198 THE MECHANISM OF GASEOUS EXCHANGE 



a negative or a positive pressure in the intercellular system, for in the former case, 

 according to Barthe"lemy ', the stomata on the leaves of Nymphaea alba, Ranunculus, 

 Ficaria, &c., close, although it may be easily proved by means of the apparatus 

 already described that this does not always occur. 



Porous cork. By means of the apparatus already described (Figs. 22 and 23) the 

 direct passage of gases through lenticels may be demonstrated. Twigs are preferable, 

 one end of which is closed by an india-rubber cap, so that the passage of gases 

 through the opened vessels is prevented. On subjecting the twig to pressure, 

 air-bubbles will be driven out from the intercellular spaces on the cut surface 

 immersed in water. With a pressure of three-quarters to one atmosphere, air-bubbles - 

 may stream from certain of the open ends of the vessels, and this can be observed 

 without interruption if the bark is removed from the immersed portion of the twig. 

 Air is apparently conveyed to the tracheae by the intercellular spaces of the medullary 

 rays, and thence passes diosmotically through the walls of the vessels, but it is only 

 when the pressure is relatively high that the diosmotic transference is sufficiently 

 rapid to cause an actual current of air towards the open mouth of the vessel. It is 

 also possible by driving in air at the cut surface to cause streams of gas to pass out 

 through the lenticels 2 . The cessation of the stream of bubbles when the lenticels 

 are saturated with water shows that the gases pass directly through the open inter- 

 cellular canals of these organs. 



Even when distinct lenticels are absent, as in Vitis> Lonicera, &c., gases may, 

 according to Klebahn, still diffuse through the intercellular spaces of the spongy 

 cork occurring in the periderm, and also between the suberinized endodermal cells. 

 The details of the formation and distribution of porous cork are given by Klebahn 

 along with a critical account of the previous literature. According to his own 

 experiments, porous cork, when once formed, functions permanently as an open 

 gaseous channel. In many plants, however, the size of the intercellular spaces 

 alters in the growth of successive years, and it is probably owing to the altered 

 rates of diffusion thereby caused that many authors have concluded that lenticels 

 close in winter. Klebahn has, however, shown that even in winter a pressure of 

 5 to 6 cm. of mercury causes bubbles of gas to be driven through the lenticels 

 (Klebahn, 1884, p. 563). In order to regulate the loss of water by evaporation, 

 it would be of distinct advantage if the changes of form which the dead cells 

 undergo as they dry were such as to close more or less completely the inter- 

 cellular spaces of the periderm, but this is a point to determine which detailed 

 experimental investigation is needed 3 . 



1 Barthelemy, Ann. d. sci. nnt., 1874, v. ser., T. xix, p. 150. See also Wiesner, Sitzungsb. d. 

 Wien. Akad., 1879, Bd. LXXIX, p. 38 (Sep.). N. J. C. Muller's researches (Jahrb. f. wiss. Bot., 

 1869-70, Bd. vii, p. 161) leave undetermined the causes to which the different effects produced 

 according to the direction of the gaseous current are due. 



2 Experiments by Stahl, Bot. Zeitung, 1873, p. 613 ; G. Haberlandt, Sitzungsb. d. Wien. Akad., 

 1875, J3d. LXXII (July session).; Klebahn, Ber. d. Bot. Ges., 1883, p. 113, and Jenaische Zeitschr. 

 f. Nat.-wiss., 1884, p. 562 ; Hales, Statics, 1748, p. 91, and PI. vii, Fig. 32. Von Hohnel was the 

 first to prove that air bubbles pass out of the vessels when the pressure is high (Jahrb. f. wiss. Bot., 

 1879, Bd. XII P- 49 N - Details by Strasburger, Bau u. Verrichtung d. Leitungsbahnen, 1891, p. 716. 



3 Cf. Klebahn, 1. c., 1884, p. 574 ; Devaux, Ann. d. sci. nat, 1891, vii. ser., T. XIV, p. 340. 



