254 GASEOUS EXCHANGE [ch. 



dioxide which a 3-Iitre jar of water would absorb from the air 

 at ordinary temperatures, could be used up by ten shoots of 

 Elodea in two minutes. His view is that the substratum serves 

 as the chief source of carbon dioxide for submerged plants, the 

 amount of this gas given off into the water from soil containing 

 organic matter being greater than that obtained by diffusion 

 from the air. 



Whether the excess of carbon dioxide is, in general, derived 

 from the substratum, or whether it is due to the oxidation of 

 carbonaceous substances in the water or to other causes, the 

 fact remains that hydrophytes growing under natural conditions 

 live in an environment particularly rich in carbon dioxide. This 

 advantage tends to be neutralised, however, by the slow diffusion 

 of gases in water. There is also the further drawback that the 

 absorption capacity of water sinks as the temperature rises, so 

 that, in warm weather, when the life processes of the plant are 

 proceeding most vigorously, the supply of carbon dioxide is 

 reduced^. Assimilation is nevertheless remarkably active among 

 water plants, several features which they commonly show being 

 well suited to the prevailing conditions; one of these is the deve- 

 lopment of chlorophyll in the epidermal cells, so that the epi- 

 dermis forms part of the assimilating system, which is thus not 

 shut off from the surrounding medium by a layer whose func- 

 tion is purely protective, as in the case of terrestrial plants. 

 Cuticle is relatively little developed, and the cell-walls seem to 

 offer no more hindrance to the direct passage of dissolved gases 

 than if they were merely thin plates of water^. That the waxy 

 cuticle of such leaves as those of the submerged Potamogetons 

 is no obstacle to the entry of liquids, has been proved by plas- 

 molysis experiments in which the whole leaf was used^. 



Submerged plants show various characteristics which have 

 the effect of increasing the surface relatively to the volume of the 

 leaf, and thus bringing a large proportion of the assimilating 

 cells into direct contact with the dissolved carbon dioxide. The 



iGoebel, K. (1891-1893). 2 Qevaux, H. (1889). 



3 Sauvageau, C. (1891^). 



