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.