422 THE ECOLOGY OF PLANTS. 



or thrice forked into narrow segments, which have small teeth on 

 their edges. Water Violet and Bladderwort are free-floating 

 plants. 



4O2. Marsh Plants. Under this heading we include 

 those plants whose roots and rhizomes, or shoot-bases, are in 

 water or mud, while their leaves, or foliage-bearing shoots, 

 and of course the flowers, grow in the air. Between typical 

 aquatics, marsh plants, moisture-loving plants, and land 

 plants there is every stage of transition. We may, however, 

 distinguish between associations of permanently submerged 

 water plants and those characteristic of marshes and bogs in 

 which the substratum alternates between long periods of 

 wetness and shorter periods of more or less complete drying- 

 up. The characteristic feature of marsh and bog plants is 

 that their lower parts, buried in the mud, are adapted to 

 aquatic life, while their upper parts, exposed to the air, 

 either resemble those of land plants or are adapted to with- 

 stand drought. 



The reduced leaf -surf ace and thick cuticle of Rushes, 

 Sedges, Horsetails, etc., have often been attributed to the 

 existence in stagnant mud, especially in peat-bogs, of acids 

 produced by the decaying organic matter, this acidity making 

 it difficult for the roots to absorb water and necessitating a 

 reduction in the transpiring surface of the plant. But actual 

 analyses show that in some cases at any rate the pond-mud 

 in which Eushes and Sedges grow contains no acids or only 

 traces of acidity, and moreover the presence of acids, in 

 certain quantities, actually increases the rate of water ab- 

 sorption by plants. 



In order to meet these objections it has been suggested 

 that these "marsh xerophytes " owe their mixed characters 

 to the persistence of ancestral features in spite of a 

 striking change of habitat, and that they are now " hydro- 

 phytes wearing a xerophytic mask." Perhaps the bad 

 aeration of the roots, which usually show marked hydro- 

 phytic characters, especially in the existence of abundant 

 air-spaces, has something to do with the xerophytic structure 

 of the shoot in both bog and marsh plants, and the " physio- 

 logical drought " theory (i.e. the view that the acidity of the 

 substratum prevents absorption and acts in the same way as 



