MOVEMENTS AND PRESSURE OF THE INTERNAL GASES 205 



Other gaseous currents in plants. Disregarding the irregular gaseous currents 

 caused by bending or swaying movements, changes of temperature, &c., there 

 appear to be other external agencies which induce streams of gas in particular 

 directions wherever these are readily produced. The currents induced in this manner 

 may, according to Raffenau-Delile ', become so strong in Nelumbium speriosum 

 on bright days that the gas streaming out of the stomata causes the drops of water on 

 the leaf to run to and fro. According to Merget 2 , when a hot body is held near 

 a leaf of Nelumbium, a current of air passes inwards from the surface of the le if. 

 The same takes place in a dead leaf, and it is powerful enough to overcome a pres- 

 sure equal to that of a column of i to 3 centimetres of water. This relatively trifling 

 difference of pressure suffices in Nelumbium or Nymphaea to maintain a current of 

 air in the plant through the rhizome from one leaf to another 3 . 



The precise manner in which gaseous currents of this character are set 

 up is not yet clear, and hence it must remain uncertain whether the necessary 

 energy is derived from a difference of temperature, or whether a difference of 

 potential is maintained by processes connected with transpiration, or whether 

 other causes or combinations of causes come into play. A trifling propulsive 

 force will not suffice to produce any strong gaseous currents through the stomata 

 and intercellular spaces when the resistance offered is great, and still less when 

 the entry of gases takes place by diosmotic means (Sect. 30). 



Composition of the enclosed gases. Here the results obtained correspond i i 

 general with what might be expected on theoretical grounds (p. 99). Thus the 

 composition may vary more or less markedly from the normal percentage, while 

 the combined action of certain factors may cause the indifferent gas nitrogen to be 

 present in different relative amount to that in the air. Under normal conditions the 

 enclosed gas is never composed of pure nitrogen, as some authors have erroneously 

 supposed 4 . A detailed consideration of the observed percentage compositions is 

 hardly necessary, since external conditions may modify the amounts present. 

 Moreover, in obtaining the air for analysis, an admixture with larger or smaller 

 quantities of absorbed or dissolved gases may frequently occur. For similar reasons 

 the percentage composition of the dissolved gas varies, but maintains a certain 

 correspondence with that of the intercellular air. A few further remarks will be 

 made later (Sect. 96), and the reasons for the non-formation of gas vacuoles in 

 the interior of a turgid cell have already been given (Sec. 29). 



In intact plants of Elodea, Ceratophyllum, &c., owing to the ease with which 

 diosmotic exchange takes place, the enclosed air differs but little in composition 

 from that of the atmosphere during day and night 6 . Nevertheless, when exposed 



1 Raffenau-Delile, Ann. d. sci. nat., 1841, ii. ser., T. xvi, p. 328. 



2 Merget, Compt. rend., 1873, T. LXXVII, p. 1469; 1874, T. LXXVIII, p. 884. 



3 Barthelemy, Ann. d. sci. nat., 1874, v. ser., T. xix, p. 152. Cf. also Lechartier, ibid., 1867, 

 v. ser., T. vin, p. 364. 



4 This is stated by E. Schulze (Lehrb. d. Chem. f. Landw., 1853, I, p. 58) to be the case in 

 grass haulms and other hollow stems, and by Barthelemy (Ann. d. sci. nat., 1874, v. ser., T. XIX, 

 p. 167) in Pontederia; but these results are contrary to those of all other investigators. 



8 Devaux, Ann. d. sci. nat, 1889, vii. ser., T. IX, p. 99. As the stream of bubbles continues, 

 the escaping air becomes gradually richer in oxygen, for reasons that are easy to see (cf. Sect. 5*). 



