STABLE AND MIGRATORY PLANT FORMATIONS 7 1 



receiver of the finely comminuted drift. Moreover, the hilly 

 ground of the so-called siliceous soils is in its distribution in 

 this country chiefly northern and western, where the atmo- 

 spheric humidity is comparatively great and the summer and 

 winter temperatures approach one another most closely. 

 The Q. sessiliflora woods have their chief distribution on the 

 Carboniferous shales of the Pennine region and the harder, 

 Silurian shales of Wales, rocks which form hilly country, 

 from their structural relations or metamorphism, but on 

 prolonged weathering yield clays almost indistinguishable 

 from the clays of the lowland oak woods. In such cases any 

 difference in the soil would be chiefly due to the effects of 

 physiography on the maturity and depth of soil, drainage and 

 climate, and the oaks as a whole may be said to show a 

 preference for clay-soils, soils retentive of moisture and with 

 large reserves of potash. 



Qiierciis sessiliflora is reported to have a less Continental 

 and more Atlantic distribution than Q. pedunculata^ and to rise 

 to greater height on the European mountains. The dis- 

 tribution of the two types of oak in this country seems 

 therefore to have been guided as much by the physiography, 

 and their past and present relations to climate, as by the 

 specific nature of the soil. Q. pedunciilata is the lowland 

 type requiring deeper soils, greater stability in the amount 

 and nature of the soil-waters, and higher summer temperatures 

 for full development, while the Q. sessiliflora woods form a 

 montane belt intermediate between those of Q. pedunculata 

 and the birch, where the physical conditions are more exacting 

 but the competition less. 



It is probable that the heat and light requirements, the 

 ability to stand wind-exposure, the kind of root-system, and 

 the capabilities of the different species of flourishing under 

 stronger or weaker solutions of salts and acids in the soil- 

 waters, are the chief factors guiding the distribution of the 

 British forest-trees. 



Ashwoods and beechwoods are considered chief associations 

 of a plant-formation of calcareous soils by Moss, Rankin, and 

 Tansley (3). They are also known on the Continent to show 

 a preference for limestone. The ash is very deep-rooting, and 

 is considered to flourish best under stronger mineral solutions 



