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Gymnosperms and Angiosperms Updated: |
Topics for March 9
Evolutionary trends in plants
Gametophyte and sprorophyte trends.
Independence from liquid water trend.
Gymnosperms: Conifers and their relatives.
Vascular Tissues, Seeds, Pollen
Angiosperms: The flowering plant.
The flower: An efficient means of reproduction.
Evolutionary trends of land plants.
1. Shift from gametophyte to sporophyte: In the simplest land plants, the gametophyte stage is the most conspicuous. In higher plants, the gametophyte exists only as part of the reproductive structures.
2. Increasing independence from water: Land plants became increasingly independent of liquid water by various adaptations to avoid water loss, etc.
Life cycle of plants.
Plants alternate between a haploid phase (the gametophyte) and a diploid phase (the sprorophyte).
The gametophyte is dominant in the simpler non-vascular plants.
The sporophyte is dominant in higher plants. (see figure 19.2).
Evolutionary trends in plants.
1. The haploid phase is dominant in simpler plants; the diploid phase is dominant in higher plants.
2. Simpler plants are very dependent on liquid water; higher plants are less dependent on liquid water.
3. Higher plants produce seeds (a life stage adapted to dispersal) and flowers (efficient reproduction).
Gymnosperms: Conifers and their relatives.
Conifers are woody trees and shrubs with needle-like leaves.
Conifers have cones (hence their name).
Cones are the reproductive structures of the conifers: Cones are diploid tissue produced by the dominant sporophyte stage.
The haploid gametophyte stage develops and produces gametes inside the cone.
Seeds: an important evolutionary advance in the conifers.
Cones produce seeds. The seeds develop on "exposed" parts of the sporophyte, hence the name "Gymnosperm" or "naked seed. Seeds are effective propagules for dispersing the population.
Seeds are very resistant stages, and may persist for years, maintaining the population.
Pollen: An important evolutionary advance. (figure 19.9)
Gymnosperms (and flowering plants as well) produce pollen as a package for the dispersal of sperm. Gymnosperms disperse pollen on wind currents.
Pollen grains are male gametophytes. They transport the sperm cells (inside the pollen grain) by wind or insects: no liquid water needed.
Cones: male and female reproductive structures.
Female cones are diploid tissue produced by the dominant sporophyte stage.
Meioses occurs inside the female cone to produce megaspores.
Megaspores develop, while still attached to the cone, into female gametophytes.
Inside the female gametophyte, eggs are produced inside a special structure, the ovule.
Cones: male and female
Male cones are diploid tissue attached to the dominant sporophyte.
Inside male cones, meiosis takes place to produce microspores.
Microspores develop into male gametophytes: pollen grains.
The pollen grains contain the male gamete: sperm. Pollen grains are very durable.
Conifers: independent from liquid water.
Conifers possess several features which allow them to occupy habitats that have only soil water.
Seeds: resistant dispersal propagules.
Pollen: male gametophytes that effectively disperse sperm through the air.
Vascular tissues that distribute water and food throughout the plant.
Sporophyte and gametophyte of gymnosperms. (see figure 19.9, p314)
All the conspicuous parts of the plant belong to the sporophyte generation: The plant and the cones are diploid.
The gametophyte is confined to specialized parts of the cone.
The female gametophyte remains inside the female cone.
The male gametophyte is released, as pollen, from the male cones.
Cones: The reproductive structures of gymnosperms.
Female cones are larger. Meiosis produces megaspores (haploid), which will develop into female gametophytes, which are retained in the female cone.
One of the cells of each female gametophyte develops as an egg.
After fertilization of the egg, the zygote and some of the surrounding diploid tissue from the sporophyte develop as a seed.
Seeds: specialized dispersal stage of the life cycle.
The seeds produced by the sporophyte contain:
A seed coat of diploid tissue from the original diploid parent.
A developing diploid embryo inside which began as a zygote after fertilization of the haploid egg by a haploid sperm. The sperm is likely to have come from a different plant.
Thus, two different diploid layers.
Male cones and pollen.
Gymnosperms produce male cones. Meiosis inside the male cones produces microspores.
Microspores develop into pollen: the male gametophyte.
One of the cells of the gametophyte develops into a sperm: the remainder of the gametophyte helps deliver the sperm.
How efficient is pollen as a means to deliver sperm?
Not very efficient, because the wind is fickle. Conifers must produce vast quantities of pollen.
Very efficient, because pollen is entirely independent of liquid water. No liquid water is needed for the sperm to reach the egg and complete the life cycle.
Summary of the two trends in gymnosperms:
The gametophyte is much reduced. It exists as a haploid female gametophyte inside the female cones and the haploid pollen which delivers the sperm. The rest is sporophyte.
The life cycle is fully independent of liquid water. No water is needed for the pollen to reach the female cone and deliver the sperm. (The sporophyte of course needs soil water for its metabolism.)
Types of gymnosperms.
The most important gymnosperms are the Conifers: firs, spruce, cedar, etc.
Cycads: Once very important, but today just a few survivors. They have massive cones and superficially resemble palms.
Ginkgoes: Only one species left. The male ginkgo is a popular ornamental plant.
Gnetophytes: Primarily desert plants, including the California Ephedra sp.
Angiosperms: The dominant land plants today.
The defining characteristic of the angiosperms is the flower. Only angiosperms produce flowers.
Flowers are an efficient targeting system for efficient delivery of pollen: The flower recruits an individual animal type as its own special pollen delivery system.
Flower structure.
The flower is primarily diploid tissue produced by the sporophyte.
Inside particular parts of the flower, meiosis produces megaspores, which develop into female gametophytes.
Inside a different part of the flower, meiosis produces microspores, which develop into the male gametophyte: pollen.
The megaspore and the female gametophyte. ("Double fertilization".)
Megaspores are produced by meiosis inside a structure at the base of the flower known as the ovary. After a series of intricate nuclear divisions, two functional gametophyte cells remain: the egg and the endosperm producing cell.
The fertilized egg becomes the embryo.
The endosperm results from a second fertilization. Endosperm "feeds" the seed.
Microspores and the male gametophyte.
As in the gymnosperms, meiosis produces microspores which develop into the male gametophyte: pollen.
Pollen is produced at the tip of specialized structures near the top of the flower. The pollen is well positioned to be picked up by animals that visit the flower.
The structure of the flower is keyed to the behavior of the animal pollinator.
Flower form and animal pollinators.
Many different animal species pollinate specific flowers.
Examples:
Insects: bees, flies, ants, moths, butterflies.
Birds: honeycreepers, humming birds.
Mammals: bats especially, but many other mammals.
Flowers, and their pollinators, share common adaptations.
Bright colored flowers attract bees, and reward the bees with nectar. Bees can see ultraviolet colors that we cannot, and flowers pollinated by bees often present such colors.
By traveling from one flower to another, bees carry pollen from the flower of one plant to the next (but the same species of plant!)
Flowers and humming birds.
Humming birds specialize on a diet of flower nectar.
Humming birds are attracted to red flowers with long, deep corollas. (Most insects cannot reach the nectar in such flowers.)
Humming birds also travel from plant to plant, transporting pollen while the gather nectar.
Not all flowers are pretty!
Some plants produce flowers that are attractive to flies.
Flowers attract flies by mimicking carrion or feces.
Flowers that attract flies may smell very strong--of carrion or feces!
Flowers and animals: coevolution and pollination.
Plants and animals share a relationship: nectar for pollination.
Individual species of plants often have a special relationship with an individual species of animal pollinator.
Such reciprocity is called "mutualism" or "symbiosis".
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