3o8 



Walker: Pond Vegetaiion. 



of Skarridso, Warming-"" found Sparganiiun ramostim in almost 

 pure masses, the only invaders being- solitary Butomus plants. 



The central and deepest part of the pond is occupied by 

 Potamogeton natans L., which at a depth of a little over two feet 

 occurs alone. This is a ver\^ shallow rooting- plant, and is not 

 dependent upon a muddy bottom. It can fix itself equally well 

 in clay, and, excepting- y^?/7ic//^ conuminis, is the only plant found 

 in all the Bramhope ponds. In the ponds containing little mud 

 the Pondweed is almost without competitors, • and occupies the 

 shallow water as well as the deeper. In one clayey pond 

 the Poia?nogefoii is accompanied by larg-e floating- masses of 

 Juncus siipiims Moench. (Plate XIX., Fig-. 5.) The fringe of 

 vegetation around this pond consists of Juncus communis. At 

 the margin the slope of the bottom is steep — about one foot in 

 two — a condition which, in the Bramhope ponds, excludes 

 CEnanthe, Eleocharis, and Sparganiiim. In the tall erect rush 

 [Juncus communis)^ which is subject to a bending strain by wind, 

 the vascular bundles and sclerenchyma are peripheral, and the 

 weak aerating tissue is axial. (Plate XIX., Fig. 3.) In 

 the floating J. supinus subjected to a pulling strain by water 

 movements, the relative position of the aerating tissue and 

 strengthening tissue is reversed. The pith, Avhich is large in 

 J. communis, and consists of stellate parenchyma, is considerably 

 reduced in J. supinus, and is formed of close parenchyma. 

 (Plate XIX., Fig. 4.) The cortex, which is small in J. coinmunis, 

 and includes a considerable amount of sclerenchyma, is large in 

 J. supinus, and consists of a weak tissue with relatively large 

 air spaces. 



In the shallow, muddy pond (Fig. i) the illumination of the 

 lower region is considerably reduced by a crowded Potamogeton 

 growth, and in warm weather, light is further absorbed by an 

 abundant plankton [Peridinium tahulatum Ehr. and Cyano- 

 phycea;-). The bottom flowering plant vegetation is therefore 

 sparse, consisting only of a few dwarfed jMyriophyllum plants. 

 In deeper clayey ponds, Potamogeton natans is thinly scattered, 

 and the plankton scarce ; this admits of a more abundant 

 bottom vegetation composed Potamogeton rufescens Schrad.f 



Upon the margin of one of the ponds wet clayey expanses 

 occur ; these sites, unsuited to perennation by marsh plants, are 



* Ibid. 



\ Mr. Bennett, of Croydon, kindly confirmed the identification of this 

 plant, and also Juncus supinus mentioned above. 



Naturalist, 



