ECOLOGY 409 



spongy and the chloroplasts motile. Stomata are entirely absent 

 from leaves that are submerged and only present on the upper sur- 

 face of floating ones, where they are nearly always open. Some of 

 these plants have broad floating leaves and dissected submerged 

 ones, often with thread-like divisions. The submerged parts are 

 devoid of special protective walls e.g. those containing cutin or 

 suberin. The cell sap has a low osmotic pressure. The submersed 

 leaves often absorb more water than the roots. The free floating 

 microscopic plants (blue-green algae, bacteria, diatoms, desmids, etc.) 

 form the plankton of our ponds, rivers and lakes. The free-swim- 

 ming higher plants (the pleuston) comprise certain liverworts 

 like Riccia and Ricciocarpus, water-ferns and such seed plants as the 

 water-lettuce and water-hyacinth. The aquatic plants including 

 the algae, mosses and flowering plants, which live attached to rocks 

 comprise the lithophilous benthos. Another class of aquatic plants 

 (benthos) include those with true roots, which attach the plant to 

 the substratum, and at most possess floating leaves. This type 

 includes the water-lilies, the water-chestnut, the splatter docks, 

 the floating-heart and the pondweeds. 



Helophytes. To this group belong plants typical to marshes. 

 A marsh is an area with wet soil, wholly or partially covered with 

 water and with annual or perennial herbs (never shrubs and trees) 

 which are adjusted structurally to a mucky soil, lacking the usual 

 supplies of oxygen. These plants likewise show an adjustment to a 

 partial or periodical submergence. Like hydrophytes, marsh plants 

 are for the most part perennial. They produce adventitious roots 

 and possess horizontal rhizomes, or runners, and frequently have 

 air chambers in roots, stems and leaves, so that they are adapted 

 to meet the scarcity of air in wet soils. They also show a striking 

 development of erect chlorophyll-bearing organs in the shape of 

 leaves, in the flags, and stems, in the rushes. 



The taller seed-like plants of the marsh-land, such as seed-grass 

 (Phragmites), the bur-reed (Sparganium), the cat-tails (Typha), the 

 blue flags (Iris), the sweet flag (A corns calamus) and the papyrus 

 (Papyrus) form associations known as fresh-water marshes, reed- 

 marshes or fens. The channels or pools of water in amongst these 

 amphibious plants are filled with true aquatic plants. 



