464 PONDWEEDS. [March, 



polygomfolias, Pour., better known to English botanists by the 

 name P. oMonrjus. This grows in moory marshes, or in grassy 

 ditches in peaty places ; and this, its place of growth, is possibly 

 one of its best and most obvious distinctive characters. 



P. natans is almost ubiquitous. Few ponds or deep waters are 

 unornamented with its floating, olive-coloured leaves. The 

 leaves vary considerably; sometimes becoming more elongate 

 than in the normal or common form, and somewhat approaching 

 to the lanceolate shape. 



P. perfoliatus and P. lucens are pretty generally found in 

 rivers and deep ditches, and in the South of England P. densus 

 is a common species. P. prcelongus and P. plantagineus"^ are 

 plentiful enough where they occur, but they are rare species 

 compared with the foregoing. P. heterophyllus, P. lanceolatus, 

 and P. rufescens, are by some, not without reason, regarded as 

 problematical species. They may be placed alongside the floating 

 form of P. polygonifolias and P. lucens ; that is, they are about 

 equally common, and equally indefinite or undefinable or un- 

 distinguishable. 



The locality of the species will be found under their respective 

 names. 



Note. The Order, genus, and species are usually defined as 

 under. 



PoTAMACEiE. — The plants belonging to this Order are herba- 

 ceous, and of perennial or annual duration. Their stems are 

 either cylindrical or very much compressed, either simple or 

 branched, often throwing out rootlets from their joints, always 

 submersed (under water) . The leaves are either broad or narrow, 

 usually of an olive colour and with their margin entire, linear, or 

 with a more or less enlarged limb with prominent longitudinal 

 ribs, or with a strong midrib. These primary longitudinal ribs 

 or nerves are connected by a series of transverse secondary nerves, 

 which are sometimes visible to the unassisted eye, and they are 

 sometimes only to be seen when magnified. The leaves are 

 mostly submersed, with occasionally, in some species, the upper- 

 most floating ; they are usually furnished with stipules, and are 

 either sessile or petiolate (have leaf- stalks) . They are usually 

 alternate, but the uppermost are not seldom {i. e. in some species) 



• This Bpecies or variety is very abundant in Ham ponds, or rather ditches, 

 Kent, near Sandwich. 



