CHAP. XXXVI OECOLOGICAL FACTORS 153 



the part of the organism to periodic changes in the buoyancy of fresh 

 water.^ 



Colour of water. Water in a pure condition is blue. Any other colour 

 may be caused by organisms,^ by suspended particles of clay and the like, 

 or, especially in fresh water, by humous acids ; yellow or brown water 

 often contains many humous acids and is acid in reaction, whereas alkahne 

 (hard) water is clear (blue). 



Movements of water. Movements of water are of great importance 

 to vegetation. They assume the form of waves (breakers) or currents, 

 and lead to a fresh supply of oxygeti ; in streaming water assimilation is 

 more active.^ Still water is very inimical to vegetation ; and for this 

 reason many species are absent from stationary depths over large areas, 

 or from enclosed calm inlets. In addition, moving water conveys addi- 

 tional nutriment ; for instance, sea-water contains but little iodine and 

 calcium, yet large quantities of these are stored by many Algae. Move- 

 ments of water are all the more essential inasmuch as many aquatic 

 plants, and particularly algae, as a rule have no far-reaching roots (in a 

 physiological sense). Finally, movements of water act mechanically, in 

 that they stretch and bend plants with a force that varies with their 

 strength. In larger plants mechanical tissue is developed*; calcareous 

 incrustations may also contribute to the stabihty of sea-weeds, but it 

 is worthy of note that chalk-forming algae and many crustaceous algae 

 grow, some in deep, and others in still water. The general shape of aquatic 

 plants is adapted to the surroundings in divers ways ; thus in rapid 

 currents we find very elongated plant-members (ribbon-like fohage, and 

 long filiform algae). 



A distinction must be drawn between currents and wave-movements, 

 as many species withstand the former but not the latter. Very many 

 species flourish only in tranquil water. 



Movements of water also favour the dispersal of propagative organs, 

 such as detached vegetative parts, spores, or seeds.^ 



Aquatic species of plants throughout have a very wide geographical 

 distribution. This is partly because the conditions of life are uniform, 

 or only slightly different, over wide areas, and the transport of marine 

 plants over great distances is very easy, and partly because many species 

 are carried far by water-haunting birds and insects, or are transported 

 by air-currents, as is the case with the smallest, mostly microscopic, 

 species. 



Differences in the aquatic flora associated with geographical situation 

 show themselves in some respects more marked in the sea than in other 

 waters ; this may be due to greater physical differences, and to the 

 universally greater constancy in amount of salt, in temperature, and 

 other characters of sea-water. 



The general morphological and anatomical adaptation of submerged 

 organs and aquatic plants has already been considered in Chapter XXVII. 



' Ostwald, 1903 a ; Wesenberg-Lund, 1900, 1908. ' See Chap. XXXVIII. 



' F. Darwin and D. Pcrtz, 1896. p. 296. * Wille, 1885. 



* Compare Hemsley, 1885; Schimpcr, 1891 ; Sernander, 1901 ; Rosenvinge, 

 1905 ; Kjellman, 1906. 



