GASEOUS EXCHANGE OF AQUATICS iii 



in general thin ; and it is the more extensive as the leaf 

 surface is often very great. Many submerged plants have 

 long ribbon-shaped leaves, e.g. the grass wracks of salt and 

 brackish water ; others have leaves divided into many narrow 

 segments, as the water crowfoot. We may note the re- 

 markable assimilating roots, often thalloid in form, of the 

 Podostemacese (see pp. 198, 294, and Fig. 23). The oxygen 

 which is formed in assimilation diffuses out in solution 

 through the whole surface. Oxygen is much less soluble 

 in water than carbon dioxide, and as a consequence of 

 this a considerable pressure may exist inside the plant. 

 Angelstein (191 1) found in Elodea an excess pressure of 

 about one-sixth of an atmosphere ; this was reduced 

 gradually in the dark as the oxygen diffused out, or was 

 used up in respiration. If a submerged plant is wounded, 

 the oxygen formed in photosynthesis escapes from 

 the wounded surface as small gas bubbles. A bunch of 

 Elodea, inverted in water under a test-tube, gives ojff so 

 much oxygen from the cut stems that a few cubic centi- 

 metres may easily be collected. The gas is not pure 

 oxygen — it contains nitrogen as v/ell — but it ignites a 

 glowing splint as oxygen does, a good demonstration of the 

 liberation of oxygen in photosynthesis. This " gas-bubble " 

 method has also been used for determining the relative 

 rates of assimilation under different conditions ; the number 

 of bubbles given off from a cut shoot in a given time is 

 used as a basis of comparison. The method must be used 

 with caution, as it is subject to many errors. It has recently 

 been improved by Wilmot (1921). 



Water in contact with air absorbs carbon dioxide 

 according to definite physical laws ; it comes to an equili- 

 brium with air when it contains about the same percentage 

 of gas as does the atmosphere. If the air were the 

 only source of carbon dioxide for submerged plants they 

 would be badly off, for diffusion into water is slow ; 

 the rapid mixing due to air currents in the atmosphere is 

 wanting, and the supply could not keep pace with the 

 plants' requirements. It is supplemented in two ways. 



