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 {Acorns 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. 
