Page:VCH Essex 1.djvu/72

 A HISTORY OF ESSEX The meteorological conditions of a district have a most important influence upon the flora, plants being peculiarly sensitive to drought or excessive moisture in the atmosphere, and also to winds and other atmospheric conditions. Perhaps next to climate the geological formations on the surface of the earth have the most potent influence upon the distribution of plants, and botanists can frequently identify a geological formation by plants which occur on its surface. In the county of Essex the distribution of plants is mainly determined by geological conditions. All plants are more or less adapted to their environment. Hence in those districts where the field boundaries are constructed of stone certain plants become more or less rare which elsewhere are com- paratively common ; and in counties from which stone is absent those plants which delight in stone walls are rare or absent. Again, if a wood is converted into arable land or pasturage, plants which need the shade and other forms of vegetable life peculiar to woodland country soon disappear. Pasturage, heath land, cultivated land and river banks have each their peculiar inhabitants. In order to show clearly the distribution of plants over any area it is necessary to divide it into districts. When possible it is better that these districts should coincide with the geological formation, or when this is impossible with the natural features of the country. Reference to the geological map of Essex will show that the surface geological formations are so scattered that any division of the county coinciding with the geological structure is practically impossible. Watson in his Cybele Britannica has subdivided Great Britain into its river beds, and although the distribution of plants does not in all cases coincide with river beds this system has many practical advantages. Professor G. S. Boulger has suggested the adoption of this system for the county of Essex, 1 and has worked out the districts of Essex on that basis ; but I do not think that for practical purposes this arrangement is as convenient in this county as the purely artificial divisions adopted by Gibson. Therefore in the list of Essex flowering plants here given Gibson's divisions have been adhered to. As the geology of Essex is fully dealt with elsewhere it is only necessary here to summarize those features of the surface geology of our county which influence the flora. Essex occupies a large part of an irregular tract known as the London basin, which has for its foundation the great chalk formation, and the chalk comes to the surface at Purfleet and Grays in south Essex ; at Quendon, Newport, Audley End and Saffron Walden in north Essex ; and at Great Yeldham and Middleton eastwards. London clay, which lies upon the chalk, is a very stiff bluish-grey clay, brown on its surface ; it shrinks and cracks in dry weather, but absorbs much water in wet weather. It occupies a large part of the county, and is exposed over considerable areas. 1 Transactions of the Essex Field Club, ii. 69. 34