Page:VCH Hertfordshire 1.djvu/100

 A HISTORY OF HERTFORDSHIRE The district drained by the Lea and its tributaries is about 500 feet in height on the north, along the downs forming the water-parting between the catchment-basins of the Thames and Ouse, the surface gradually sloping towards the south, until along the river Lea below Hoddesdon the ground is much lower and flatter than in any other part of Hert- fordshire. Most of the district is on the Chalk, much covered by boulder-clay, but south- east of Furneaux Pelham, Much Hadham, Hertford, and Hatfield it is on the London Clay and its narrow margin of Reading Beds. The old towns of Hertford and Ware are the only localities in the county for the rare Sisymbrium irio, which sprang up in London after the great fire of 1666, whence its name of London rocket. The true oxlip (Primula elatior) occurs only near the head of the Stort on the borders of Essex ; the very rare Orobanche ceerulea only at Hoddesdon, where it is parasitic on the milfoil (Achillea millefolium) ; and an old wall of Brocket Park is the only habitat of the almost equally rare Galium anglicum. Trifolium gkmeratum is known only at Easneye near Ware ; the only locality for Stratiotes a/aides, except where it has evidently been planted, is Hatfield Park ; and the ponds on Hertford Heath lay exclusive claim to Carex bcenninghausiana. Two species usually of rare occurrence (Polygonum dumetorum and Apera spica-venti) are frequent in the district. The Lea district has the largest number of rare plants of any, and much the largest number peculiar to it. The list is as follows RANUNCULACEJE Ranunculus diversifolius, H. Wats. lingua, L. hirsutus, Curtis (sardous, Crantz) Helleborus foetidus, L. viridis, L. PAPAVERACE^ Papaver Lecoqii, Lamotte FUMARIACE.S Fumaria Boraei, Jord. Vaillantii, Loisel. CRUCIFER^E Nasturtium silvestre, Br. amphibium, Br. Sisymbrium irio, L. Sophia, L. Erophila praecox, DC. Senebiera (Coronopus) didyma, Persoon Lepidium ruderale, L. Iberis amara, L. CARYOPHYLLEJE Silene anglica, L. noctiflora, L. Cerastium quaternellum, Fenlz. Stellaria palustris, Ehrh. LEGUMINOSJE Trigonella ornlthopodioides, DC. Trifolium subterraneum, L. ochroleucum, L. glomeratum, L. Hippocrepis comosa, L. Vicia (Ervum) gracilis, Loisel. Lathyrus aphaca, L. (Orobus) tuberosus, L. silvestris, L. ROSACES Rosa silvestris, Woods stylosa (v. systvla), Bait. Cratsegus monogyna, Jacy. CRASSULACE/E Sedum fabaria, Koch HALAGORE/E Callitriche obtusangula, Le Gall. OHAGRAKIEB Epilobium roseum, Schreb. UMBELLIFER^ Bupleurum rotundifolium, L. Carum segetum, Benth. (Bunium) bulbocastanum, Koch Cicuta virosa, L. Sesseli libanotis, Koch RUBIACEA Galium Witheringii, Sm. anglicum, Huds. VALERIANE./E Valerianella auricula, DC. COMPOSITE Pulicaria vulgaris, G<ertn. Filago apiculata, G. R. Sm. spathulata, Presl gallica, L. Arctium intermedium, Lange nemorosum, Lej. Centaurea calcitrapa, L. Amoseris pusilla, Gaertn. Crepis biennis, L. taraxacifolia, Ihuill. Hypochaeris glabra, L. Tragopogon pratensis, L. CAMPANULACE/E Campanula rapunculus, L. PRIMULACE* Primula elatior, Jacq. Anagallis caerulea, Schreb.