TRAP-FLORA OF RENFREWSHIRE, 5 
3. Vaccinium stage-——In this condition the rock affords an 
opportunity for the plants of the third group, such as blaeberry, 
heather, Galium saxattle, Potentilla tormentilla and grasses (such 
especially as Aiva flexuosa, Agrostis, Festuca ovina, Anthoxanthum 
and Luzula). At fifteen hundred feet Lycopodiaceze, the hard- 
fern, and Juncus squarrosus are found. 
These plants are probably divided into two groups, which act 
in different ways. Those which are essentially “cranny” plants 
fix themselves in some deep crack or crevice, and send out 
quantities of branches, which grow a short way through the tuft 
and then turn upwards above its surface. Others, such as Ga/ium 
saxatile, do not seem to care much about a crevice to fix them- 
selves in. They have exceeding long wiry shoots which are 
buried in the moss, and from these the erect or trailing aerial 
branches are formed. Galium is especially interesting, because 
there are at least two sorts of aerial branches. Some end in 
flowers and fruits; others, after forming leaves and remaining 
above the moss, appear to become, at the end of the season, 
burrowing colonising shoots. Their tips grow into and hide 
themselves in the soil. They seem, indeed, to be negatively 
heliotropic. Calluna (heather) also does not seem to care about 
the rock crannies, and spreads under the surface of the moss. 
Grasses, in this stage, seem to send their roots down through the 
humus of the moss tuft until the roots reach the stone, which they 
probably corrode rapidly. 
Blaeberry is especially interesting. Sometimes it is distinctly a 
cranny plant. I have seen a specimen with a stem half an inch 
in diameter, which had grown deep into a huge, detached, 
apparently quite solid, block of stone about two and a half feet 
thick and three feet high. But the blaeberry also forms horizontal 
pink shoots, which grow through the moss humus and no doubt 
take root. 
Thus the stone is exposed to two distinct destructive influences 
—the wedging and breaking-up effect of the cranny plants, 
and the corroding action of the grass roots and others on its 
surface. 
The development of this third stage is extremely interesting to 
study. On watching closely one often finds that the surrounding 
vegetation grows over the rock like a sort of vegetable wave. 
