Primary Woodland and its Derivatives 29 



generations of vegetation, the essential factors may be considered as : 

 (i) the amount of available sunshine in the year as affecting the working 

 energy ; (2) the degree of temperature as affecting the rate of metabolism ; 

 (3) the available water-supply which will include all inorganic food- 

 substances. In the case of primary rain-forest in the tropics, these factors 

 may be all at an optimum ; but as different cases arise introducing new 

 problems, when such factors fail individually or collectively, new types of 

 vegetation are isolated as solving them more or less effectively. Thus in 

 tropical forest with light and temperature at a maximum, plant-life can rise 

 to the limit of these physical opportunities ; but water-supply is the deter- 

 mining factor. With diminished water-supply, the forest-growth fails, trees 

 loosen canopy, may shed their leaves, present a seasonal response to alter- 

 nation of wet and dry periods, and become dry deciduous forest, grading to 

 fewer and fewer trees, and these with highly xerophytic adaptations for 

 perennation over the dry spell. 1 



With extension to extra-tropical regions (latitudinal), as northward, 

 light-supply is reduced, implying less work done in the year ; temperature 

 also reduces to the limit of frost and snow, further lowering the rate of 

 metabolism. Trees, again, present reducing features, but water-supply 

 remains the essential factor in determining the forest-growth, and so long as 

 it is available canopy may be maintained. 



Similarly altitudinal extension on tropical mountain ranges, with a 

 limit at the snow-line, gives all effects of reduced temperature, but with 

 undiminished insolation ; and so long as water-supply is effective the trees 

 persist. 



Combination of the case of the deciduous forest passing northward, 

 and of that on mountain ranges, gives the general case of northward 

 migration of the trees of the indigenous flora ; all with markedly reduced 

 habit and reduced annual output, as the highly specialized end-terms of older 

 phyla of a former tropical existence, now represented by predominantly 

 deciduous types, working with a short season, and enduring the cold of the 

 northern winter (frost-period), with the last relics on the northern mountains, 

 suffering from all disadvantages at the limit of tree-life (Salix, Betula) ; but 

 within the range of this country, still sufficing to give continuous forest- 

 formation, so long as soil is available and the water-supply permanent. 2 



From such general considerations it follows that, coming as it does 

 within the zone of North Temperate deciduous forest, with full water-supply 

 throughout the year, the entire area of the Oxford district was originally 

 general woodland. The tree-types are now comparatively few, as the 

 migrants of N. Europe, in turn the strays and residual forms of older and 

 more tropical series. Oak (Quercus pedunculata) is the predominant type, as 

 the species (with allied Q. sessiliflora) farthest north of a forest-race of trees 

 still culminating on the forest-ranges of tropical mountains of S. Asia, 3 and 

 passing north at lofty elevation (i-z miles) on the Himalaya ; similarly 

 associated in Hill Forest with Alnus y Betula, Corylus, Carpinus, Fagus^ 

 Salix, Populus, as comparable residual types also passing farthest north of 

 their series. Many of these are isolated generic monotypes, flowering in 

 early spring in order to get seed matured within the year. Much the same 

 applies to Ulmus montana and Fraxinus, as the last northern strays of 

 essentially tropical families ; in a lesser degree to Acer campestre ; while 

 Tilia^ also a residual stray, and alien Castanea, still manage to produce 

 some seed though only flowering at midsummer. 



1 Schimper, loc. cit., p. 351. 



2 Tansley (1911), Types of British Vegetation, p. 65. 



3 Cf. Wallich (1830), Plantae Asiaticae Rariores, fig. 46, Quercus spicata with erect fruiting- 

 spike, i ft. long, with over 100 acorns. 



