Pethybridge & Praeger — Vegetation South of Dublin. 155 



The lower limit of U. Gallii is sometimes climatic, the lessening of 

 exposure allowing- the stronger-growing U. europceus to overshadow 

 and beat out its rival ; sometimes edaphic- artificial, the incoming of 

 the drift leading to cultivation, and consequent destruction of the 

 natural vegetation. The upper limit is sometimes caused by exposure, 

 in which case U. Gallii slowly gives way to the heaths ; or frequently 

 it is formed by the lower edge of the wet peat-cap which clothes most 

 of the higher hills. 



TJie Pteris Associaiion. (P.) 



The distribution of the Bracken Pteris Aquilina is very interesting, 

 and presents a number of problems. While the associations of Ulex 

 europmis, U. Gallii^ and Calluna maintain a definite order in altitude, 

 and represent climatic zones in a small way, the Bracken forms colonies 

 in one and all of these, disputing the ground with each ; and often, it 



Fig. 2. 



would appear, beating them back. These colonies vary considerably 

 in size, and only the larger ones will be found indicated on the map. 

 For the success of Pteris a certain depth of light, well-drained soil, 

 peaty, loamy, or sandy, is necessary. There are also very marked 

 indications of its preferring shelter ; and again and again in the 

 mountains one notices this plant predominant on hill-sides which are 

 sheltered from the prevailing westerly winds, and covering the eastern 

 sides of hollows and river-glens, while absent from the western. 

 Thus, in Glencullen, the west bank of the little side glens is often 

 thickly occupied by Pteris, while the eastern bank is in the posses- 

 sion of Calluna. The same tendency may be noticed on Carrick- 

 gollaghan, Ballycorus Hill, the hill west of Ballybetagh, and elsewhere. 

 The most striking example is on the ground lying east of Prince 

 William's Seat. Here a section across Kaven's Rock and the adjoining 

 hill displays a formation like this (fig. 2). Pteris^ then, far from replac- 

 ing Calluna, or either species of Ulex, when conditions are unfavourable 

 for their growth (as these three species do to one another), appears as 



K.I. A., PROC. vol,. XXV., SEC. B.] 



