Pkthybridge & Praegkr — Vegetation South of Biihlin. 159 



sometimes, as notably on Kippure, by tlie existence of imderground 

 water-channels, at the bottom of a solid covering of peat. In this 

 district, as elsewhere in the British Isles, the Calluna association is 

 the most marked association in the whole vegetation, and no other plant- 

 group is so entirely under the dominance of its leading member. Tlie 

 lower limit of the Calluna heath is generally the e:ffect of the cessation 

 of thick peat. As regards its upper limit, this is generally a question 

 of drainage, and in the more level bogs of the broad ridges and flat 

 domes of the granite hills, Calluna becomes stunted and thin, and 

 Cyperacece more and more conspicuous. The upper limit of Calluna is 

 not determined in our district by elevation; where drainage allows, the 

 plant remains, on the whole, dominant right up to the tops of the 

 mountains. It, however, becomes here much dwarfed in stature, and 

 its dominance is partially threatened by much moss and Empetrum. 

 The present distribution of Calluna is, in some measure, artificial. 

 Turf -cutting is largely carried on in the peat which underlies the 

 wetter associations of the liigher grounds; and everywhere Calluna 

 follows the better drainage conditions produced by these human 

 operations. That drainage brings in this association in the midst of 

 tlie sopping wet Scirpus association is also well seen in the streaks of 

 Calluna accompanying the streams which run through the latter 

 association, the better-drained banks of the streams being immediately 

 seized upon by the Calluna. 



In well-drained ground, especially near its lower limit, Calluna 

 grows luxuriantly — a dense, uniform growth, two to three feet in height, 

 with a continuous undergrowth of mosses, ^mongwhioh Listera cordata, 

 Melampyrum pratense, and other plants grow as dependent species. As 

 we proceed upward, the change from the Calluna type of moor to the 

 Scirpus type is often gradual. The heather becomes by degrees more 

 stunted, and Cyperacece gradually increase. Thus, there is often a wide 

 band which Avas mapped as CS (i.e. Calluna dominant, with much 

 Scirpus cesspitosus), followed by another wide band of SC (i.e. Scirpus 

 dominant, with much Calluna). This latter type, indeed, is the 

 characteristic one of much of the high ground of the Dublin moun- 

 tains, covering large areas; only here and there is the ground so 

 unfitted for the growth of Calluna that a practically pure Scirpus 

 association results. 



As regards the conditions under which this association is developed 

 in our area, it will thus be seen that, as far as soil is concerned, a 

 certain amount of peat is necessary, and the drainage of this must be 

 fairly good. The association is influenced too by its exposure to wind. 



