easy to understand why the closure of the stomata, even in leaves 

 with hardly any cuticle, may suffice to prevent any production of starch, 

 for owing to the exceedingly minute percentage of carbon dioxide present 

 in ordinary air, the assimilation of this gas is active only when gaseous 

 exchange is extremely rapid (Sect. 57). Starch may appear, however, in 

 leaves with blocked stomata in an atmosphere containing 20 to 40 per cent, 

 of carbon dioxide. Blackman 2 has indeed shown definitely that this gas 

 diffuses in appreciable amount through the thick cuticle of leaves viNerinni 

 oleander and Prunus lanrocerasits in such an atmosphere. Similarly, 

 when the surface bearing stomata is smeared with vaseline, the carbon 

 dioxide produced by respiration diffuses outwards more and more rapidly 

 through the cuticle as it accumulates, whereas at first it escaped almost 

 entirely through the stomata. From these considerations, as well as from 

 the large amount which can be assimilated when the stomata are open, 

 it follows that the cell-walls bordering upon the intercellular spices must 

 be readily permeable to gases. 



The feebly cuticularized epidermis of aquatic plants is readily per- 

 meable to water, and it allows gases also to diosmose with ease. The 

 formation of bubbles which accompanies the assimilation of carbon dioxide 

 (Sect. 32) demonstrates directly that large amounts of this gas diffuse into 

 the interior even when the surrounding water contains but little ? '. 



o 



periods of time, for the necessary energy might be derived from intramolecular respiration, and as 

 a matter of fact, certain aerobic plants (Chara, A'itclla, &c.) continue to show rotation for long 

 periods of time (one or more weeks) in the absence of free oxygen (cf. Ewart, Journ. Linn. 

 Soc. Bot., Vol. xxxi, 1896, p. 421, and Kiihne, Zeitschr. f. Biol., Bd. xxxvi, p. i, 1898). By 

 means of the bacterium method it can, however, be shown that oxygen diosmoses outwardly 

 through the cuticulnrized walls of an assimilating hair-cell lying in water, but more rapidly through 

 the uncuticularized end-walls, and the difference becomes increasingly marked as the cuticularization 

 is more complete (cf. Ewart, 1. c., p. 365). Diosmosis will be equally possible in air, provided 

 that the properties of the cuticular film are unaltered by immersion and that its water-percentage 

 remains constant.] 



1 On the permeability of cork for gases, see Lietzmann, Flora, 1887, p. 3"6; Wiesner tind 

 Molisch, Sitzungsb. d. \Vien. Akad., 1889, Bd. XCVIII, Abth. i, p. 678; Wiesner, ibid., 1879, 

 Bd. i.xxix, Abth. i, p. 4 (Sep.X 



2 Blackman, Phil. Trans., 1895, Vol. CLXXXVI, p. 556 ; Annals of Botany, 1895, Vol. IX, p. 164. 

 Hence it was that Boussingault found that CO 2 -assiinilation took place in atmospheres containing 

 a large percentage of carbon dioxide, although the stomata of the leaves had been closed by vaseline 

 (Agron., Chim. agric., &c., 1868, T. IV, p. 375. See also Garreau, Ann. d. sci. nat.. 1849, iii. ser., 

 T. xni, p. 343). 



The amount of a gas absorbed by a given volume of water depends upon the solubility as well 

 as the partial pressure of the former. Hence at I5C. water contains, when fully saturated, 63.8 N, 

 34-0 O, 2-2 CO 2 , whereas the air above contains 21 O, 79 N, 0-04 COo. A litre of such water 



