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A HISTORY OF HERTFORDSHIRE with difficulty and yielding but a small detritus. The result would be that hygrophiles or moisture-loving plants would be supplanted in the struggle for existence by xerophiles or heat-loving plants which thrive with a smaller amount of moisture. That is the principal change which has taken place in our flora since the epoch of the Hitchin lake-bed which immediately preceded the arrival of man in that district, and this change is still going on, every bit of land which is drained and brought under cultivation, and every drop of water abstracted from our under- ground Chalk reservoir in excess of that which percolates into it, hastening it on. We have now scarcely any purely eugeogenous soils. Of the eighteen botanical provinces into which Hewett Cottrell Watson, in his Cybele Brifannica, divided Britain, Hertfordshire is in two, the Thames and the Ouse, and each of these provinces comprises two geognostic types, dysgeogenous and subeugeogenous. Much the greater part of the county is in province 3, Thames; only a small portion in the north being in province 4, Ouse. In the Thames province it is only a small portion of the county, the London Clay area in the south, which is subeugeogenous ; and in the Ouse province the very small area of the Gault Clay in the extreme west which is subeugeogenous may be disregarded for any practical purpose. Very much the greater portion of the county, in both the Thames and the Ouse provinces, therefore partakes of the dysgeogenous type of each of those provinces. A list of 89 ' dysgeogenous species ' (xerophiles) of British flowering plants and of 138 'eugeogenous species ' (hygrophiles) has been given by John Gilbert Baker in a paper read before the British Association in 1855.* Of these we have in Hertfordshire 30 xerophiles, being about 33 per cent, of those enumerated by Mr. Baker, and only 10 hygrophiles, or about 7 per cent, of the species which he enumerates. But this is not all : our 30 xerophiles are comparatively common their relative frequency in our six botanical districts may be expressed by the number 104 ; on the other hand our to hygrophiles are comparatively rare their relative frequency in our botanical districts being represented by the number 17. What is meant by this will be seen from the following tables, which give the occurrence of each species in each of the six botanical districts to be described presently. These lists might easily be extended, but it is thought better only to include those species which are enumerated by Mr. Baker. In these and all other tables of flowering plants the sequence of species is the same as in Sir J. D. Hooker's Students F/ora, and the names adopted by him are used. In some cases the names used in Pryor's Flora are added as synonyms. 1 ' The Flowering Plants and Ferns of Great Britain : an attempt to classify them according to their geognostic relations' (1855). This paper, which was printed as a separate pamphlet, is mainly based upon J. Thurmann's Essai de phytostatiyue . . . Jura . . . (Berne, 1849). 46