Heordshire | Page 6

the Thames into (3) Thame, (4) Colne, (5) Brent, (6) Lea; both the larger provinces and the smaller districts thus being founded on the natural divisions of a country, drainage areas or catchment basins.
In the following brief notes a few of the rarer or more interesting flowering plants of each district are enumerated.
1. The Cam.--This is the most northern district. It is almost entirely on the Chalk and is very bare of trees. The few plants which are restricted to it are very rare. A meadow-rue, Thalictrum Jacquinianum, and the cat's foot (Antennaria dioica) occur only on Royston and Therfield Heaths; Alisma ranunculoides and Potamogeton coloratus only on Ashwell Common; and of the great burnet (Poterium officinale) the sole record is that of a plant gathered near Ashwell in 1840.
2. The Ivel.--This district is S.W. of that of the Cam, and the Chalk Downs of that district are continued through it. Its rarer plants are Melampyrum arvense, which occurs only in one spot S. of Ashwell; Smyrnium olusatrum, which has been found near Baldock and Pirton; and Silene conica, which was found near Hitchin in 1875. The white helleborine (Cephalanthera pallens), the dwarf orchis (_Orchis ustulata), and the musk orchis (Herminium monorchis_) occur on the Chalk Downs.
3. The Thame.--A very small tongue-like protrusion[a] of the extreme W. of the county, in which are the Tring Reservoirs. Two of the species confined to the district, Typha angustifolia and _Potamogeton Friesii_, are water-plants which occur only in these reservoirs or in the canals which they supply. A rare poplar, Populus canescens, grows by the Wilstone reservoir, and the man-orchis (Aceras anthropophora) on terraces cut in the Chalk near Tring.
4. The Colne.--A large district, comprising almost the whole of the western portion of the county. Diplotaxis tenuefolia, Silene nutans, and Hieracium murorum grow only on old walls in St. Albans. Colney Heath is our only habitat for a very rare loosestrife, _Lythrum hyssopyfolium, and also for Teesdalia nudicaulis_, while there is but one other locality, a different one in each case, for four of its plants, Radiola linoides, Centunculus minimus, Cuscuta epithymum, and Potamogeton acutifolius. The pasque-flower (Anemone pulsatilla) grows abundantly on the Chalk slopes near Aldbury. The rarer orchids of the district are the bog-orchis (Malaxis paludosa), the narrow-leaved helleborine (Cephalanthera ensifolia), and the butterfly orchis (Habenaria bifolia).
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 c?rulea, 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 filixf?mina[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