132 C. E Moss, W. M. Rankin and A. G. Tansley. 
cinerea). Pteris aquilina and Vaccinium Myrtillus , both of which 
can tolerate fairly deep shade, may be dominant both under trees 
and in the open. Aim flexuosa is the characteristic grass; while 
Teucrium Scorodonia, Galium saxatile, Potentilla erecta, and, in 
locally damp places, Molinia ccerulea, Erica Tetralix, and Salix 
repens occur. 
The oak-birch-heath association gives a much more open and 
less shady wood than the typical oakwoods, except where beech is 
locally dominant, in which case quite the opposite condition obtains. 
The foliage of the birches allows more light to pass to the ground 
than does that of the oaks ; and the trees are often separated by 
considerable intervals, in which the heath-plants have the ground 
to themselves. In this case, there are frequently wide stretches of 
pure heath without trees, or with only isolated trees or shrubs ( e.g ., 
birch or hawthorn), in the intervals of the woodland. Many 
extensive areas of sandy soil, as is well-known, bear no woodland 
at all, but are covered entirely by heath-vegetation. 
It is indeed a question whether the oak-birch-heath association 
is not always, as it certainly is often, a stage in the degeneration of 
oak forest to heathland. Graebner 1 came to the conclusion that 
the majority of the great heaths of the north-west German plain, 
which are floristically and ecologically closely akin to our own 
south-eastern heaths, occupy the positions of ancient woodland 
that naturally degenerated and finally disappeared before the 
invading heath-plants. Graebner believes he has established 
the occurrence of a continuous natural process at work on the 
woodlands of light soils where there is a rainfall of 70 cm. (28 inches) 
or more. According to him, under such conditions, the surface 
layers are being continually impoverished in mineral salts by 
washing out or “ leaching,” and the typical plants of the forest- 
floor are thus starved and give way before the invasion of mosses 
and shade-bearing heath plants. The matting together of the 
surface-layers of soil by the rhizoids and rootlets of the invaders 
prevents the access of oxygen to the soil, and leads to the accumu¬ 
lation of “ acid humus ” or “ dry peat ” (Rohhumus) in place of the 
original “ mild humus ” of the woodland soil. Thus we have the 
formation of a type of wood with a heathy vegetation, poor in 
species, on a soil composed of a mixture of sand and dry acid humus 
or peat. Finally, according to Graebner, either by the leaching of 
the soil to such a depth that the roots of the trees can no longer 
1 Die Heide Norddeutschlands, Leipsig, 1901. 
