35.3
Structure des graines et développement précoce du sporophyte
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Q1: What is double fertilization and how does it create the seed?
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.
Q2: What are the main structural components of a seed?
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.
Q3: How do monocot and dicot seeds differ in structure?
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.
Q4: What is the embryonic axis and what structures does it contain?
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.
Q5: What happens to the seed coat and cotyledons during germination?
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.
Q6: How does the sporophyte embryo develop its chromosome composition?
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.
Q7: What role does the endosperm play in seed development and germination?
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.