92 PLANT GEOGRAPHY 



The refraction of light by water gives to aquatic 

 plants, even at moderate depths, the characters of shade- 

 plants, such as long internodes, epidermal chlorophyll, 

 and the absence of palisade-tissue. The buoyancy of 

 the water renders such mechanical tissue as wood 

 unnecessary, and the absence of transpiration and the 

 absorption of food through the whole of the uncuti- 

 cularised surface cause a great reduction in the vascular 

 system. Even those water-plants that are rooted in 

 the mud probably depend upon their roots for little 

 beyond mere attachment. The general presence of large 

 intercellular spaces, though it may serve to float the 

 plant and thus aid it in obtaining air, is doubtless largely 

 concerned in the storage of gaseous food-elements. The 

 rounded leathery floating leaves, and the much-divided 

 or long ribbon-like submerged ones, found often on the 

 same plant, and offering a minimum of resistance to 

 running water, are familiar external modifications occur- 

 ring in many plants not otherwise related to one another. 



The geographical distribution of aquatic Spermato- 

 phytes is far wider than that of average land-plants, 

 especially in an east and west direction. Most British 

 species range through the North Temperate Zone. In 

 the remarkable tropical order Podostemacece, Dicotyle- 

 dons resembling mosses or liverworts, adapted to life in 

 rapid streams, and of most uncertain systematic position, 

 Podostemon and Tristicha extend from Brazil to Madagas- 

 car and South Africa, the former also occurring in North 

 America. 



Although not so conspicuous by their size as are 

 the Spermatophytes, the" Cryptogamia contribute in- 

 teresting types to fresh- water floras. Thus, in the 

 Benthos, or submerged but rooted group, we have the 

 Characece, so abundant on calcareous mud, such mosses 

 as Fontinalis and Hypnum frequenting both running and 

 stagnant waters, and the Quillworts (Isoetes) of our lakes 

 with their erect, rush-like leaves. In the Pleuston, or 

 ' free-swimming group, such Hepaticae as Riccia, and 

 HydropteridecB such as Azolla and Salvinia occur with 

 Duckweeds (Lemna), Utriculavia, Aldrovanda, etc. The 

 green Algae or Chlorophycece, which are of very primitive 

 construction and cosmopolitan in occurrence, include 

 many rooted forms and others that are free-swimming. 

 The Plankton, or floating micro-organisms of our lakes, 



