TRAP-FLORA OF RENFREWSHIRE. 3 
3. Vaccinium (blaeberry) and wiry grasses.—From this 
stage there is a difference in the flora above the zone of 
cultivation, for peat formation sets in. In the low-lying 
lands, or zone of cultivation, this is followed by 
4. A herbaceous stage. 
5. Whin or broom and bracken. 
6. Rosaceous stage. 
1. Lichen stage—(a) Crust lichens.—The commonest forms are 
Lecidea geographica, L. contigua, L. lavata, L. lapicida, L. rivulosa, 
L. fumosa, Lecanora tartarea, and Vartolaria dealbata. 
The last two appear, in my specimens, to grow over stone that 
has been already occupied. 
These crust lichens are, for some reason, not well-developed. 
Where they do appear, however, they absorb most of the rain 
water, which is thus prevented from entering or corroding the 
rock; they also affect atmospheric weathering in other ways. 
Dust accumulates on them; the last two lichens form a quite 
perceptible crust, covering the stone, and of quite an appreciable 
thickness. The growth is slow. Variolaria grew five hundred 
and twenty micro-millimetres from the 1st February to the end of 
September. 
(6) Leafy lichens.—Parmelia physodes, P. saxatilis, P. sulcata, 
and Cladonia cervicornis are the most important. Though these 
are found on natural rocks, they are not common. It is on walls 
or dykes formed of these rocks that they are especially abundant. 
On such dykes Physcia parietina, P. tenella, and others occur, 
and possibly a close search would show them on the rocks 
also. 
These Parmelias, and especially P. su/cata, are extremely 
tenacious of their ground. Their growth is about five millimetres 
perannum. When once established, they spread circularly, and on 
lifting off the patch, one finds clear evidence that they not merely 
overgrow their neighbours, but also overgrow ‘their own remains. 
Layers of dead pieces of former fronds are found deZow those that 
are active, and the younger rhizoids clearly fix themselves on the 
older branches. Thus the rock is protected not merely by the 
thickness of one frond, but also by a series of dead pieces. They 
also act as water-retaining and dust-collecting agents ; they also 
intercept heat, light, etc., and afford shelter to small animals. 
