The seed

The seed, resulting from sexual reproduction, creates a new generation of plants that bear characteristics of both parents. The plant must survive often through conditions that would be damaging to a growing vegetative organism. The seed is a means of protecting against extreme conditions of temperature and moisture, and is thus the overwintering stage.

Seed structure
The basic seed structure is shown in Figure 7.7. The main features of the seed are:

• embryo, in order to survive the seed must contain a small immature plant protected by a seed coat;
• testa, the seed coat, is formed from the outer layers of the ovule after fertilization;
• micropyle, a weakness in the testa, marks the point of entry of the pollen tube prior to fertilization;
• hilum, this is the point of attachment to the fruit.

The embryo consists of a radicle, which will develop into the primary root of the seedling, and a plumule, which develops into the shoot system, the two being joined by a region called the hypocotyl. A single seed leaf (cotyledon) will be found in monocotyledons, while two are present as part of the embryo of dicotyledons. The cotyledons may occupy a large part of the seed, e.g. in beans, to act as the food store for the embryo.

In some species, e.g. grasses and Ricinus (castor oil plant), the food of the seed is found in a different tissue from the cotyledons. This tissue is called endosperm and is derived from the fusion of extra cell nuclei, at the same time as fertilization. Plant food is usually stored as the carbohydrate, starch, formed from sugars as the seed matures, e.g. in peas and beans. Other seeds, such as sunflowers, contain high proportions of fats and oils, and proteins are often present in varying proportions. The seed is also a rich store of nutrients that it requires when a seedling, such as phosphate.

The seed structure may be specialized for wind dispersal, e.g. members of the Asteraceae family, including groundsel, dandelion and thistle, which have parachutes, as does Clematis (Ranunculaceae). Many woody species such as lime (Tilia), ash (Fraxinus), and sycamore (Acer) produce winged fruit. Other seed-pods are explosive, e.g. balsam and hairy bittercress. Organisms such as birds and mammals distribute hooked fruits such as goosegrass and burdock, succulent types (e.g. tomato, blackberry, elderberry), or those that are filled with protein (e.g. dock). Dispersal mechanisms are summarized in Table 7.1.

Seeds are contained within fruits which provide a means of protection and, often, dispersal.