BIOLOGY OF THE OAK

Trunk and Branches -The oak is one of the largest of British trees and may reach a height of 130 ft. or more. It can be recognized at a distance by its rounded outline and spreading branches, which bend and twist in an irregular zigzag manner. The trunk of an oak growing in the open usually divides into branches at a height of 10 to 12 ft., but when the tree is growing in a wood surrounded by others the trunk may rise 20 to 30 ft. before branching. The bark of the trunk is dark brown and is marked with deep ridges and furrows. The twigs are slender and bear small brown buds which are spirally arranged. At the tip of a twig the buds are clustered together, but occur at fairly wide intervals along the stem behind the tip. Frequently the apical bud dies and the growth is carried on by a lateral bud. It is for this reason that the branches of the oak are so often crooked.

The Leaves

The leaves are oblong in general shape, and their edges are cut into large rounded lobes. Certain differences that occur regularly in the leaves and fruits of oak trees have caused botanists to distinguish two varieties of oaks. These varieties are now usually looked upon as two distinct species, named respectively the common oak, Quercus pedunculata, and the sessile oak, Quercus sessiliflora. The leaves of the common oak have a very short stalk, whereas those of the sessile oak have stalks from ½ to I in. long and are slightly hairy on the under side.

The Flowers and Fruit

The oak flowers appear with the first leaves in April or May, and the two kinds of flowers, staminate and pistillate, are borne on the same tree. The staminate flowers occur in catkins consisting of a pendulous central stalk along which the flowers are arranged singly or in clusters. Each flower is composed of five to twelve pendulous stamens which are enclosed in a perianth of five to seven lobes. The female flowers of the common oak occur on a stalk which is more or less erect and bears four to five flowers, each of which arises in the axil of a small bract. A shallow cup of overlapping bracts, the future acorn cup, surrounds each flower, which consists of a six-toothed perianth enclosing the small ovary. This is formed of three joined carpels and bears a short style with three stigmas. The axis bearing the female flowers of the sessile oak is shorter than that of the common oak, so that the flowers are clustered between the base of the leaf stalk and the shoot. After fertilization the overlapping scales surrounding the female flower develop into the acorn cup, and the ovary becomes the acorn. The acorns of the common oak usually occur in pairs on a slender stalk an inch or two long, but those of the sessile oak have no stalk, but are attached by the cup to the twig itself. Although the ovary has three loculi, each containing a pair of ovules, usually only one of these ovules develops into a seed.

The common oak is the more usual type in the Midlands and South of England, and is usually found growing on heavy loam or clay soils. The sessile oak is more frequently found on the siliceous soils of the North and West of England. Two other species of oak which are often found in Britain are the turkey oak, Quercus cerris, and the evergreen or holm oak, Quercus ilex. The turkey oak has rather narrow leaves with long pointed lobes, and the stalkless acorn cup is covered with long loosely arranged bracts. The evergreen oak is a small tree with shiny dark-green leaves which have plain or spiny edges. The acorn of the evergreen oak is long and narrow.

It is suggested that other forest trees should be studied by first-hand observation by following the scheme given below.

QUESTIONS ON TREES

Stem.

What is the average height of the tree ?

Describe the outline of the tree. Is it rounded, pointed, oblong, etc. ?

Is the trunk short ? Can the trunk still be recognized as a main axis after branching has taken place ?

Is branching regular or irregular ?

What is the colour of the bark of the trunk ? Is the bark smooth or rough ?

What is the colour of the bark of the twigs ?

Leaves.

Is the leaf arrangement alternate, spiral, opposite or decussate ?

What is the shape of the leaf ? What is the arrangement of the veins ?

Describe the colour, shape and size of the buds.

Flower.

Does the tree flower before or after the leaves are out ?

On what branches are the inflorescences found ?

Is there a perianth, calyx, corolla ? Describe the colour and number of the parts. Is the flower scented ? 1

How many stamens are there in a flower ? Describe the stamens. 1

Is the gynascium apocarpous or syncarpous ? How many carpels are there ? Describe the placentation. Describe the style and stigma.

Fruit. 1

Describe the structure of the fruit. To what type does it belong ? 1

Mow many seeds are there in each fruit ? 1

How are the fruits or seeds dispersed ?

Sorry, comments are closed for this post.