The fertilized egg cell develops into an embryo which, with certain reserve tissues, forms the seed which is enclosed in a protective covering called the seed coal. The Fruit is a ripened ovary, its walls forming the seed vessel, and contains one or more seeds. In most conifers the seeds are contained in a cone consisting of carpels attached to a central axis that becomes woody. The fruits of broad-leaved trees are classified as true fruits if their origin is a single ovary, or accessory fruits if other parts of the flower are involved, or if their origin is the entire inflorescence (mulberry). True fruits are further divided into dry and fleshy fruits. Dry fruits with a dry and hard outer wall include the following:
Samara — usually a winged, one-seeded fruit with thin, membranous to leathery coat (birch, elm, ash).
Nut — a one-seeded fruit with hard, woody wall not connected with the seed (ha/el, lime, hornbeam).
Legume— usually the product of two carpels splitting along two lines of suture when ripe and containing several seeds; characteristic of most members of the family Leguminosae. Capsule — a single or several valved fruit splitting in various ways when ripe and containing several seeds (poplar, willow, chestnut-).
Fleshy fruits have soft, fleshy walls. They include: Drupe — usually a one-seeded fruit with a coat comprising three layers: a thin outer layer, middle fleshy layer and a hard bony layer — the stone encasing the seed (cherry, walnut). Berry — a fruit with thin membranous covering and fleshy centre with one or several seeds embedded in the pulpy mass; few trees bear true berries.
Of the accessory tree fruits the most common is the pome. A product of the fusion of a fleshy receptacle with ovary wall. The fruit proper is the ‘core’ containing several seeds. The apple, pear and mountain ash, etc. bear pomes.
The seed is made up of a membranous or hard outer covering — the seed coat or testa and the inner nucleus, which contains only the embryo and food reserves for the initial period of growth stored in the seed leaves (cotyledons) of the embryo (oak, beech, horse-chestnut, maple, ash, etc.). Some seeds contain a separate food reserve (endosperm) which surrounds the embryo and provides food for its growth (many conifers, etc.). Often clearly discernible on the seed is the scar where it was attached to the fruit. Usually this is light in colour (horse-chestnut and many Leguminosae). When germinating, it is through this scar that the seed absorbs the most water and through or near it that the first root and shoot emerge.
Germination takes place if conditions are suitable, I.e. in adequate temperature and humidity. The absorption of water promotes not only the growth of the- embryo plant and the rupturing of the seed coat, but also the conversion of the reserve food supplies stored within the seed. The quantity of water the seed absorbs is enormous— 40 to 100 per cent of the dry weight of the seed. Another condition necessary for germination is warmth of a certain degree lasting for a certain period. The minimum temperature required for germination is about 5°C, the optimum about 25°—27°C, depending on the species. Also absolutely necessary for successful germination is an adequate supply of oxygen needed by the seed for conversion of the food stores and for respiration.
If conditions are favourable, the seeds of most woody plants germinate within three to four weeks of sowing. But in some trees the period of germination is longer when sown in spring. If the seeds have become too dry during the winter storage they may not germinate at all, or else not until the following spring. This is true of many seeds, particularly of large seeds or those that fall from the tree in the autumn (silver fir, Douglas fir, white pine, common yew, oak, beech, chestnut, maple, etc.). With these species it is best to sow the seeds in the autumn, or else store them in damp sand at a temperature of 0°—3°C, preferably in a cellar. They are sown together with the sand during the following spring.