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On the Number of Botanical Species to a Square Mile of Ground. 

 By Hewett C. Watson, Esq. 



In the 'Phytologist' for this present month Mr. Coleman intimates 

 his opinion, founded upon observations in the county of Hertford, 

 " that if a square mile be taken at a venture, its flora may be considered 

 a good one if it amount to as many as 200 species" (Phytol. iii. 220). 

 Were it possible for a botanist to map out the whole of Britain into 

 square mile sections, and fully ascertain the species of each section, 

 I think it probable enough that 200 species to a mile of rural surface 

 might prove over rather than under the average number. In a general 

 sense, I may therefore concur with the opinion expressed by Mr. Cole- 

 man, although there is some degree of discrepancy between us. 



On some of the elevated moors of Scotland, where the surface is 

 pretty uniform, and is still left almost in a state of nature, many 

 square mile sections might likely be found each containing less than 

 one hundred, possibly even less than fifty species of flowering plants. 

 But in England I doubt not that many square mile sections might 

 also be taken, within which a botanist would find more than 300 spe- 

 cies, and in several of them more than 400. 



By way of putting this to a practical test, since reading the valuable 

 paper by Mr. Coleman, I have reckoned up the species on a square 

 mile of surface immediately around my own house. I find that nearly 

 400 species can be enumerated as certainly wild within the extent of 

 the mile ; and if adding to the list about a score of introduced and 

 unsettled species, the number may be raised to 410 species or upwards. 

 By enlarging the boundaries so as to extend the single mile into a 

 space of twenty-five square miles, I find that my enumeration will 

 give 660 species, including aliens and unsettled species or varieties. 

 The whole county of Surrey would probably be found to include up- 

 wards of 800 species, fully double the number in the single square 

 mile. 



I take the square mile immediately around my own abode, as being 

 that one with the productions of which I feel myself most familiar. It 

 presents considerable diversity of situation for plants, with little 

 variety of soil ; the latter being either clayey loam or gravel, with a 

 few very small patches of sand. The mention of a dozen generic 

 names will indicate tolerably well the variety of situation ; namely, 

 Hydrocharis, Actinocarpus, Calluna, Jasione, Verbena, Lycopsis, 

 Helminthia, Briza, Silaus, Hyacinthus, Circaea, and Orobus. 



