64 THE ADAPTATION OF WATER-PLANTS 



operations through a greater area of the fluid from which it derives 

 its suppUes of carbon dioxide, as in Ranunculus aquatiiis, Hottonia, 

 Utricularia, and Myriophyllutn. In other plants whose submerged 

 leaves approach in size and shape the terrestrial type, as in Ana- 

 charts, Potamogetoji crispus, P. densus, and P. perfoliatus, the same 

 ends are subserved by the translucency of these organs, together 

 with their comparative smallness and greater number. Moreover, 

 many of them grow in slow-running streams, and their flexible 

 stems permit them to assume a sloping position, so that the upper 

 leaves shut out little or no light from the lower ones. Submerged 

 leaves have no stomata, the carbon dioxide which they require for 

 food being absorbed directly from and with the surrounding water, 

 or in combination with calcium as bicarbonate of lime, the latter 

 salt being in solution and having a higher percentage composition 

 of its component carbonic acid than monocarbonate of lime 

 (CaCog), from which it has been formed. A number of hydro- 

 phytes, including Myriophylliim, Ceratophyllum, Ranunculus aqua- 

 tiiis, and several species of pond weeds, are capable of absorbing 

 from the bicarbonate this extra portion of carbonic acid, leaving 

 the nearly insoluble (in water) monocarbonate as an incrustation 

 on the stems and leaves. 



In some cases, the lime, in addition to forming in this way an 

 external coating to the cell-wall, permeates its substance as well. 

 Apart from its geological importance as a factor in the formation 

 of calcareous beds, consequent on the death and decay of a suc- 

 cession of such vegetable growths, the precipitated lime acts as a 

 further protection to the tender tissues of the living plants. Float- 

 ing leaves are, as a rule, entire (e.g., NyinphcBa, Nuphar, Villarsia, 

 Hydrocharis), and have no stomata on their lower surface, where 

 they are in contact with the fluid, but have abundance on the 

 upper surface, which is exposed to the air. 



Aerial leaves, which are lifted entirely out of the water, are 

 usually erect, and have the stomata pretty equally distributed on 

 both surfaces (water-flag, grasses, and sedges). The water butter- 

 cup {Ranunculus aquatiiis), already cited as an example, strikingly 

 illustrates the various types of leaves characteristic of aquatic 

 plants. Some varieties of it have all the leaves submerged and all 

 finely divided ; others have both submerged and floating leaves, 



