Page:VCH Hertfordshire 1.djvu/90

 A HISTORY OF HERTFORDSHIRE Although the best division of a county for botanical purposes is into river-basins, there are some characteristics of our flora which are more prominently brought out by a lithological division. The Upper Chalk occupies very much the greater part of Hertfordshire, with much boulder-clay upon it in the east, and with clay-with-flints and gravel, chiefly Glacial, in the west. These divisions coincide so nearly with the Lea river-basin on the east and the Colne river-basin on the west that they need not here be further alluded to. The Upper Chalk ends off and the Middle Chalk crops out along the Chiltern Hills on the north-west, and this portion of the county, being chiefly chalk downs, has a peculiar flora, essentially xerophilous. The pasque-flower (Anemone pulsatilla] occurs in Hertfordshire only on the Middle Chalk, growing abundantly, though very locally, in some of the chalk combes, chiefly on slopes facing south-west. The combe south of Barton, just outside our boundary, is known as the ' Pulsatilla Banks,' and this name might well be given to the westerly slopes of Aldbury Owers near Tring. The Middle Chalk is also with us peculiarly the home of the orchids. The dwarf orchis (Orchis ustulata), the man orchis (Acer as anthropophord)^ and the butterfly orchis (Habenaria bifolia) seem to be restricted to it, but this is probably due merely to the bareness of this division of the Chalk, some orchids only thriving on a calcareous soil. Carum bulbocastanum is almost entirely restricted to the chalk hills on the north ; and Fumaria paruiflora^ Astra- galus hypoglottis, Senecio campestris, Tbesium linophyllum, and Bracbypodium pinnatum are absolutely restricted to them, with the exception of one record of the last-named species which is open to question. In the south-east, overlying the Chalk, are Eocene beds, the London Clay ending ofF and the Reading Beds cropping out from underneath it in a range of hills which form the north-western edge of the London Tertiary Basin. This is our subeugeogenous district, and it presents a marked contrast to the dysgeogenous Cretaceous area. As stated in the article on the geology of Hertfordshire, ' its soils, its agriculture, and its flora are of an essentially Middlesex type. In the Colne and Brent dis- tricts it forms grass lands devoted to hay-farming and grazing, interspersed with woods chiefly of oak, ash, elm, and fir trees ; in the Lea district, on the south, owing to the rich alluvial soil, market gardens and nurseries predominate ; while on the east, owing to the covering of boulder-clay, the land is chiefly under arable culture, partaking of the character of the corn-growing districts of the adjoining county cf Essex.' The hygrophiles Medicago falcata, Arnosera pusilla, Cbenopodium glaucum, Potamogeton acuti- fohus, and Carex paradoxa are restricted to this Eocene area. The counties by which Hertfordshire is surrounded are Cambridge- shire, Bedfordshire, Buckinghamshire, Middlesex, and Essex. There are no species of flowering plants in these counties which are not recorded for Herts. Three of these, Ranunculus Jiuitans, Salix rubra, and Potamoge- ton zosterifo/ius, occur in all the adjoining counties. The following Herts species are not recorded from any one of them : Ranunculus jloribundus, Silene conica, S. nutans, Rosa sihestris, Pyrola rotundifolia, Cuscuta epilinum, 48 '