Like flowers, fruits occur only in angiosperms. As the embryo and endosperm develop following double fertilization, the ovary increases in size and is gradually transformed into a fruit. During this process, the ovary wall grows and differentiates, resulting in the fruit wall, or pericarp. The ovule enlarges into a seed, and tissues of the inner (if present) and outer integuments become the seed coat. With the exception of parthenocarpy (the development of fruit without seeds), fruits are developed only after double fertilization. As the embryo develops, parenchymatous extra-embryonic tissues of a flower undergo complex taxon-specific histologic modifications designed for embryo protection and seed dispersal. The other floral parts, i.e. style, perianth and androecium, usually dry up and fall off. At the same time both the cells of the pericarp and those of the seed coat divide and grow. The cells often contain chloroplasts capable of intense photosynthesis, which may provide significant amounts of assimilates for the growing heterotrophic embryo.
The pericarp is usually comprised of the expanded and often highly modified ovary wall. In fruits derived from an inferior ovary, a hypanthium participates in the pericarp formation and, in some cases, so do the perianth and even bracts of the inflorescence. Three zones are often seen in the pericarp, an outer exocarp, an inner endocarp and a mesocarp. However, in many plants the endocarp, the mesocarp or both, may be missing in the pericarp. In some fruit types like berries, and in many dry fruits, these zones are difficult to recognize altogether.
The morphology and internal structure of fruits are very diverse. These differences are taxon-specific and they are, to a considerable extent, related to dispersal and germination strategies. The classification of the fruits may be based on the consistence of the pericarp as to whether it is dry and hard, or soft and fleshy. Fruits may also be classified on the basis whether they dehisce or not when ripe and whether they are single-seeded or multiple-seeded. In single-seeded indehiscent fruits, the unit of dissemination is fruit itself rather than seed.
One of several available fruit classifications is given below:
These are characterized by the expansion of parenchyma in the pericarp during fruit development. Thin-walled and highly vacuolated parenchyma cells predominate in the pericarp of fleshy fruits. These cells remain intact long after fruit maturation.
The pericarp of these fruits consists mostly of dead desiccated cells at maturity. In regions of the growing pericarp, some cells die earlier than others and, as a result, they become crushed. At the final stages of maturation, one or more layers of cells undergo sclerification, giving the fruit a characteristic hardness.
The seed functions as the main reproductive unit in both angiosperms and gymnosperms and provides continuity between the successive generations. Seeds are the means of plant dissemination (dispersal). In addition, seeds help the tender embryo to survive under cold, dry and other unfavorable conditions, and supply the embryo with nutritive materials. The angiosperm seed is composed of three main parts: embryo, endosperm (the source of stored nutrients) and a protective seed coat. The embryo and endosperm are separate products of double fertilization, whereas the seed coat has a maternal (ovular) origin. The endosperm may be transient and as such, it may degenerate more or less completely before the seed becomes dormant. In such a case, only the embryo contains food reserves (e.g. proteins, starch and/or oils). But in most flowering plants the endosperm tends to remain. The ratio of volumes occupied by the embryo and endosperm in the mature seed varies greatly among different plants. In most monocotyledons the endosperm is preserved in dormant seeds and becomes the source of nutrients for the embryo in the germinating seed.
The embryo (the incipient sporophyte) is the most important part of the seed. It consists of two (dicotyledons) or one (monocotyledons) leaf-like cotyledons, and a hypocotyl – the stem-like axis below the cotyledons. The hypocotyl forms two apical meristems at its poles, the root apical meristem (RAM) at the lower pole and the shoot apical meristem (SAM) at the upper pole. These two meristems give rise to all postembryonic structures in the primary growth of the plant. In dicotyledons, the SAM is situated between the cotyledons at their bases, whereas in monocotyledons it occupies a lateral position with respect to the vertically oriented cotyledon. The embryo is situated opposite the micropyle, which facilitates the penetration of the seed coat during germination.
The seed coat in mature seeds is dry and usually consists of dead cells. The withering away of different layers of the seed coat occurs asynchronously in a developing seed. One or several cell layers may become non-living when the cell walls are thin, and the seed is still enlarging. Such layer or layers become crushed and may finally disappear. But other layers remain alive and grow in pace with the seed expansion. Their cells may undergo sclerification and lignification after the cessation of seed growth. The pattern of secondary thickenings in the sclereids and the shape of sclereids themselves in the supporting layers of the seed coat vary greatly and are taxon-specific. In different plants the transformation of the integumentary parenchyma into seed coat sclerenchyma may occur in different cell layers. The presence of one or more layers of calcium oxalate crystal-containing cells is also a frequent phenomenon. The cells of the outer layer of the seed coat often secrete a conspicuous amount of slime at the terminal stage of seed development. When moistened, the slime becomes sticky and adheres to the soil. It may also facilitate seed dispersal by passing animals. The frequent brown or black color of seeds is due to pigments accumulated in the cells of the seed coat. In single-seeded fruits where the embryo is protected by the pericarp, the seed coat is often obliterated or appears as a very thin, structureless membrane between the embryo and the pericarp.
During desiccation of seeds at the final stage of their ripening, the seed surface relief, or micromorphology, acquires a characteristic pattern that is stable and taxon-specific. Therefore, scanning electron microscope studies of seed surface have proved to be of great importance in solving taxonomic problems. Among the characters that may determine taxon-specific seed surface patterns are cellular arrangement, shape of cells, the outline (straight, lobed or irregularly curved) of their anticlinal walls, the surface relief of their outer periclinal walls, the sculptures of the cuticle and epicuticular waxes, etc.
After the arrival of a desiccated dormant seed in favorable conditions, e.g. appropriate soil moisture, temperature and aeration, it usually germinates. Germination is preceded by the absorption of large amounts of water. The imbibition of seeds is, as a rule, accompanied by the rupture of the seed coat usually at the micropylar end of the seed. The mobilization of food reserves coincides with water absorption. Water-soluble products of hydrolysis are transported to the activated apical meristems. First, the root appears out of the micropyle and starts its downward growth into the soil. The root develops root hairs and, frequently, lateral roots. Only after this do the other organs of the embryo start to emerge. The absorptional activity of the emerging root helps to assure the young seedling an adequate supply of water and mineral nutrients when the shoot breaks through the surface of the soil.
In some plants, the cotyledons together with the incipient shoot apex appear above ground due to the intercalary growth of the hypocotyl. In such epigeal (above ground) germination, the cotyledons turn green and become the first photosynthesizing organs of the seedling. In other plants, the cotyledons that are thick and rich in storage substances remain within the seed coat underground. In this hypogeal (underground) germination, the shoot apical meristem is pushed through the soil surface by the elongating epicotyl (the internode above the cotyledons) and the first photosynthesizing organs of the seedling are the leaves situated above the cotyledons. During germination the formation of new organs proceeds. Leaf primordia are initiated at the shoot apex. As the leaves unfold, the stem portions between them elongate into internodes and, thus, the shoot system is established. The embryonic root becomes the primary root of a seedling. The primary root produces lateral (branch) roots that are called secondary roots. As a result, the root system is initiated and the new sporophyte becomes established.