Every plant association has a characteristic appearance, due to the preponderance of one particular species which has been more sucoessful than the others in the struggle for the necessities of life. This species is called the dominant of the association. Thus the ling may be the dominant in a heath association, the reed grass in a swamp and the oak, beech, or ash in a wood, according to the nature of the soil. Generally the dominant is of greater size than the subordinate species, and this tends to produce
I different layers of vegetation. Thus in an oak wood there is a tree layer, a shrub layer in which hazel, briars and brambles occur, a herb layer occupied by such plants as bracken, woodand wood sorrel, and sometimes a layer of moss and liverworts is found beneath these herbaceous plants.
There is also a zonation in thesystems of an association. Such zonation is well shown in a dry oak wood where the bluebell, soft grass and bracken grow together. The soft grass is shallow rooting, and its rhizome runs in the surface soil. Below it the bracken rhizome is found, and beneath this lie the bulbs of the bluebell. In this way these plants utilize the same area of ground and yet avoid competition for water and salts in the soil.
Where different layers of vegetation are competing with each other for light, as in a wood, there are seasonal differences in the time of greatest activity and in the time of flowering. This seasonal variation is well shown by the oak woods which occur on damp clay soils. The lesser celandine and dog ’s mercury are small plants which grow in the most shaded part of the wood, and put forth theirin February and March. In this way they avoid the shade cast by the oak trees, which do not develop their until later in the spring. The celandine and dog ’s mercury flower in March and May and their leaves wither and die early in the summer, when they are shaded by the oak trees and by taller herbaceous plants. The food manufactured by the leaves before withering is stored by the tuber of the celandine and in the rhizome of the dog ’s mercury, and by means of these storage organs the plants perennate until the following spring. The wood grows up later in the spring and is found chiefly in the better lit areas of the wood, but its leaves also wither early in the summer. In the summer, plants such as herb robert, wood avens and the common bugle, which are able to live in the shade cast by the oak foliage, grow up and flower. In the autumn and winter, when the herbaceous plants have died down, toadstools show their maximum development.