LAKES, RIVERS, ETC. 



245 



plants are distinguished from land plants in 

 being submerged wholly or in part in water. 

 There are also moisture-loving plants (see 

 below), from which they are distinguished, 

 hence the term Hydrophytes. Certain modifi- 

 cations, briefly mentioned in Section I, are 

 required by water plants to fit them for life in 

 water. 



The watery element necessitates the increase, 

 in size and number, of the air-spaces. The 

 chlorophyll granules are aggregated in cells 

 nearer the surface than in land plants to ac- 

 complish photosynthesis more effectively. 



Water plants have a thin cuticle which is 

 not cutinized or waxy, so that absorption takes 

 place at all points, and stomata are not re- 

 quired as a rule, or where present are on the 

 upper surface, and do not open and close. 



The stems of water plants, owing to the 

 support received from the water, do not require 

 thickening, or a series of vessels of woody 

 elements (xylem) for the conduction of water 

 from the root to the rest of the plant, as 

 absorption is possible at all points, so that the 

 supporting tissues are reduced; but as it is 

 necessary for the plant to convey food from 

 the leaves the phloem is well developed. 



Since absorption is not confined to the roots, 

 these are also not well developed, and may be 

 absent, as in Bladderwort, or where present 

 be merely organs of attachment, or serve to 

 maintain equilibrium. 



Whilst many aquatic plants have primitive 

 characters, some have highly-organized flowers, 

 as Water Buttercups, and these are probably 

 derived from earlier land types and have 

 adapted themselves vegetatively to life in the 

 water. A large proportion of Monocotyledons 

 (over 30 per cent) are aquatics. 



Hydrophilous Types. Between land plants 

 that do not need a considerable amount of 

 water and those that are aquatic there are 

 intermediate types, which are distinguished 

 owing to their demand for a greater or less 

 amount of moisture. Those that are inter- 

 mediate, such as meadow plants, woodland 

 plants, cornfield plants, are Mesophytes, re- 

 quiring a medium amount. There are others 

 that require a larger amount of moisture, such 

 as marsh plants and wet meadow types, and 

 the riparial types of aquatic plants. 



Marsh plants, like water plants, are not able 

 to transpire readily, and have emergency exits 

 for the expulsion of water in drops at special 

 points. 



Zonal Character of Aquatic Vegetation. 

 Aquatic vegetation shows a well-marked zona- 

 tion or arrangement of different types in zones 

 or parallel bands. This applies whatever be 

 the character of the aquatic habitat lake, 



river, &c. though there may be an 

 of one or more types, and the one may take 

 the place of the other, showing the manner in 

 which aquatic vegetation may invade the land, 

 or vice versa. Of freshwater types of forma- 

 tion there are four main divisions. These are 

 based upon the relative force of the current, or 

 the richness or otherwise of the water in 

 mineral salts. 



Thus there are slow-flowing rivers which 

 are rich in mineral salts (alkaline), and these 

 are usually lowland. Then there are stagnant 

 or foul waters, which are usually devoid of 

 flowering plants, and are colonized by Blue- 

 green Algae, as sewage waters. Some nearly 

 stagnant waters may be rich in lime, and 

 contain a characteristic phanerogamic flora. 

 Where the water is poor in mineral salts, as 

 in lakes and tarns in upland regions, usually 

 on siliceous rocks, certain rare and local types, 

 as Quillwort, Lobelia, Shore Weed, &c., grow. 

 These upland lakes are usually moorland 

 highland lakes. There are also quickly-flow- 

 ing streams on hill slopes, which may either 

 be rich or poor in lime, and each has its own 

 particular flora. 



In all these cases the plants show a zonal 

 arrangement. In the middle grow the plants 

 that float or have floating leaves, as Duckweed, 

 Water Buttercup. Around the central zone 

 grow half -submerged or totally submerged 

 types, as Pond weed, Hornwort, &c. ; whilst in 

 what is called the third or Reed Swamp ann- 

 otation grow the Reeds, Rushes, Sedges, Purple 

 Loosestrife, &c. , which are only half -sub- 

 merged. 



Uniformity of Conditions, or Diversity. 

 Water preserves a more or less uniform tem- 

 perature, and is less modified than land by the 

 alternation of summer and winter, so that 

 there are no aquatic Tropophytes, in the same 

 sense at any rate as with land plants. The 

 conditions of light are more or less uniform 

 in freshwater formations, but are influenced by 

 the weather in the shape of clouds, fog, and 

 extremes of sunshine. 



Water and its Effect upon Plants. Water 

 may vary in respect of its constituent salts, 

 and be either in general salt or fresh water. 

 There is also an intermediate type which is 

 neither saline nor fresh, but brackish, as along 

 the coast in salt marshes or estuaries. 



Fresh water may be hard or soft, poor or 

 rich in mineral salts. The water of upland 

 lakes is usually poor in mineral salts, but peat 

 bogs are usually charged with humous acids 

 and acid in reaction. Other upland waters 

 situated upon siliceous sandy rocks are poor in 

 mineral salts, and clear or pure. It is in such 

 pools or lakes that Desmids are especially 



