172 ALIEN FLORA OF BRITAIN 



common on old walls, but it has also been observed 

 by the writer on rocks in Somerset, and perhaps when 

 more attention is turned to the precise status of such 

 plants it may be found in similar situations in other 

 parts of Northern Europe. Pending such time it is 

 more prudent to class it among introductions. 



Ulmus campestris, L. Very common as a hedge- 

 row tree in England. It rarely ripens its seeds, has 

 no vernacular name, and shows other signs of being 

 an introduction. It is native on the Continent. 



[Urtica dioica, L. This plant is now so widely 

 distributed in the North Temperate Zone, and is 

 so extremely common in waste and cultivated ground 

 in England, that it is difficult to determine which 

 of its apparently wild habitats, if any, are reliable 

 evidence of its indigenous state. The mere fact, 

 however, that it grows plentifully about the homes 

 of such wild mammals as still survive in England 

 seems to place its native status beyond the possibility 

 of criticism.] 



Urtica pilulifera, L. A common weed of cultivated 

 and waste ground in some parts of Europe. Its 

 popular name of Roman Nettle suggests that it was 

 introduced in the time of the Romans. It has 

 certainly been known from the earliest botanical times, 

 but never under conditions that could suggest its being 

 native. 



Urtica urens, L. A common weed >of cultivated and 

 waste ground in Europe, including England. Its origin 

 is uncertain. 



