Walker: Pond Vegetation. 



307 



reached. The dearth of oxyg"en in the black putrefying" mud 

 at this depth renders necessary a well-developed intercellular 

 space system which resembles that of an Equisetum stem. 

 A transverse section of the underg-round stem shows a large 

 central air canal, with smaller ones in the cortex equal in 

 number to the vascular bundles. Fibres are sparing-ly developed 

 and the stem is consequently weak ; for this reason probably 

 the plant is confined to very muddy ponds. At a depth of about 

 nine inches the Dropwort merges gradually into a zone, consist- 

 ing- principally of Sparganium ramostL?n Huds. This is also 

 a mud-loving- plant, with a slender and somewhat weak rhizome, 

 which lies only a little distance below the surface of the mud. 

 The plant is, however, taller and more robust than the Drop- 

 wort and can exist in a g-reater depth of water. Attached to the 

 base of the aerial shoots are numerous roots ; these are of two 

 kinds : long, stout roots which grow directly downwards and 

 serve to attach the plant, and short, slender roots (water roots. 

 Warming""), which spread out near the surface of the mud and 

 are for water absorption. Scattered through the Bur-reed zone 

 are numerous plants of Eleocharis paliistris Br. The thin aerial 

 stems of this plant offer but slight resistance to water move- 

 ments and wind. Correlated with this is a shallow burrow- 

 ing" rhizome and short roots which can maintain the plant in 

 shallower and looser mud than in the case of the Bur-reed. 

 Eleocharis is present in all the shallow ponds at Bramhope, 

 whether clayey or extremely muddy. The Sparganium zone 

 ends at a depth of about eighteen inches and the Eleocharis six 

 inches before this. 



The thin underground stems of the Bur-reed offer a strong- 

 contrast to the stout rhizomes of many terrestrial Mono- 

 cotyledons with large sword-shaped leaves, and take up com- 

 paratively little space in the muddy floor of the pond. This 

 admits of invasion by other plants with narrow aerial shoots 

 which occupy the gaps between the Bur-reed shoots. In the 

 Bramhope ponds, as already noted, Eleocharis palustris invades 

 the Bur-reed zone. In a number of ponds at Filey, Smithf 

 found Equisetum limosum and Alisnia plantago occurring with 

 the Bur-reed [Sparganium simplex). In mill dams in the vicinity 



* Eug. Warming-, Botaniske Exkursioner. 3. Skarridso. Vidensk. 

 Meddel. fra den naturh. Foren. i Kbhun. 1897, p. 182. 



t William G. Smith, 'Notes on the Veg'etation of Ponds.' 'The 

 Naturalist,' 1903, p. 389. 

 1905 October 2. 



