5. The Brent.—­The smallest district, a protrusion[b] of the county in the S. entirely on the London Clay, and chiefly interesting owing to the presence of Totteridge Green and its ponds.  In these ponds grow the great spearwort (Ranunculus lingua) and the sweet-flag (Acorus calamus), the former, however, not being indigenous.  The star-fruit (Damasonium stellatum) formerly grew on Totteridge Green, and Chenopodium glaucum at Totteridge, but neither has lately been seen.

6. The Lea.—­The largest district, comprising the whole of the E. of the county.  The London rocket (Sisymbrium irio) occurs only in the old towns of Hertford and Ware; the true oxlip (Primula elatior) near the head of the River Stort; a very rare broom-rape, Orobanche caerulea, at Hoddesdon, where it is parasitic on the milfoil; and an almost equally rare bedstraw, Galium anglicum, on an old wall of Brocket Park.  A rare trefoil, Trifolium glomeratum, is known only at Easneye near Ware; and Hatfield Park is our only locality for the water-soldier (Stratiotes aloides) except where it has evidently been planted.  Two species, usually of rare occurrence, Polygonum dumetorum and Apera spica-venta, are frequent in the district.

The indigenous flowering plants of Hertfordshire number 893 species, 679 being Dicotyledons and 214 Monocotyledons.  If to these be added 199 aliens, etc., the total number of species recorded is brought up to 1,092.  The flora is essentially of a southern type, the northern species being few in number.  Owing to the dry soil, xerophiles largely prevail over hygrophiles.

The Ferns and their allies the horsetails and clubmosses are not well represented, both the soil and the air of the county being too dry for them.  Another cause for the present scarcity of ferns is the proximity of Hertfordshire to London, for they have been uprooted and taken there for sale in cart-loads.  We have twenty-four species of ferns and fern-allies, but not one really rare.  The principal varieties are Scolopendrium vulgare, var. multifidum; Athyrium filixfaemina[c], var. convexum; and Polypodium vulgare, var. serratum. Equisetum silvaticum is our rarest horsetail; and our only clubmoss is Lycopodium clavatum.

The Mosses are much better represented than the ferns, 175 species having been recorded.  The bog-mosses are represented by six species—­Sphagnum intermedium, cuspidatum, subsecundum, acutifolium, squarrosum, and cymbifolium. Tetraphis pellucida occurs in Sherrard’s Park Wood, and Polytrichum urnigerum in Hitch Wood. Seligeria pusilla has been found in an old chalk-pit in Brocket Park, and S. paucifolia on chalk nodules in the Tunnel Woods near Watford. Campylopus pyriforme occurs in Berry Grove Wood, Aldenham, and C. flexuosus in Dawley’s Wood, Tewin.