BOTANICAL FEATUEES OF lEELAND xxxvii 



Other plants travel from their ports of arrival far over the country. 

 Traffic by road does not much help their dispersal, but inland 

 navigation and railways are important factors in the dissemination 

 of aquatic and of light-soil species respectively. Of the recent 

 immigrants by water, the most famous is the Water-Thyme, Elodea 

 canadensis, whose remarkable history need not be traced here. Acorus 

 Calamus, introduced at Moira between 150 and 200 years ago, has 

 spread to the Lagan Canal, and is now abundant from Lough 

 Neagh to Belfast ; and recently it has appeared on the Boyne. 

 It is important to note that inland navigation is responsible 

 for the spread of certain native aquatics as well. Equisetum 

 variegatum has been carried up and down the Eoyal Canal till it 

 now lines the banks for miles between Dublin and Mullingar, 

 though it is one of the rarest plants elsewhere in the counties 

 through which the canal passes. The same applies to several of 

 the Charas that threaten to choke the waterways ; and Butomus, 

 Sagittaria, and other plants must, in certain counties, be sought 

 along the canals, and not in the natural waters. 



The spread of certain recently introduced species along th^ 

 railway-systems of the country is so remarkable as to deserve 

 mention. Linaria minor was first noticed near Cork about 1819. 

 The first edition of "Cybele" reports it as abundant at Cork, and 

 adds Enniscorthy, Carlow, and Dublin. Subsequent works record 

 numerous other stations in the south and east, almost always on rail- 

 way tracks. At the present time it is known to occur over almost 

 the entire southern three-fourths of Ireland, chiefly on the systems 

 of the Midland Great Western and Great Southern and Western 

 railways. Diplotaxis muralis has a similar history. First observed 

 at Portmarnock in Co. Dublin in 1837, it has spread in all 

 directions, and now ranges, almost always on railway tracks, from 

 Waterford to Sligo, and from Louth to the boundaries of Kerry. 

 Arenaria tenuifolia furnishes a more recent instance. First 

 observed in Kildare as late as 1897, it is now known to occur 

 right across the Central Plain from Dublin to Galway, and from 

 Eoscommon and Meath southward to Limerick and S. Tipperary, 

 being strictly confined to railway tracks. The three plants above- 

 named have, in England, a wide distribution on light soils. The 



