EXTERNAL COMMUNICATIONS OF THE AER1FEROUS SYSTEM 197 



g, to which the glass cylinder / containing water is fitted. By pouring mercury 

 into J, or by means of water pressure, the air in g may be compressed and thus 

 driven through the petiole. The cut surface of the petiole may be observed by 

 means of a lens or low-power microscope, and as v. Hohnel ' has shown, the gas 

 bubbles may be seen to pass out from the intercellular spaces of the pith and 

 cortex even under moderate pressure only. In the leaves of Nymphaea, Funkia, 

 Calla aethiopica, Arum maculatum, Rumex, &c., a pressure of one-eighth to one-fifth 

 of an atmosphere is sufficient, and hence air can be driven through the leaves by 

 means of the lungs. Even when the lamina of Nelumbium spedosum is immersed 

 in water, bubbles of air may pass out from it. If when it is exposed to air, one 

 blows through the petiole, the currents of air escaping from the stomata cause 



d 



FIG. 22. 



FIG. 23. 



the mobile drops of water on its upper surface to roll to and fro, as was first observed 

 by Raffenau-Delile. 



If the stomata are occluded by water, usually no air- bubbles can be driven 

 through them by a pressure of two to five times greater intensity. This is because 

 the water filling the stomatal pore is held with considerable force, owing to 

 the narrowness of the opening 2 . That the stomata are still open is shown by the 

 fact that water may be gradually drawn into the intercellular spaces of the mesophyll, 

 causing the leaf to become a darker green colour 3 . As might be expected, the 

 widening of the stomata allows streams of gas to pass through them more rapidly 4 ; 

 but it is, however, not always immaterial whether the stream of gas is caused by 



1 V. Hohnel, Jahrb. f. wiss. Bot., 1879, Bd. XII, p. 52. A similar convenient apparatus was 

 employed by Strasburger, 1891, I.e., p. JiS. 



2 A thread of water occupying a capillary tube o-oi mm. diameter is only driven out by 

 a pressure of about 22 cm. of mercury. Cf. Nageli u. Schvvendener, Mikroskop, 1877, 2. Aufl., p. 366. 



3 Dutrochet, Memoires, Bruxelles, 1837, p. 172. Similar experiments by Unger and Sachs. 



4 See N. J. C. Miiller, Jahrb. f. wiss. Bot., 1872, Bd. VIII, p. 103. 



