Transcript Seeds
Seeds Parts of a seed • Cotyledon: The seed leaves. They often look different to the plant’s true leaves. They can either be above ground (epigeal) and be photosynthetic or remain below ground (hypogeal) and act as storage organs. • Testa: Is the seed coat. This needs to be penetrated by water before the seed can germinate. Sometimes the testa will need to be exposed to fire or an animal’s digestive system before germination can happen. • Plumule – gives rise to the first true leaves. You should already have the following: Epicotyl (above cotyledon attachment point) - Grows into the leaves of the plant. Also known as plumule. Hypocotyl (below cotyledon attachment point) - Becomes the stem Radicle - Becomes the root Germination • Factors that affect seed germination are: – Water – Oxygen – Light – Temperature. Water • Uptake of water is called imbibition. • Imbibition causes the seed to swell and break the seed coat. • When seeds are formed, most plants store a food reserve with the seed, such as starch, proteins, or oils • When the seed imbibes water, hydrolytic enzymes are activated which break down these stored food resources into useful chemicals. Oxygen • Oxygen is needed for cellular respiration (the same process that humans get energy from). • Some seed coats are don’t allow oxygen in and will only germinate once the coat is ruptured or worn away. • Seeds get oxygen from the air pockets in the soil that surrounds them. • If a seed a is buried too deeply or is in water logged soil it may not get enough oxygen and fail to germinate. Temperature • The temperature needed to germinate varies from plant to plant. • Some need cold temperatures to germinate and others need warm temperatures. • Some seed may even need a bushfire to take place before they will germinate. Light or darkness • Most seeds are NOT effected by light (after all they often start off underground). • However some seeds will only germinate when they receive enough light such as those that live in dense dark forests. Scarification • A process which mimic natural processes that weaken the seed coat before germination. – Seeds may be soaked in (sometimes boiling) water – Exposed to smoke or fire. – The seed coat may be sanded away. Dormancy • Seed may need special conditions to germinate. • Some seed will remain dormant even if they have all the components they need to germinate. This dormancy may be broken by a change in temperature or light levels. • Dormancy prevents all seeds germinating at the same time (which increases the risk that all seedling couple get wiped out in a disaster). • It also allows time for the seed to be dispersed far away from the parent plant so it won’t be in competition.