The endosperm is formed by the fusion of the haploid male nucleus with the polar nuclei (diploid) to form the triploid endosperm. Endosperm usually develops before the embryo does. After double fertilization, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” that has a milky consistency. This liquid mass, the endosperm, becomes multicellular when cytokinesis partitions the cytoplasm by forming membranes between the nuclei. Eventually, these “naked” cells produce cell walls, and the endosperm becomes solid. Coconut “milk” and “meat” are examples of liquid and solid endosperm, respectively. The endosperm functions to provide nutrition in the form of starch and in some cases even proteins to the developing embryo.
In some cases, the growing embryo completely uses up the endosperm and the resultant seed is called as ex-albuminous seeds. In other cases, the growing embryo does not completely use up the endosperm and the resultant seed contains the endosperm which is highly nutritious and this kind of a seed is called as albuminous seed. Based on the cell divisions that the endosperm undergoes, it can be of three types: Nuclear, Cellular and Helobial.
The fusion of one of the male nuclei along with the female gamete in the ovule leads to the formation of the zygote. The first mitotic division of the zygote splits the fertilized egg into a basal cell and a terminal cell.The terminal cell eventually gives rise to most of the embryo. The basal cell continues to divide, producing a thread of
cells called the suspensor, which anchors the embryo to the parent plant. The suspensor helps in transferring nutrients to the embryo from the parent plant and, in some species of plants, from the endosperm. As the suspensor elongates, it pushes the embryo deeper into the nutritive and protective tissues. Meanwhile, the terminal cell divides several times and forms a spherical proembryo (early embryo) attached to the suspensor. The cotyledons begin to form as bumps on the proembryo. A eudicot, with its two cotyledons, is heart-shaped at this stage. Only one cotyledon develops in monocots.
Soon after the rudimentary cotyledons appear, the embryo elongates. Cradled between the two cotyledons is the embryonic shoot apex. At the opposite end of the embryo’s axis, where the suspensor attaches, an embryonic root apex forms. After the seed germinates—indeed, for the rest of the plant’s life—the apical meristems at the apices of shoots and roots sustain primary growth.
- Formation of Seed
Structure of mature seed
The seed develops from the ovules inside the fruit. The integuments of the ovule will undergo further transformation, replication, and elongation and will become the seed coat—of variable texture, consistency, and colors, depending on the type of plant. The embryo consists of an elongate structure, the embryonic axis, attached to fleshy cotyledons. Below where the cotyledons are attached, the embryonic axis is called the hypocotyl (from the Greek hypo; under). The hypocotyl terminates in the radicle, or embryonic root. The portion of the embryonic axis above where the cotyledons are attached and below the first pair of miniature leaves is the epicotyl (from the Greek epi; on, over). The epicotyl, young leaves, and shoot apical meristem are collectively called the plumule.
The embryos of eudicot (dicot), possessing two cotyledons are packed with starch prior seed germination as they absorb carbohydrates from the endosperm when the seed was developing. However, the seeds of some eudicot species, such as castor beans (Ricinus communis), retain their food supply in the endosperm and have very thin cotyledons.
The embryos of monocots possess only a single cotyledon. Grasses, including maize and wheat, have a specialized cotyledon called a scutellum. The scutellum, which has a large surface area, is pressed against the endosperm, from which it absorbs nutrients during germination. The embryo of a grass seed is enclosed within two protective sheathes: a coleoptile, which covers the young shoot, and a coleorhiza, which covers the young root. Both structures aid in soil penetration after germination.
During the last stages of its maturation, the seed dehydrates until its water content is only about 5–15% of its weight. The embryo, which is surrounded by a food supply (cotyledons, endosperm, or both), enters dormancy; that is, it stops growing and its metabolism nearly ceases. The length of time a dormant seed remains viable and capable of germinating varies from a few days to decades or even longer, depending on the plant species and environmental conditions.
Environmental conditions required to break seed dormancy vary among species. Seeds of some species germinate as soon as they are in a suitable environment. Others remain dormant, even if sown in a favorable place, until a specific environmental cue causes them to break dormancy. The requirement for specific cues to break seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling. Seeds of many desert plants, for instance, germinate only after a substantial rainfall. If they were to germinate after a mild drizzle, the soil might soon become too dry to support the seedlings. Some seeds have coats that must be weakened by chemical attack as they pass through an animal’s digestive tract and thus are usually carried a considerable distance before germinating from dropped feces.
Seed Germination and Seedling Development
Germination depends on imbibition, the uptake of water due to the low water potential of the dry seed. Imbibing water causes the seed to expand and rupture its coat and also triggers metabolic changes in the embryo that enable it to resume growth. Following hydration, enzymes begin digesting the storage materials of the endosperm or cotyledons, and the nutrients are transferred to the growing regions of the embryo. Germination requires certain conditions, such as the softening of the seed coat, moisture, and adequate warmth, to occur. During germination, the hypocotyl begins growing downward to become the root; the cotyledon(s) will develop into the shoot, stems, and leaves.
The process of germination results in the sprouting through the ground’s surface of the seedling, which will develop into the mature plant with flowers. The cycle then begins again.
- Fruit Form and Function
While the seeds are developing from ovules, the ovary of the flower is developing into a fruit, which protects the enclosed seeds and, when mature, aids in their dispersal by wind or animals. Fertilization triggers hormonal changes that cause the ovary to begin its transformation into a fruit. If a flower has not been pollinated, fruit typically does not develop, and the entire flower usually withers and falls away.
The wall of the ovary becomes the wall of the fruit known as the pericarp. The pericarp can be further divided into three layers: Epicarp, Mesocarp, and Endocarp. The pericarp acts as a protective covering for the developing seeds until dispersion occurs.
Fruits are divided into two types based on their development: True Fruit which develop from an ovary, False Fruit which does not develop from an ovary.