MOVEMENTS AND PRESSURE OF THE INTERNAL GASES 201 



important part in the transport of water, but it is nevertheless probable that 

 a certain negative pressure, partly due to the evaporation of water, may 

 arise in intercellular spaces, for these also may suck in water to a certain 

 extent (Sect. 29). It has, however, not yet been determined whether the 

 intercellular spaces aid by any such means in the absorption of water. The 

 marked negative pressure of the intercellular air which Goebel 1 observed 

 on cloudy days in the leaf-stalks of Nymphaea, Nelumbium, Colocasia, &c., 

 might have been produced in various ways, for independently of the action 

 of darkness, when the intercellular air is rich in oxygen, the more rapid 

 diosmosLs of the respiratory carbon dioxide suffices to induce pronounced 

 negative pressure. It is probably mainly in the latter manner that the slight 

 negative pressures observed in the 

 intercellular spaces of tubers 2 , &c., 

 and of submerged water plants, are 

 produced in the absence of any tran- 

 spiration. Detailed investigations 

 are necessary in such cases to deter- 

 mine the precise means by which 

 the intercellular spaces become per- 

 manently filled with air. 



The existence of rarefied air in 

 plants was first shown by experiments 

 performed by Hales 3 . If the base 

 of a leafy twig is immersed in water, 

 as shown in Fig. 24, and the cut side 

 branch, b, is attached by an india- 

 rubber connexion to the glass tube a, 

 filled with air and having its lower end 

 in water or mercury, the latter rises 

 in the tube a, showing that air has 

 been drawn in through the cut surface 



of the side branch. In these and similar experiments the mercury may be raised 

 to a height of 3-5 cm., although the result produced is chiefly or entirely due to the 

 rarefication existing in the intercellular air. By using barked twigs, or by inserting 

 manometers in the wood of trees, much higher negative pressures are usually 

 obtained 4 . 



FIG. 24. 



1 Goebel, Pflanzenbiol. Schilderungen, 1893, 2. Th., p. 251. 



2 Devaux, Ann. d. sci. nat., 1891, vii. ser., T. xiv, p. 393. On the feeble development of the 

 intercellular spaces in certain tubers, see Sect. 31. 



3 Hales, Statics, 1748, p. 90. Similar experiments by Meyer, Physiol., 1838, Bd. n, p. 73 ; 

 Sachs, Experimentalphysiol., 1865, p. 261; Barthelemy, Ann. d. sci. nat., 1874, v. ser., T. XIX, 

 p. 150; Bonnier, Rev. gen. de Bot., 1893, T. v, p. 13, &c. 



* See Th. Hartig, Bot. Zeitung, 1861, p. 18 ; Bohm, Versnchsst., 1877, Bd. XX, p. 371 ; Pappen- 

 heim, Bot. Centralbl., 1892, Bd. xi.ix, p. 2 ; Bonnier, 1893, 1. c. 



