THE ECOLOGY OF PLANTS. 439 



416. Woodland Vegetation. The vegetation of a wood 

 consists of (1) the "canopy " or foliage surface of the dominant 

 and sub-dominant trees themselves, (2) the " underwood " of 

 shrubs and small trees, (3) the undergrowth of herbaceous 

 flowering plants, (4) the carpet of mosses, liverworts, and 

 fungi, (5) the shrubby and herbaceous plants at the edges of 

 the wood, and finally the very important but often overlooked 

 teeming population (vegetable and animal) of the soil itself. 



A wood is a very complex plant community, showing much 

 greater division of labour and specialisation among the con- 

 stituent plants than is seen, for instance, in a moor or a marsh. 

 There is a keen struggle for existence, yet each plant has its 

 own special work, which one might almost say benefits the 

 wood as a whole as well as itself. The formation of a wood 

 can only take place if the conditions are favourable with 

 regard to light, moisture, shelter, and soil, while the climatic 

 conditions in a wood differ markedly from those obtaining in 

 open country. 



The more decidedly xerophilous trees, such as Pines, can 

 grow in poor shallow soil, but most deciduous trees require 

 rich and fairly deep soils, which have to be prepared before- 

 hand by the growth and decay of successive generations of 

 smaller plants, whose dead and decayed remains form the 

 beginning of the humus essential for the enriching of the 

 soil. In high places fully exposed to wind, trees are either 

 absent or stunted or grow in a distorted manner, which shows 

 clearly the direction of the prevailing wind. 



Light is one of the most important factors in the biology 

 of a wood. It has been stated that the light intensity in a 

 Birch wood in summer is about one-fifth that of full day- 

 light on a sunny day, that in an Oak wood about one- 

 twentieth, and that in a Beech wood one-eightieth, but the 

 results obtained with photographic paper (Art. 124) of 

 course vary greatly, owing to differences in habit of the trees, 

 their closeness of growth, etc. However, woods consisting of 

 Beech, Sycamore, Ash, and Elm may be called shade areas, 

 while those consisting of Oak and Birch are light areas. 



In the former cases the trees, especially the Beech, by their 

 dense branching and close leaf arrangement, cast a deep shade 

 in which few other trees or shrubs or even herbs can grow, the 

 seedlings of all except shade-enduring plants being killed off. 



