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Proceedings of the Royal Irish Academy. 



This is sucli as to preclude the growth of any trees, the only exception 

 being, perhaps, a few individuals of Pyrus Atccuparia, which one may 

 find in tlie high ground, with their crowns kept well below the level 

 of the tops of the gullies in which they grow. The Calluna plants 

 themselves, too, lie, at any rate in the more exposed places, with their 

 axes parallel to the direction of the prevalent winds, i.e., from west 

 to east, as can be clearly seen when, after burning, the exposed side of 

 some rounded hill presents the appearance of some gigantic head, the 

 coarse hair of which has been combed down in one direction. It may 

 be mentioned here that this burning of the heather is a regular 

 operation performed for the purpose of providing the tender young 

 tops of the new Calluna for grouse-feeding. But burning is also 

 carried on, though apparently less methodically, in the two TJlex asso- 

 ciations. This results in a stimulus to the sward-producing plants for 

 the first few years at least, and so produces rather more food for sheep- 

 grazing. The heat produced by this burning does not seem sufficient 

 to kill seeds which may be lying on the surface or in the soil, for on 

 several occasions, as on Carrickgollaghan, we noted young seedlings of 

 TI. Gallii in thousands springing up on ground from which the old 

 bushes had been recently removed by burning. 



As regards precipitation, the Calluna moor, along with the Erio- 

 phorum and Scirpus moors, occupies localities where this is probably 

 at its maximum in our area ; but whereas on the two latter types of 

 moor, the rain which falls remains and forms a sopping wet bog, on the 

 Calluna moor, as stated before, the drainage is fairly good. The upper 

 regions, at certain parts of the year at any rate, are constantly enveloped 

 in mist and cloud, so that the amount of available sunlight is then 

 seriously diminished ; this occurs chiefly during the colder part of the 

 year. In the summer, however, tlie Calluna moor is exposed at times 

 to the scorching rays of the sun, and must, therefore, be able to with- 

 stand considerable periods of drought. 



A glance at the list of plants composing the association shows that, 

 with the exception of the hemi-parasite Melamptjrum pratense, and 

 possibly its congener Pedicularis sylvatica, all the species are perennial. 

 A number of these are evergreens, and their leaves, such as those of 

 Calluna itself, Erica cinerea^ E. Tetralix, Vaocmium Vitis-Idmy 

 Empetrum^ &c., show well those peculiarities of structure characteristic 

 of xeropliytes.^ 



1 For a discussion and description of these structural peculiarities, see Kerner 

 and Oliver, " The Natural History of Plants," vol. i., p. 300 ; Miall, " Round the 

 Year," p. 208; Niedenzu, Engler's Bot. Jahrbucher, 1889-90, p. 134. 



