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Chapter 24

Chapter 24 Detailed Notes

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Wilfrid Laurier University
Tristan Long

Chapter 24: Plants 24.1 Defining Characteristics of Land Plants - Not all plants photosynthesize to obtain energy. - Land plants are mainly all photoautotrophs (use light as a source of energy). - Cells have cell walls consisting of cellulose.All are sessile (unable to move) - Different life cycles known as alternation of generations. - Plants have an alteration of generations. They have 2 multicellular stages in their life, diploid and haploid. The diploid generation produces sporophyte and the haploid generation produces gametophyte. - Final defining feature of land plants is that the embryo is retained inside the gametophyte tissue o The reason for retention of embryos in parental tissue and for the rather complex life cycle will become clearer after we’ve looked at the evolution of plants and their transition onto land 24.2 The Transition to Life on Land: - Idea that evolution involves organisms “trying” to adapt or that natural selection gives organisms what they need to survive is one of the major misconceptions about evolution - Natural selection cant see what a species needs, and organisms cant try to adapt 24.2a Early Biochemical and StructuralAdaptations Enhanced Plant Survival on Land - Poikilohydric plants – Plants that had little control over their internal water content. These plants are drought tolerators which can withstand dry and moist environments - Stomata: pores in the cuticle covered surfaces that open and close to regulate water loss, and the main root of CO2 into leaves o When they need to open, pumps potassium, allowing change in gradient and opens Overview of the ‘Plant Body’  Almost all plants are photosynthetic autotrophs  Shoot system above ground  Photosynthetic leaves, stems  Root system below ground  Nonphotosynthetic roots  Plant bodies are dendritic (spreading and branched in form) The Plant Cell: General Properties  Primary cell wall (surrounding plasma membrane and cell contents)  Cellulose fibres in matrix of hemicellulose  Rigid but flexible  Large vacuole (30-80% of internal volume) Tonoplast membrane  Used for storage and to maintain turgor pressure against the cell wall  Plasmodesmata  Cytoplasmic connections between adjacent cells Plant Cell Wall Structure • Cellulose is an branched polymer made of 7,000 - 15,000 glucose molecules per polymer (most abundant organic substance on Earth!) • Hemicellulose is a branching polymer consists of chains of 500-3,000 sugar units o Give rigidity and flexibility • Pectins particularly abundant in the non-woody parts of terrestrial plants o More pectin = more elastic 24.2c Lignified Water-Conducting Cells Provided Strength and Support for Plants to Grow Upright - Land plants were able to synthesize lignin, a polymer of phenylpropanoids (absorb UV), which was deposited in the cell walls –particularly water-conducting cells- providing support and rigidity to those tissues allowing the plants to grow upright. These water conducting cells make up a tissue called xylem - Vascular Plants – Contain vascular tissue (xylem, which helps plants stand upright.And Phloem, which conducts sugars through the plant body). - Also have apical meristems – regions of constantly dividing cells near the tips of shoots and roots. 24.2d Root and Shoot Systems WereAdaptations for Nutrition and Support - As plants moved to land they portrayed a less complex haploid stage, and the opposite with the diploid sporophyte stage. Diploid became more dominant because if there was a mutation in haploid, it won’t be fixed as easily as it is in diploid - Roots and stems: structures that are fundamental adaptations for absorbing nutrients from soil and support o Roots: anchoring structures that also absorb water and nutrients in association with mycorrhizal fungi  Root systems are cylindrical absorptive structures with a large surface area that favours the rapid uptake of soil water and dissolved mineral ions o Shoot systems have stems and leaves that arise from apical meristems and function in the absorption of light energy from the sun and CO2 in the air o Leaves  Microphylls: narrow leaves with only one vein or strand of vascular tissue  Megaphylls: broader leaves with multiple veins - Structure is specialized for underground growth o Absorb water and dissolved minerals o Conduct water and minerals to aerial plant parts o Anchor and support aboveground parts o Store nutrients produced by photosynthesis - Exodermis o Outer layer of root cortex - Endodermis - Inner layer of root cortex - Pericycle o Between stele and endodermis o Can function as meristem 24.2e In the Plant Life Cycle, the Diploid Phase Became Dominant - The sporophyte generation begins after fertilization, when the zygote divides by mitosis to produce a multicellular diploid organism. Body will eventually develop capsules called sporangia which produce spores by meiosis 24.2f Some Vascular Plants Evolved Separate Male and Female Gametophytes - Homosporous plants – Plants that make only one type of spore. (acts as a bisexual) o Produces sperm and eggs o Some have ways to produce sex organs on different gametophytes or to otherwise prevent self-fertilization - Heterosporous plants – produce 2 types of spores (smaller = mircospores male) (bigger=megaspores female) o 24.3 Bryophytes: Nonvascular Land Plants - Bryophytes: liverworts, hornworts and mosses both important for colonization of bare land. Helps build soil on bare rock and stabilizes soil surface. - Bryophytes do not contain lignin so they can’t live on land. Usually are very small and live near waters. But they are also poikilohydric which allows them to withstand inhospitable places. - They lack vascular tissue. Sperm must swim between plants to the egg. Definitely adapted to land - They produce flagellated sperm that must swim through water to reach an egg - no vascular tissues for movement of water and nutrients- in wet habitats o lack xylem and phloem - no roots, only filamentous rhizoids (anchors) - produces gamete inside protective organ called gametangium in which eggs form are like flask shaped structures called atheridia - are not a monophyletic group. They evolved as separate lineages in parallel with vascular plants 24.3a Liverworts May Have Been The First Land Plants - Liverworts are the first Land plants - Make up the phylum Hepatophyta - Have rzhizoids, not stomata - Gemmae are small cell masses that form in cuplike growths on a thallas, which can help reproduce asexually. They can grow into new thalli when rainwater touches them out of the
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