28 Plant-life of the Oxford District 



III. PRIMARY WOODLAND AND ITS DERIVATIVES 



What we are accustomed to call a tree is the typical plant of the 

 modern land-surface, representing the response of plant-life to conditions of 

 subaerial environment over a period of hundreds if not thousands of millions 

 of years. Such trees, again, have not been created or evolved to suit the 

 conditions of the land alone, but are essentially derivative and migrant to 

 the land from an older order of submarine existence as seaweeds. Just, in 

 fact, as man has persistently felt himself to be a stranger and sojourner on 

 this world, because his body, being inherited in all essentials of structure, 

 metabolism, and sense-organs from the oceanic Fish, is equally far from 

 being constructed to suit subaerial conditions. That is to say, in the modern 

 tree one sees the finished article of long ages of an evolution as long as our 

 own, going back to the sea where all life as we know it began. 



Given an autotrophic plant-organization, of branching shoot-system 

 displaying photosynthetic laminae to air and incident light, growing-points 

 with permanent meristem, and a system of secondary increase adding 

 effective mechanical strength to resist the physical chances and movements 

 of the medium, it is evident that, under optimum conditions of tempera- 

 ture and light-supply, vegetation will continue to increase indefinitely on the 

 area occupied, and be, in fact, practically immortal, since without senile 

 decay. Ail these growth-factors having been evolved during submarine 

 existence, the translation of plant-life to the land-surface does little more at 

 first than continue such organization, and progressively adapt it to sub- 

 aerial conditions. 



Where conditions of sunlight, temperature, and the necessary water- 

 supply, are at a maximum, plant-life is still dominant^ and at its 

 optimum. Thus tropical evergreen rain-forest is not only the primary 

 station for higher plant-life on the land, but it is still constituted by the 

 most highly specialized types of tree-vegetation, as a massed jungle of 

 dominant and competitive plant-forms. 1 Where the ground is fully 

 occupied, such tree-growths can only obtain pre-eminence by outstripping 

 their fellows in height. Hence upward extension is exaggerated in com- 

 petition for favourable light-supply, which is a constant taking the year 

 round. In the struggle to rise higher over adjacent competitors the 

 arboreal form attains its full differentiation as ' high-forest ' in ' close 

 canopy '. . Simultaneously with extension to higher levels of the atmo- 

 sphere (100-300 ft.), the massive trunk is evolved in response to the 

 demand for resisting lateral displacement by the wind, and the effective 

 support of the mass of the branches and foliage. 



Such a generalized arboreal phase of plant-life is sufficiently familiar 

 as a commonplace to the inhabitants of even this part of the world's 

 surface, in the forms of indigenous woodland. For this the biological 

 conditions require to be more clearly expressed, and the limitations of such 

 a phase of vegetation more particularly defined. The actual amount of 

 plant-life in aggregate mass possible on a given area is dependent on such 

 factors as: (i) the amount of solar energy falling on unit-area; (2) the 

 amount of water available in the soil ; (3) the constitution of the substratum in 

 relation to the supply of food-ions in solution ; (4) the possibilities of seasonal 

 change including effects of frost, drought, and storm. For example, in 

 the general case of a suitable soil with sufficient mineral matter, particularly 

 nitrogen and phosphorus-content in addition to the humus-remains of past 



1 Schimper (1903^ Plant-Geography, Eng. Trans., p. 304. 



