Introduction: After fertilization a plant zygote develops into an embryo through cell division and differentiation. In fowering plants, a second fertilization event gives rise to a primary endosperm nucleus. The primary endosperm nucleus undergoes cell division, but its sole role is to nurture the developing embryo. In some seeds the endosperm persists in the mature seed, and functions as storage tissue for the germinating seedlings. In other plants the endosperm is totally consumed before the seed matures. In these seeds the food storage function is taken over by the cotyledons which are part of the embryo. These seeds are termed exalbuminous.
Embryo Development: Below are links to views of four stages of development of shepard's purse, Capsella bursa-pastoris.
Globular: At this stage the embryo is already divided into two parts: a suspensor and the embryo proper. The suspensor functions during the development of the ovule, but has no function in the mature seed.The embryo proper is the part of the embryo that goes on to develop into the new sporophytic plant. At this stage we already see the outer layer of cells of the embryo proper organized into a protoderm. This primary meristematic tissue goes on to develop into the epidermis. Endosperm tissue should be visible in the area of the ovule not occupied by the embryo.
Heart-shaped: At this stage can be seen the earliest development of the cotyledons. The two lobes at the top of the embryo proper go on to become the cotyledons in the mature embryo. Some evidence of the organization of the procamium and ground meristem can can be observed here. The procambium is the primary meristem that matures into vascular tissue. The ground meristem surrounds the procambium and matures into ground tissue. Protoderm is the outer most cell layer covering the entire embryo proper. Endosperm tissue should be visible in the area of the ovule not occupied by the embryo.
Torpedo: At this stage the coteledons have developed to the point where they are forced to curve to fit within the confines of the ovule. The three primary meristems are all clearly visible. Endosperm tissue should be visible in the area of the ovule not occupied by the embryo.
Mature: As the seed becomes mature, the embryo becomes dormant. Students need to know all parts of the labelled diagram. Endosperm tissue has been completely consumed by this stage.
Link to diagram of all four stages
Bean Seed and Seedlings: The bean seed is an exalbuminous seed (it lacks endosperm at maturity). Most of its volume is occupied by two large cotyledons that serve as food storage tissue. The stem of the embryo is divided into the part above the cotyloedons (the epicotyl) and the part below the cotyledons and above the root (the hypocotyl). In this seedling the hypocotyl elongates and arches pulling the cotyledons and the apical meristem out of the ground, thus, protecting that meristem.
Link to View of Bean Seed
Link to view of seed and seedling (what comes from where)
Link to view of Seedling Stages
Pea Seed and Seedlings: Like the bean seed, mature peas seeds have no endosperm. Most of the volume is occupied by two large cotyledons that serve as food storage tissue. Unlike bean seedlings, in peas, the hypocotyl does not elongate. The cotyledons remain in the soil and the epicotyl elongates. As it does it arches to pull the apical meristem out of the ground, and in this way protects the apical meristem.
Link to view of pea seed
Link to view of embryo and seedling
Link to view of Seedling Stages
Corn Grain and Seedlings: The corn grain is more than a seed it is a fruit. The integuments of the ovule do not mature into a seed coat. The pericarp matures into a hard tissue that functions to protect the embryo.The corn grain is albuminous. Much of its volume is occupied by endosperm that serves as food storage tissue. The embryo itself has one cotyledon (the scutellum). Like all gasses the embryo is well developed. The stem has a number of leaf primordia (and is called a plumule). Also, as in all grasses the plumule is enclosed in a sheath-like structure called a coleoptile. The coleoptile is the first part of the embryo to break the ground. Once it emerges the shoot grows right up through it. This it corn's adaptation for protecting the apical meristem of the shoot. The coleorhiza ensheaths the root and is a mirror Images of the coleoptile.
Link to View of Corn Grain
Link to detail view of Corn Embryo
Link to view of Seedling Stages