1906. 
Reviews. 
129 
Again, Bracken and IJng are competitors on undulating ground ; 
Calluna occurs on the slopes exposed to the prevailing westerly winds, 
while Pteris occupies the sheltered slopes. There is evidence that the 
Bracken requires a fairly deep, well- drained soil, and, given this, its 
deep rhizomes, with their rapid apical growth, are capable of invading 
the more primitive Calluna and Gorse vegetation. These conclusions, 
arrived at on a favourable area, will serve as useful guides to other 
workers. The paper on the South Dublin District will be a land- 
mark as a study of Ulex Gallii and U. eiiroptcus. The authors have 
demonstrated that over a wide area these form two distinct zones^ 
U. Gallii the higher, U. europcciis the lower. The reasons for this are 
suggested, but we hope that the authors will be able to make it the 
subject of more detailed study. The same distribution was pointed out 
to me some years ago by Mr. C. E- Moss as a result of his observations 
in Yorkshire, but the limited amount of Gorse did not allow of any 
definite conclusion being drawn up. Some readers of this paper may 
think that the distinction by colour on a map of this scale of two plants 
so closel}' allied as to be almost varieties is rather "fine." If such be 
the first impression, a careful reading of this paper will correct it. The 
two dominant plants — U. europcEus and I/. Gallii— qs^, as it happens 
nearly related, but the ecological features of the two associations are 
distinct in regard to plants present, and, better still, on comparing their 
growth—'' The most marked change (in the U. Gallii association) is the 
smaller structure of the plants, the absence of tall-growing species, and 
of such as have large leaves; in short, a tendency to more pronounced 
xerophytism in character" (p. 154). Cases of nearly allied plants 
forming dominant species in associations characteristic of very different 
conditions are by no means rare, and our general impression is that 
varieties and sub-species of the systematists are in most cases the out- 
come of growth of the parent species under different external con- 
ditions. The study of the two Ulex associations in this paper will serve 
one good purpose if it only directs the attention of systematists to this 
aspect of ecology. 
The Moorland zone includes the vegetation which caps the mountains. 
The six most important associations shown by colours on the map 
are : — Calluna, Vaccinium, Juncus, Eriophorum, Scirpus, and Racomi- 
trium. The Calluna association is a familiar one in Britain, and in 
Dublin District occupies the greater part of the moorland. The authors 
record observations on its conditions of life, the chief of which is that 
it requires a well-drained, peaty soil. Vaccinium, which forms an asso- 
ciation covering square miles in the Scottish Highlands, occurs only in 
a few places in the Dublin District, although the species of Vaccinium 
and other plants of the association are common in the other hill associa- 
tions. The Juncus associations, which have already been referred to in 
other papers, were found so extensive in the Dublin District that a special 
colour has been assigned to them. The authors distinguish a Juncus- 
Polytrichuni association, generally in wet places where springs emerge 
