4 TRAP-FLORA OF RENFREWSHIRE. 
The two Cladonias (cervicornis and pyxidata var. neglecta) form 
compact little tufts. On investigating these tufts, one finds a 
bewildering mass of remnants—remains of the older part of the 
Cladonias, mosses which have begun to grow between them, and 
fragments of liverworts, etc. 
The net effect of their preliminary attack upon the rock is to 
form a rough surface covered by a more or less thick, firmly 
attached crust of vegetable lichenic material. 
2. Moss stage.—Clochodrick stone is essentially in the first 
period (or crust stage). The second stage is _ especially 
characterised by mosses. Of these the commonest genera are 
Rhacomitrium lanuginosum, R. heterostichum, R. actculare, Cera- 
todon purpureum, Dicranum species. One finds along with these 
mosses, Cladonia rangiferina, Stereocaulon denudatum, Sphero- 
phoron coralloides, Cladonias various species (C. gracilis var. 
chordata, C. furcata and its rare variety spinosa), Cetrarta 
aculeata, etc. 
In the more exposed places, Andraeas and Polytrichum species 
occur. In the wetter places, Brachythecium, Hypnoids and 
Bartramia pomiformis, Diplophyllum albicans and /unger- 
manntacee. 
Some of the Cetrarias, Bartramias, Rhacomitria and Dicrana 
are able to form nearly pure tufts without any other plant. This 
is unusual. Generally one finds complex tufts consisting of many 
different species, with lichens, mosses and liverworts all mingled 
together. Very often worms, small insects, animalcule, fungi 
(Agaricus semiglobatus and A. hypnorum), also appear in them. 
But, whatever the constitution of the tuft botanically, its economic 
importance is quite extraordinary. The amount of rich earthy 
material is very great. Exact measurement is almost impossible, 
because one cannot say definitely where one should begin to 
measure. The moss stems are often two to three inches high, and 
the underlying earthy matter may be one inch thick (more or 
less). 
That particular area of the rock is now defended by the tuft 
from frost ; its surface is not dragged off by water. No ablation 
occurs. The moss stems are like a sponge laid upon it, and even 
when they are saturated with moisture, the water simply flows 
over them and does not touch the stone. 
