186 HELOPHYTES SECT, v 



example, those living on Sphagnum must necessarily have the power of 

 raising themselves as their substratum grows. 1 



3. As in aquatic plants, internal air-containing spaces develop in stems, 

 leaves, and roots, and are adapted to meet the scarcity of air in wet soil, 

 whose air-content is sometimes further decreased by special conditions, 

 such as the accumulation of organic remains, production of peat, 2 the 

 interweaving of roots, and other means by which there is formed a covering 

 that shuts off the supply of air. In order to secure adequate aeration 

 for the plant the following special devices exist : 



(a) Aerenchyma* : a tissue which, like cork, has its own phellogen, 

 but consists of thin-walled, non-suberized cells, and includes large, air- 

 containing, intercellular spaces. The tissue shows itself as a white, 

 spongy envelope. It occurs in Epilobium hirsutum and other species, 

 Lythrum Salicaria, Lycopus europaeus, the mimosaceous Neptunia 

 oleracea, and others. 



(b) Respiratory roots (pneumatophores). In some trees and shrubs 

 there are developed erect roots which thrust their tips above water and 

 convey air to the intercellular system of parts in the mud by means of 

 their pneumathodes, that is to say, by means of lenticels or other com- 

 munications with the atmosphere. 4 They occur particularly in mangrove- 

 swamps ; also on certain palms, Taxodium distichum, 5 and possibly on 

 Jussieuea. 6 



4. Marsh-plants commonly have mesophytic leaves ; but in a remark- 

 able number of them xerophytic structure is encountered. 7 



5. The seeds and fruit of many marsh -plants are provided with air- 

 containing spaces and other devices that facilitate their dispersal by 

 water. 8 



FORMATIONS AND ASSOCIATIONS 



Qualities of the soil play a great part in evoking floristic, and, more 

 or less, oecological distinctions in the communities : 



Saline swamps are not only floristically, but also anatomically and 

 morphologically so peculiar, that they differ widely from fresh-water 

 swamps, and they will be considered in Section VII, dealing with halo- 

 philous vegetation. 



Fresh-water swamps, which alone will be dealt with here, display many 

 differences according as the water is troubled or not, according as the soil 

 is muddy, sandy, or gravelly, and so forth. The constituent formations 

 have been but little investigated. These are probably several, but 

 certainly there are two : Reed-swamp and Bush-swamp. There are also 

 communities on the ' boundary ' zone of wet land which live an amphibious 

 life, and are hydrophytic in adaptation, and for these Schroter and 

 Kirchner set up a third formation consisting of amphiphytes. They 

 write 9 : ' Every point on the boundary zone is annually inundated for 

 a shorter or longer period, which is longer the nearer it stands to the lake. 

 . . . Thus this zone represents a very gradual transition from terrestrial 



1 See P. E. Muller, 1894. 2 See Chap. XVI. 



3 Schenck, 18896; Gobel, 1889-92, ii. 



4 Gobel, 1886; Wilson, loc. cit. ; Schenck, 18890; Schimper, 1891 ; G. Karsten, 

 1891. 5 Kearney, 1901. 6 Gobel, 1889. ' See Chap. XLVI. 



8 Guppy, 1891-3 ; Ravn, 1894. * Schroter (und Kirchner), 1902, p. 42. 



