Plant Biology: Nastic Movements, Reproduction, Pollination, Nutrition
The Nasties: Nastic Movements
Nastic movements in plants are responses to stimuli, determined by the plant’s organ structure. These reactions are rapid and reversible, triggered by various stimuli like light, temperature, or touch. Examples include photonasty (response to light) and thigmonasty (response to touch).
Reproduction in Bryophytes
Bryophytes are seedless, nonvascular plants that require a moist environment for growth and reproduction. The dominant phase is the gametophyte (sexual phase), with the sporophyte growing on it. The gametophyte has a false stem formed by phyllids and rhizoids. Male gametes (anterozoids) are biflagellate and form in a special organ. Female gametes (oospheres) form in bottle-shaped archegonia. Fertilization occurs in the presence of water. The resulting diploid zygote develops into a multicellular sporophyte embryo through mitosis. The sporophyte becomes a brown structure with a foot, a stalk, and a capsule containing spore mother cells. These cells undergo meiosis to produce haploid spores, which are released and germinate in favorable conditions, forming a filamentous protonema, from which the gametophyte develops.
Reproduction in Pteridophytes
Pteridophytes are seedless vascular plants with roots, stems, and leaves. The vascular system transports water, minerals, and nutrients. The sporophyte consists of a rhizome (stem) and leaves called fronds. Sporangia develop on fronds in groups called sori, where meiosis occurs to form spores. Spores are released and germinate in suitable conditions, forming a heart-shaped prothallus (gametophyte) with rhizoids for attachment.
Pollination
Pollination is the transfer of pollen grains from pollen sacs to stigmata containing oospheres. Self-pollination occurs between flowers of the same plant. Cross-pollination occurs between flowers of different plants of the same species. Anemophilous plants rely on wind for pollination, producing large quantities of lightweight pollen. Zoophilous plants rely on animals (insects, birds) for pollination, often having developed specific shapes, flavors, and showy petals to attract pollinators.
Nutrition in Cormophytes
Cormophytes are evolved plants with specialized structures for nutrient uptake and transport. Water and mineral absorption occurs through roots, forming crude sap transported through the xylem. This transport is aided by transpiration (water evaporation through stomata), cohesion of water molecules, and root pressure. Photosynthesis occurs in leaves, using CO2 entering through stomata to produce organic matter. This organic matter forms elaborated sap, transported through the phloem for cellular respiration and growth.
Gas Exchange
- Stomata: Openings in the epidermis, abundant on leaf undersides, allow CO2 and O2 diffusion. Stomatal opening and closing are controlled by turgor changes in guard cells, influenced by ion concentration, light, CO2 levels, hormones, temperature, and humidity.
- Lenticels: Natural openings in the suber layer, allowing gas exchange between internal tissues and the atmosphere.
- Root hairs: Extensions of epidermal cells, facilitating the absorption of dissolved gases from soil water.