Walker : Po?id Vegetation, 



309 



occupied by a shallow rooted annual, Water Purslane [Peplis 

 Portiila L.), Plate XX., Figf. 2. The annual decay of this plant 

 slowly contributes towards the formation of mud, and thus would 

 prepare the way for such mud-lovingf plants as Sparganimn and 

 (Enanthe fistulosa\ these in their turn, as the muddy floor rises, 

 would slowly encroach upon the pond, and ultimately, through 

 their own incomplete decay and the obstruction by their shoots 

 of twig-s and other foreign matter blown against them, bring 

 about its filling up and destruction. Several small ponds at 

 Bramhope have already become almost obliterated, possibly in 

 this way. 



In another pond, with banks too steep for Sparganium and 

 Eleocharis a mass of Raniuictilas aquatilus L. occurs ; this is 

 very conspicuous in spring, but as summer advances it becomes 

 obscured by a dense Potamogeton vegetation. Glyceria fliiitans 

 Br. occupies clayey places in some of the ponds, and where the 

 water is muddy it has few competitors. 



The well-known xerophilous characters, both of external 

 aspect and internal structure, exhibited by many swamp plants 

 (and plants of undrained ponds'^) are conspicuous in the rushes, 

 CEnanthe fistalosa and Eleocharis palustris. Schimperf points 

 out that in the case of peat bogs, a condition of physiological 

 dryness prevails, and, therefore, only xerophytes thrive in these 

 situations. E. S. Clementsi: suggests that the xerophilous 

 characters of amphibious plants are due, rather to the persist- 

 ence of stable ancestral structures, than to the inhibition of 

 absorption through the presence of humic acids§ and the bad 

 aeration of the roots. This explanation is difficult of application 

 in the case of the Water Dropvvort, whose terrestrial ancestors 

 probably resembled CEnanthe crocata. The submerged shoots of 

 the Water Dropwort possess leaves with a flattened lamina 

 resembling those of a typical umbelliferous plant. It seems 

 probable that the xerophilous character of (Enanthe fistiilosa is 

 directly due to the root conditions, particularly bad aeration 



* H. C. Cowles, ' The Physiographic Ecolog-y of Chicag-o and Vicinity. 

 Bot. Gaz., Vol. 31 (1901), p. 145. 



fSchimper, A, F. W., 'Plant Geog-raphy upon a Physiolog-ical Basis.' 

 Translated by W, R. Fisher, 1903, p. 8. 



J Clements, F. E., ' Research Methods in Ecology,' p. 127. The original 

 paper was not available. 



§ Schimper. Ibid, p. 4. 

 1905 October 2. 



