256 GASEOUS EXCHANGE [CH. 



assimilation, through the disintegration of carbon dioxide 1 . 

 The greater part of the oxygen, in the case of terrestrial plants, 

 is at once returned, by means of the stomates, to the atmosphere 

 whence it came. But water plants show a marked tendency to 

 retain this element, and we find that their tissues are generally 

 penetrated by an elaborate system of intercellular lacunae, by 

 means of which the oxygen evolved in the assimilating cells 

 presumably finds its way to other parts of the plant, where it 

 may be used for respiration 2 . The aerating system arises very 

 early, as it also does, indeed, in many terrestrial plants; we 

 find, for instance, that, in the growing apex of the stem of 

 Elodea, there is a network of intercellular spaces reaching to 

 within two or three cells of the summit, while, in the winter- 

 buds of Myriophyllum, a complete ring of large air canals occurs 

 only i mm. from the stem apex 3 . In many water plants the 

 air system is so elaborately developed that almost all the cells 

 are in contact with the internal atmosphere by means of some 

 part of their surface. Unger 4 , who has measured the quantity 

 of air contained in various plant tissues, finds that 71-3 per cent. 

 of the volume of the leaves of Pistia Stratiotes^ L., the floating 

 River Lettuce, is occupied by air, while in land plants, especi- 

 ally xerophytes, the percentage is much lower; for instance, 

 the leathery leaves of Eucalyptus Preissiana^ Schauer, contain 

 9 6 per cent, of air, and the succulent leaves of Begonia hydro- 

 cotylifolia y Hook., only 3-5 per cent. 5 



The exception that proves the rule that the tissues of water 

 plants are characterised by the unusual development of inter- 



1 CloeZjS. and Gratiolet, P. (i 850); Cloez,S. (i 863); and later literature. 



2 The gases bubbling from wounds in the green shoots of submerged 

 plants in sunlight have been described as containing about 90 per cent, 

 of oxygen. Tieghem, P. van (1866). 



3 Devaux, H. (1889). 4 Unger, F. (18542). 



6 Unger, F. (18542). The leaf of Pistia has no elongated stalk, while, 

 in the case of Eucalyptus and Begonia^ Unger includes the petioles in 

 the calculation. This might tend slightly to exaggerate the difference 

 in the percentages, but, even if corrected for this detail, the figures would 

 doubtless remain sufficiently striking. 



