ae WOODLAND ASSOCIATIONS 53 
Variation of Vegetation in the Oak Woods 
The vegetation of the oak woods varies greatly from place 
to place. The variation in the vegetation is due to the varia- 
tion of the various ecological factors. It is impossible, in the 
present state of knowledge, to give anything like a complete 
account of these factors; and it is still more difficult to state 
the action of the various factors either on the vegetation as a 
whole or upon the individual plants. However, some operating 
factors may be recognised ; and doubtless future work by plant 
physiologists will suggest what are the effects of these factors 
on the vegetation and on the individual plants. 
In the woods of Quercus sessiliflora, important ecological 
factors are the water-content of the soil, the kind and quantity 
of humus present, and the amount of light which penetrates 
the leaf-canopy of the trees and shrubs. These factors are cor- 
related in the most complex manner, and can perhaps best be 
illustrated by considering various woodland habitats where any 
one of them becomes pronounced. It must be remembered, 
however, that the habitats chosen are connected by all possible 
intermediate stages; and it is the sum of these minor habitats 
which constitute the more general habitat of the association 
as a whole. 
(1) Marshy places. Where springs arise and by the 
sides of the various rills and streams, wet and marshy places 
occur where the soil is well aérated; and consequently any 
humus that is present is mild (not alkaline) humus and not 
acidic humus. In such places, the oak (Quercus sessiliflora) 
tends to become very rare, and the alder (Alnus glutinosa) 
and the ash (frasinus excelsior) to become correspondingly 
more abundant. Birches (Betula pubescens, and B. pubescens 
var. parvifolia) may however remain. At the lower altitudes, 
the crack willow (Sala fragilis) is sometimes found; and at 
the higher altitudes, where, however, the soil-water may be 
more or less acid, Salix awrita is locally abundant. S. cinerea 
is usually an abundant species; and where the two last are 
found together, hybrids (S. awrita x cinerea) occur. The bird 
