28 FLOWERING PLANTS 
The Grass moor is dominated by grasses. This shows two 
well-marked divisions: there is the dry grass heath, where — 
there is almost no peat and the chief grass is Molinia; and the 
wet grass heath with peaty soil, the dominant grass being 
Festuca. This latter has far more plants than the former. 
Grass heaths differ even within the limits of a county ; those 
of North Yorkshire have more plants than those of the West 
hiding. 
Woodland Pine forests are not nearly as extensive in 
Vegetation. Britain as they were centuries ago, or as they are 
even now in other parts of the northern hemisphere. North 
Canada, North Scandinavia are still covered by pine-woods, 
and in the Pyrenees and the Alps there is still a belt of pines 
above the oak. As compared with the oak or beech, the pine 
has more marked xerophytic conditions ; the stomata are sunk 
below the surface of the leaf, and it is calculated that the pine 
only gives off about one-sixth of the amount of water tran- 
spired by an average oak in the same time. The fact that the 
leaves of pines do not decay rapidly and form humus quickly 
may account for the scantiness of the undergrowth. — 
The birch is often found associated with the pine. It can — 
grow on many kinds of soil ; for instance, in the clefts of rocks, 
on dry gravel or sand, and even on wet moors. It requires — 
plenty of light, and as its crown is open, much light comes 
through, and the undergrowth is very varied and abundant. 
The birch may almost be considered a transition between 
Mesophytes and Xerophytes ; on the whole it is xerophytic, 
but not as markedly so as the pine, and its undergrowth 
includes a far larger number of Mesophytes than does that of 
the pine. a 
The oak and beech are distinct Mesophytes; the former 
requires more light than the latter, and as its crown 1s less 
thick and shady than that of the beech, its undergrowth is far 
more varied and ticher. The upland oak-wood may usually 
be distinguished from the lowland; the former is usually dry, 
rocky, deficient in humus, and characterized by an under- 
growth of late-flowering Xerophytes; the latter is damp, 

