35.3: Seed Structure and Early Development of the Sporophyte

Seed structures are composed of a protective seed coat surrounding a plant embryo, and a food store for the developing embryo. The embryo contains the precursor tissues for leaves, stem, and roots. The endosperm and cotyledons—seed leaves—act as the food reserves for the growing embryo.

The embryo contains a double set of chromosomes, one set from each parent. Fertilization of the haploid egg by the haploid sperm gives rise to the zygote, which develops into the embryo.
The endosperm is a feature common to most flowering plants, and it is created during the process of double fertilization. Here, two sperm enter into each ovule. One sperm fertilizes the egg; the other fertilizes the central cell, producing the endosperm. Conifers and other gymnosperms do not undergo double fertilization, and therefore do not have a true endosperm.
Seed structure differs between monocots and dicots, two types of flowering plants.

Monocots, such as corn, have a single large cotyledon called the scutellum, which directly connects to the embryo vascular tissues. The endosperm acts as the food reserve. During germination, the scutellum absorbs enzymatically-released food materials and transports them to the developing embryo.

The monocot embryo is surrounded by two protective sheaths. The first, the coleoptile, covers the young shoot. The second, the coleorhiza, encases the young root. Both structures facilitate soil penetration after germination.

Dicot seeds may be endospermic or non-endospermic. In endospermic dicots, such as tomatoes, the food reserves are present in the endosperm. During germination, the cotyledons absorb the enzymatically-released food material from the endosperm and transport it to the growing embryo.

Seeds develop from fertilized flowers. Specifically, seeds develop from ovules found in the ovary of the flower, through a process called double fertilization. During this process, one sperm fertilizes the egg and one sperm fertilizes the two polar nuclei, resulting in a sporophyte embryo and an endosperm—a store of nutrients.  

The sporophyte embryo has a double set of chromosomes, one set from each parent, formed by the fertilization of a haploid egg by a haploid sperm. 

The embryo consists of the precursor tissue for stems and roots, as well as one or more embryonic leaves—called cotyledons. The embryo is surrounded by a food reserve in the form of an endosperm, cotyledons, or both, and is enclosed within a protective seed coat to form a seed. 

Plants are generally classified as monocot or dicot, depending on the number of cotyledons in their seeds. 

Typically, monocot seeds have a single large cotyledon called the scutellum. During germination, the scutellum absorbs enzymatically-released food materials from the endosperm and transports them to the developing embryo. 

Dicot seeds, by contrast, have two fleshy cotyledons. Many mature dicot seeds lack endosperm and reserve food material in the cotyledons. Some dicots, however, have seeds with a nutrient-storing endosperm, similar to most monocots. 

The embryonic axis of monocot and dicot seeds consists of the plumule, radicle, and hypocotyl. All of these structures participate in the seed germination and early development of the seedling, or sporophyte. 

During germination, the seed coat ruptures and gives way to the radicle, the first organ of the young sporophyte. In response to light, the cotyledons separate, the hypocotyl straightens, and the epicotyl expands and forms its first true leaves.

As the food reserves are depleted, the cotyledons wither. Eventually, the new leaves expand and begin to synthesize their food through photosynthesis.