90 Plant-life of the Oxford District 



River and Ditch Flora. 



The control of the aquatic vegetation is less marked than that of the 

 land-surface exploited for crops ; but effects due to interference in special 

 cases follow the normal ecology of ponds and ditches, where these are to be 

 drained, or partially cleaned to maintain the flow of the water ; as even 

 the locked river may be let out in sections when the locks are being 

 repaired or enlarged, and a channel is kept free of weeds for navigation. 



Cleaning the main river takes place in late summer, or when the vegeta- 

 tion has reached its maximum (July-Aug.) ; enormous quantities of weed 

 being collected into barges, the cutting being effected by pulling a drag- 

 knife over the bed of the stream. Notices by the Thames Conservancy 

 prohibit such weeds being left where they may decompose and contaminate 

 the water. The material, collected to the extent of many tons, includes 

 dense growths of trailing Pond-Weeds (Potamogeton lucens, perfoliatus, 

 pectinatus, etc.), together with stretches of submerged leaves of Sagittaria^ 

 Scirpus lacustris, to a less extent of Oenanthe fluviatilis and Elodea, as well 

 as intrusive vegetation from the bank-side, cut by hook (Sparganium 

 ramosum^ Acorus Calamus^ and rhizomes of intrusive sub-aquatics as 

 Epilobium hirsutum y Lycopus, Agrostis stoloniferd). As these plants are 

 largely rooted in a gravel bottom, such cleaning does little to affect the 

 buried rhizomes, and little damage is really done to the flora ; the vegetative 

 growth being renewed on a cleared site in the succeeding season. 1 



Branches of the river utilized as Mill-streams (Wytham and Osney Stream, 

 The Old River) were formerly kept clean by weeding once or even twice in 

 the course of the summer. Now they are much neglected; the town-mills 

 are supplied with water taken off at Medley, and others have auxiliary power. 

 Such streams, left uncleaned, fill up with dense growths of Sagittaria, Spar- 

 ganium ramosum, S. simplex, together with Nuphar and luxuriant intrusive 

 bank-growths of Epilobium hirsutum, Rumex Hydrolapathum, Stum angusti- 

 folium, Carices and Scirpus, or may be readily blocked by fallen Willows. 

 When wholly neglected and stagnant, they give Batrachian Ranunculi, and 

 a dense mantle of Lemna-forms (L. minor, L. trisulca, Spirodela polyrrhiza). 



When kept well-cleaned, providing a good fast-flowing stream of clear 

 water over a gravel bottom, such ditches produce a quite distinct flora, vegetating 

 actively in early spring, and filling up with a dense growth, as banks of 

 Callitriche verna (3-6 ft.), stretches of submerged Ranunculus trichophyllus, 

 with Oenanthe fluviaiilis, Elodea, Myriophyllum and Ranunculus fluitans ; as also 

 algae as Chara, Vaucheria and Enteromorpha intestinalis in long trails (3-6 ft.). 

 A good example is afforded by the Railway ditch alongside the Willow Walk, 

 when cleaned out in summer and giving a new crop of clean aquatics over the 

 mild winter-months. 



As an example of a wholly artificial construction, the City Reservoir, 

 adapted from pits formed by removing ballast-gravel to make the Railway 

 embankment, at Hinksey, is of particular interest. In late summer it commonly 

 affords a remarkably beautiful collection of wholly subaqueous vegetation, 

 growing in fairly still clean water, at a depth of 6-10 ft. When the surface 

 is quiet and the sun shining brightly, massed growths of submerged leafy shoots of 

 Callitriche, Oenanthe fluviatilis, Myriophyllum, and Nuphar, are shown, together 

 with the vertically erected foliage-trails of Potamogeton lucens, P. perfoliatus, and 

 the similarly erected cable-petioles of Nymphaea alba (alone flowering at the 

 surface), also the conspicuously erected foliage of submerged Scirpus lacustris 

 and the ribbon-leaves of Sagittaria. 



1 This is particularly striking in the case of Nuphar, the stout rhizomes of which form a solid 

 mat at the bottom of the stream, and only the leaves and flowers are cut away. 



