Seed Study Guide

📖 Core Concepts Seed – an embryo plus a food reserve, wrapped in a protective seed coat; formed from a fertilized ovule. Embryo – diploid plant from the zygote; gives rise to cotyledons, epicotyl, hypocotyl, radicle, and plumule. Endosperm – normally triploid tissue that stores starch, oils, or proteins for the seedling. Seed Coat (Testa/Tegmen) – maternal tissue derived from ovule integuments; protects the embryo. Double Fertilization (angiosperms) – one sperm + egg → zygote (embryo); second sperm + central cell → primary endosperm nucleus. Dormancy – a reversible block that prevents germination until conditions are favorable; can be physical, chemical, morphological, physiological, or combinations. Germination – uptake of water → metabolic re‑activation → radicle emergence. --- 📌 Must Remember Three seed parts (angiosperms): embryo (diploid), endosperm (triploid), seed coat (maternal). Albuminous (endospermic) vs. Exalbuminous (non‑endospermic): presence or complete absorption of endosperm into cotyledons. Physical dormancy = hard seed coat → water‑impermeable; broken by scarification, fire, or digestive passage. Physiological dormancy = hormonal inhibition (high ABA); broken by stratification, temperature cues, or after‑ripening. Morphological dormancy = under‑developed embryo; must grow to a species‑specific length before germination. Thermodormancy = germination only at certain soil temperatures; disappears with seed aging. Viability standard: ≥ 90 % germination in 20 days = high‑quality seed (agricultural benchmark). --- 🔄 Key Processes Double Fertilization Pollen tube delivers two male gametes. Gamete 1 + egg → zygote → embryo. Gamete 2 + central cell → primary endosperm nucleus → endosperm. Seed Coat Formation Outer integument → testa (outer coat). Inner integument → tegmen (inner coat). Scarification (breaking physical dormancy) Mechanical: sandpaper, nail file, or brief hammering. Chemical/thermal: soak in hot water (≈ 80 °C, 10 min) or dilute acid. Stratification (breaking physiological dormancy) Mix seeds with moist medium. Refrigerate (≈ 4 °C) for 4–12 weeks depending on species. Germination Sequence Water uptake (imbibition) → metabolic re‑activation → radicle protrusion → hypocotyl/epicotyl elongation. --- 🔍 Key Comparisons Physical vs. Chemical Dormancy Physical: impermeable seed coat; broken by scarification, fire, digestion. Chemical: germination inhibitors; removed by leaching or enzymatic breakdown. Albuminous vs. Exalbuminous Seeds Albuminous: distinct endosperm persists (e.g., wheat, corn). Exalbuminous: endosperm absorbed into cotyledons (e.g., beans, peas). Monocot vs. Dicot Embryo Structures Monocot: 1 cotyledon, coleoptile (plumule sheath), coleorhiza (radicle sheath). Dicot: 2 cotyledons, no coleoptile/coleorhiza. Gymnosperm vs. Angiosperm Seed Development Gymnosperm: single fertilization; no double fertilization; haploid female gametophyte supplies nutrients. Angiosperm: double fertilization; triploid endosperm formed. --- ⚠️ Common Misunderstandings “All seeds are fruits.” Only the mature ovary (fruit) encloses true seeds; many “seeds” (sunflower, peanut) are actually dry fruits. “Hard seed coat = dormant forever.” Hard coats cause physical dormancy that can be broken by scarification or environmental cues. “All endosperm is starchy.” Endosperm consistency varies: starchy, oily, cartilaginous, horny, ruminated. “Dormancy = inability to germinate.” Dormancy is reversible; seeds can germinate once the block is removed. --- 🧠 Mental Models / Intuition “Seed = embryo + pantry + armor.” Think of embryo as the chef, endosperm as the pantry stocked with food, and seed coat as the armor protecting them. “Dormancy layers = security system.” Physical dormancy = locked door; physiological dormancy = alarm system; both must be disabled for entry (germination). “Double fertilization = two‑step recipe.” First step makes the main dish (embryo); second step prepares the side dish (endosperm). --- 🚩 Exceptions & Edge Cases Vivipary – mangrove seeds germinate while still attached to parent; bypasses typical dormancy. Combinational Dormancy – both hard coat and physiological inhibition; either barrier may be broken first. Secondary Dormancy – seeds that were non‑dormant can re‑enter dormancy after exposure to adverse conditions (e.g., high temperature). Non‑seed plants – ferns, mosses reproduce via spores, not seeds; do not follow the seed‑centric concepts. --- 📍 When to Use Which Scarify → hard‑coated (physically dormant) seeds: legumes, many tropical trees. Stratify (cold‑moist) → seeds with physiological dormancy requiring chilling: many temperate forest herbs, fruit trees. Hot‑water soak → fire‑adapted species or hard‑coated seeds where heat mimics fire cues. Leaching in running water → chemically dormant seeds with water‑soluble inhibitors (e.g., some legumes). Smoke treatment → fire‑dependent species (e.g., many Australian natives). --- 👀 Patterns to Recognize Seed coat → hilum + micropyle: hilum = scar from ovary attachment; micropyle = former pollen entry point. Embryo polarity: upper (chalazal) pole → growth zone; lower (micropylar) pole → suspensor. Endosperm type clues: “oily” seeds are often small, glossy (castor bean); “starchy” seeds are grainy (wheat, rice). Dormancy clues in species name: “thermo‑dormant” → temperature‑dependent; “physio‑dormant” → hormonal control. --- 🗂️ Exam Traps Distractor: “Seeds are always the same size as the embryo.” – False; many seeds have large endosperm making them much larger than the embryo. Trap: “Physical dormancy is broken by cold temperatures.” – Physical dormancy requires mechanical/thermal damage, not just cold. Misleading choice: “Gymnosperm seeds undergo double fertilization.” – Incorrect; they have single fertilization only. Near‑miss: “All monocot seeds have a coleoptile.” – True for most, but some reduced‑cotyledon monocots (e.g., orchids) lack a prominent coleoptile. ---