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 A HISTORY OF HERTFORDSHIRE PHANEROGAMIA The most interesting questions to determine with regard to the flowering plants of our county are how and when they were introduced, and what changes have taken place, or are doing so, in the character of the flora. It is essentially of a southern type, possessing but few northern species, and these are ' mostly rarities and numerically quite insufficient to modify the aspect of vegetation.' 1 To show what is meant by this it is necessary to state that the flora of Britain is a derived one, having originally been introduced from the continent of Europe somewhere about the Glacial period, with many subsequent accessions. Most of our commoner species have come from central Europe, whence they have spread over the whole of the British Isles, some northern species having a Scandinavian origin and some southern species having migrated from France and Spain. It is these which greatly predominate over the northern species in Hertfordshire. The introduction of some of our existing species may date from before the Glacial period, part of our small arctic flora may have been introduced from the Scandinavian peninsula during this period, but by far the greater number of our widely diffused plants appear to have followed the retreat of the ice towards the close of the Glacial period, migrating into this country from the great Germanic plain. Although at that time the present main features of the surface of the county had been impressed upon it, sub-aerial denudation has been actively going on for the countless ages during which man has been upon the scene, and a vast amount of material has been removed. But this erosion has been effected by our existing rivers flowing in the same general direction as they do now, though at higher and higher levels as we trace them back in time. The flora of the county would not necessarily be thereby affected, but it has doubtless been modified to some extent by the clearing of forests and the draining of land. Hertfordshire was undoubtedly much more densely wooded in past times, even within the historic period, than it is now ; the sources of our rivers were much higher ; streams ran down many valleys which are now dry ; and early man had to seek the higher ground away from the morasses which have left evidence of their former existence in beds of peat, or perhaps as elsewhere to seek safety from the wild beasts which prowled over the country by erecting his dwellings over lakes which have long ceased to exist. That the flora of Hertfordshire between the close of the Glacial period and the advent of man was not widely different from what it is at the present day may be gathered from the following list of flowering plants determined by Mr. Clement Reid from the ancient lake-bed at Hitchin : 2 Ranunculus aquatilis (aggregate), R. sceleratus, R. repens, Montia 1 Flora of Hertfordshire, p. 558. 'The Palaeolithic Deposits at Hitchin,' Trans. Herts Nat. Hist. Sac. vol. x. pp. 18, 19 (1898). 44