Transcript Slide 1
Germination • Germination is the resumption of growth of the embryo plant inside the seed. • Germination is the resumption of growth of the embryo plant inside the seed. Annuals and ephemerals • Plants that complete their life cycle in one growing season or less – Seed, germination, growth, flowering, death – Over-winter as seeds – In hot climate survive drought as seed • There are winter and summer annuals • Winter annuals germinate in autumn • Mainly found in Mediterranean climates • Annuals - Plants that perform their entire life cycle from seed to flower to seed within a single growing season. All roots, stems and leaves of the plant die annually. Only the dormant seed bridges the gap between one generation and the next. Ephemeral • Plants lasting a very short time • A plant that grows for a short time only. Many desert plants are ephemeral as they grow seasonally only for brief periods and then came back in a more or less long dormancy phase. • This is an especially short-lived annual plant that completes its life cycle in two-three weeks. The seeds are encased in a waterproof coating that prevents desiccation for years if necessary. These plants essentially avoid drought by occurring as seeds most of the time. Annuals and ephemerals • Annuals • Includes many important weeds e.g. – Hairy bittercress , mayweed, fat hen (annuals) • Important garden plants – Most bedding plants • Many important crops – All cereals (wheat, barley, rice) • Important note – many annuals will over in mild conditions or under protection • Ephemerals • More than one generation per season – Many weeds – e.g. Groundsel, Biennials • Plants that complete their life cycle in two growing seasons – Germinate and vegetative growth in year one – Flower, set seed and die in year two – Over-winter as seeds Biennials • A few common plants – e.g . Foxgloves (Digitalis spp) • Many important food plants – Onions, leeks – Cabbages, swede, turnip – Carrots, parsnip • Important notes – – Some biennials will survive a third year – If growing conditions are poor will flower and set seed in year one (known as “bolting”) • Biennials - Plants which require two years to complete their life cycle. First season growth results in a small rosette of leaves near the soil surface. During the second season's growth stem elongation, flowering and seed formation occur followed by the entire plant's death. • Foxglove is an example of a biennial plant. In the first year after it is planted, only roots and stems grow. In the second year flowers and seeds are produced, and then the plant dies. Perennials • Plants that last for more than two years – Not all are long lived – Some can last for thousands of years • Perennials - Plants that persist for many growing seasons. Generally the top portion of the plant dies back each winter and regrows the following spring from the same root system Herbaceous Perennials • Do not have a permanent hard (wooden) structure above ground – Foliage dies back under adverse condition – Very common – e.g. Docks, dandelions, – Many bulbs e.g. Ransomes, daffodils, bluebells • Some have a permanent soft structure above ground – e.g. Grasses, Woody Perennials • Have a permanent hard (wooden) structure above ground • Trees and shrubs – Can be single stemmed (trees) – Can be multi-stemmed (shrubs) • Can evergreen or deciduous – Evergreen – keep leaves all year round – Deciduous – shed leaves in adverse conditions • Trees and shrub have significant influence on our lives – Fuel, material, shelter, visual, food Germination What is required for germination to take place? • • • • Water Temperature Oxygen Seed to be viable • Dicot seed germination Monocot Hypogeal germination - hypo means below • In hypogeal seed germination, the cotyledons stay below the soil. The stem above the cotyledons, epicotyls, elongates and raises the plumule above the ground Epigeal germination - epi means above while hypo means below • In epigeal seed germination, the cotyledons serve as leaves and, together with the shoot, are pushed above the ground. Time lapse seeds • http://www.bbc.co.uk/learningzone/clips/anintroduction-to-seed-germination-andgrowth/63.html • http://www.youtube.com/watch?v=d26AhcKe EbE • Beech tree • http://www.youtube.com/watch?v=rswfAzmL zHA • http://www.bbc.co.uk/learningzone/clips/anintroduction-to-seed-germination-andgrowth/63.html • Seed germination occurs when the requirements have been met already. • These include proper temperature, oxygen, adequate water, a preceding dormant period, and correct photoperiod (the duration of daily exposure to light of an organism) • Presence of adequate water is very vital since during the process of seed germination, the embryo absorbs water, enabling it to resume growth and development. • Water is always a requirement in order for vigorous metabolism to take place. Once metabolism starts, the root is then developed, allowing the seed to find its own water. • The plumule develops and from it grows a shoot that allows the seed to absorb light. • Air plays a vital role in germination. Seed should get ample oxygen for germination. Inadequate oxygen supply will inhibit the germination, leading to seed death. Thus seed should not be buried deep into the soil, as it hampers the air supply and hence the germination. • Remember breaking seed dormancy in tree seeds? What was needed? What do the experiments show about the requirements for germination? Pyrogallol (absorbs oxygen) Oxygen present Oxygen present No light Oxygen present Oxygen present No oxygen moist 4oC A dry moist moist moist Warm B Warm C Warm D Warm E Temperature: • Different species of seeds require different temperature for germination. • Most of the seeds require a temperature little above the room temperature to germinate. Some seeds require cool climate for germination. • Exposing the seeds above or below these temperatures prevent the seeds from germination. Hence some seeds germinate during summer season and some during winter. • Conducive temperature helps to break the dormancy period of a seed. Vegetable seeds germinate slightly above the room temperature. Radish and spinach seeds prefer winter season for germination. • The outer covering of a seed is called the testa. It is usually hard and protects the softer parts of the seed. There is a tiny hole in the testa called the micropyle. When the seed is ready to germinate, water is taken in through the micropyle. The first root, called the radicle, will grow out of the seed through the micropyle. On the surface of the testa there is a scar which shows the place where the seed was attached to the fruit. • Inside both peas and beans there are two large food-storing structures called cotyledons • The food contained in the cotyledons gives energy to the embryo when the seed germinates. The embryo is found between the two cotyledons. It is made up of two parts; the radicle, or first root, which points toward the micropyle, and the plumule, or first shoot. Radicle is first to emerge. Plumule afterwards. • Primary meristems – found at tips of shoot and root. ‘Apical meristems’ • Tissues from these are known as ‘primary tissues’ • Secondary meristems – on sides of stems and roots‘lateral meristems’. Vascular and cork cambium for exam[ple. • Apical meristems allow growth in length, lateral meristems allow growth in thickness. • Secondary thickening – what was this? • Apical meristems are the site of primary growth in a plant, and can be found at the root and shoot tips • New unspecialised cells become available at the meristems, the site of mitosis • These cells become elongated and undergo vacuolation • They become specialised (differentiated) to perform a particular function • They then form part of a permanent tissue which performs a particular role within the plant • The cycle continues for growth and regeneration purposes • Lateral meristems are responsible for secondary thickening, which is required by perennial plants that grow year after year, and need the structural support to continue doing so. • Secondary thickening • Where have we seen this occurring? • A bud is a ‘condensed shoot’. Its stem is very short and its leaves overlap. • the outermost leaves are the bud scales. Meristem: • Meristematic plant tissue is characterized by frequent cell division, producing cells that become differentiated into specialized tissues (a) Shoot apical meristem (b) Leaf primordial (c) Axillary bud primordium (d) leaf (e) Stem tissue (cambium) has two types: 1. Vascular cambium that produces secondary xylem and phloem 2. Cork cambium produces some of the bark layer of a stem Growth • A. root cap • B. zone of cell division • C. zone of elongation • D. zone of differentiation • E. apical meristem • F. ground meristem • G. procambium • H. protoderm Types of bud • Terminal buds –ends of main shoots or branches. Add to length • There are also buds in the axils of the leaves – axillary or lateral buds. Make new branches • Either may produce a flower instaed of , or as well as, a leafy shoot. Tropisms • plants make a chemical in their tips that stimulates growth. • Usually this growth hormone spreads evenly around the stem, so the stem grows upward. But if light hits the plant from one direction, the growth hormone is distributed unevenly. In fact, more auxin flows down the "shady" side, causing it to grow faster than the "sunny" side of the plant. • This unequal concentration of auxin results in differential growth and the curving of the plant toward the light source. • the stem bends toward the light because the side of the stem that is away from the light grows faster than the side of the stem facing the light. • This uneven growth pushes the tip toward the light source. • The growth of new plant cells is stimulated by the plant chemical auxin. • The growing stem responds to light from a certain direction by decreasing the amount of auxin on the side of the stem facing the light, relative to the opposite "shady" side