SECTION IV 



CLASS I. HYDROPHYTES. FORMATIONS OF 

 AQUATIC PLANTS 



CHAPTER XXXVI. OECOLOGICAL FACTORS 



BEFORE considering the various communities of aquatic plants it is 

 necessary to discuss the general characters of water and its oecological 

 factors, in so far as these affect the distribution and existence of plants 

 confined to water. 1 



Air in water. Air occurs dissolved in water in variable amounts. 

 In the atmosphere 2 and in water the gases present are the same, but their 

 relative proportions are different ; the gas absorbed by ordinary water 

 contains more oxygen and much more carbon dioxide in proportion to 

 nitrogen than does the atmosphere. Just as in the case of land-plants, 

 these two gases are the only important ones, the former in respiration, 

 and the latter in assimilation. Only certain bacteria can dispense with 

 oxygen. Air can reach parts that are submerged in water with much 

 greater difficulty than it can reach parts situated in the atmosphere or 

 in ordinary soil ; indeed, stagnant water may become so poor in oxygen 

 as to almost exclude the existence of higher plants and animals. Appa- 

 rently in consequence of this, certain species occur particularly in places 

 where the water is very troubled or has a rapid flow, and where there 

 is a constantly fresh supply of water ; possibly for the same reason many 

 submerged parts (leaves) or whole plants (algae) are divided into capillary 

 segments compare the construction of gills by which means the surface 

 in contact with water is greater than if the organ presented a single surface ; 

 and perhaps for the same reason many algae and Podostemaceae bear long 

 hairs that serve as probably respiratory organs or enlarge the assimilatory 

 surface. The obstructed supply of air is also a reason, and perhaps the 

 most important one, for the large air-containing spaces 3 which occur 

 in many aquatic plants (sometimes occupying more than seventy per cent. 

 of the whole volume of a plant) ; these spaces serve, inter alia, to convey 

 air, and especially oxygen, from parts in the air to those in the water or 

 mud. Certain swamp-plants, and especially some living in mangrove 

 swamps, possess special respiratory organs which will be described here- 

 after. 4 



When access of air is prevented and water becomes poor in oxygen 

 there are formed in soil humous acids, which are characteristic of moor 

 and peat soils, and which render soil physiologically dry. 5 



As the temperature rises the power of water to dissolve gases decreases, 

 and this is perhaps the main reason for the disappearance of certain 



1 See upon this subject Oltmanns, 1905. * See p. 14. * See p. 98. 



4 See Chapter LX. * See Chapter XVI and Section VI. 



