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discussed in reference to the relative antiquity of 

 plants ; but in a single chapter it is impossible to do 

 more than call attention to certain considerations 

 which are frequently overlooked by students of 

 British species. 



It is customary to speak of the British flora as 

 consisting for the most part of species introduced 

 into this country by natural means, while some 

 plants owe their introduction to human agency or 

 are 'escapes' from cultivation. It is by no means 

 an easy task in some instances to decide whether a 

 species is native or introduced, but in some cases, 

 a few of which are mentioned, there is no doubt as to 

 the alien nature of the plants. The term 'native' 

 needs a word of explanation. It is not intended to 

 convey the idea that a plant so designated came into 

 existence on British soil and spread thence to other 

 regions ; but by native species we mean such as have 

 reached this country by migration from other lands, 

 or it may be in some instances have actually origin- 

 ated in this part of Europe. One of the best known 

 aliens in Britain is the American water-weed, Elodea 

 cmiadensis {OY Anacharis alsinastrmn), which was 

 discovered about sixty years ago in a canal near 

 Market Harborough in Leicestershire (u). In all pro- 

 bability this North American species was introduced 

 into England with timber. Once established, it 

 spread through the waterways with alarming rapidity 



