Praeger — On Types of Distribution in the Irish Flora. 55 



show a diagonal range across Ireland from the north-east towards the 

 sonth-west, having a marked absence in the north-western and south- 

 eastern areas. An example (fig. 57, Hydrocharis Morsus-rance) will 

 illustrate this type ; and to show the similarity of range of the group, 

 a statistical map is added (fig, 58) according to the usual plan, show- 

 ing the aggregate range of the following members : — 



Eanunculus circinatus. 

 Lathyrus palustiis. 

 Hydrocharis Morsus-ranae. 

 Sagittaria sagittifolia. 



Eleocharis acicularis. 

 Carex acuta. 

 Lastrea Thelypteris. 



Fig. 57. — Hydrocharis Morsus-rance. Fig. 58. — Mountain-folding of Ireland. 



On the map I have added the main lines of the old " Caledonian " 

 and "Hercynian" folding of Ireland, as demonstrated by Prof. Cole,' 

 since this, I believe, is the key to the peculiar range of these species. 

 It will be noticed that they are without exception marsh and water 

 plants, and they follow the great central trough of the island, spread- 

 ing over the basins of the Shannon, Erne, Boyne, and Bann, which 

 lie in the synclinal area, but avoiding the great anticlines of Leinster, 

 Mayo and Donegal, including the river-system of the south-east ; and 

 are absent even from the great western lake-system of which Lough 

 ■Corrib is the predominant member. 



Lastly, as to the distribution of plants which are probably or certainly 

 introduced in Ireland. As before stated, the aliens are generally 

 marked by a discontinuous range. A large number are widely spread; 



Knowledge, April, 1898. 



