種子の構造と胞子植物の初期発生

Double fertilization occurs when two sperm enter an ovule. One sperm fertilizes the egg to form the sporophyte embryo, while the other fertilizes the two polar nuclei to create the endosperm, a nutrient storage tissue. This process produces a seed containing both the embryo and food reserves necessary for germination.

A seed consists of a protective seed coat surrounding an embryo and food reserves. The embryo contains precursor tissues for stems, roots, and cotyledons (embryonic leaves). Food reserves are stored in the endosperm, cotyledons, or both, providing nutrition for the developing seedling during germination and early growth.

Monocot seeds have a single large cotyledon called the scutellum that absorbs nutrients from the endosperm. Dicot seeds have two fleshy cotyledons and may lack endosperm, storing food directly in the cotyledons. Both types have protective sheaths that facilitate soil penetration during germination and early development.

The embryonic axis consists of three key structures: the plumule (developing shoot), radicle (developing root), and hypocotyl (connecting tissue). These structures work together during seed germination and early sporophyte development to establish the seedling's root and shoot systems for nutrient and water uptake.

During germination, the seed coat ruptures to allow the radicle to emerge as the first organ. The cotyledons absorb enzymatically-released nutrients and transport them to the developing embryo. As food reserves deplete, the cotyledons wither while new leaves expand and begin photosynthesis to sustain growth.

The sporophyte embryo develops a double set of chromosomes—one set from each parent. This occurs when a haploid sperm fertilizes a haploid egg, creating a diploid zygote that develops into the embryo with genetic material from both parents, ensuring genetic diversity in the offspring plant.

The endosperm, created during double fertilization, serves as the primary food reserve for the developing embryo and seedling. During germination, cotyledons or the scutellum absorb enzymatically-released nutrients from the endosperm and transport them to support early growth until photosynthesis begins sustaining the plant.