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Protistans, plants Updated: |
Topics for March 6
Evolutionary trends in plants
Gametophyte and sprorophyte trends.
Independence from liquid water trend.
Mosses and other bryophytes
Gametophyte most conspicuous
Waxy cuticle, protected gamete development
Ferns and other seedless vascular plants
Sporophyte most conspicuous
Vascular tissue conducts water
Gymnosperms: Conifers and their relatives.
Structure and function of cones: the reproductive structures.
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.
Green algae: possible ancestors of the higher plants.
Green algae share a number of biochemical traits with higher plants, including the chemical form of chlorophyll.
Like many other protista, green algae are capable of sexual reproduction. Example: Chlamydomonas. Note that the dominant life phase is the haploid phase. Most of the time, haploid cells reproduce asexually.
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).
The bryophytes: Simple land plants.
Examples of bryophytes: mosses, liverworts, hornworts.
Mosses are partially independent of liquid water. They can grow in damp terrestrial environments.
A critical life stage is dependent on liquid water: sperm must swim to the egg producing structures.
Life cycle of a moss. (figure 19.4).
The most conspicuous stage of moss is the gametophyte: A haploid stage.
Male gametophytes produce sperm, which must swim to the female gametophyte.
Female gametophytes produce eggs, which are retained in special structures on the female gametophyte.
The fertilized egg is a zygote, dependent on the female gametophyte.
Life cycle of a moss (cont.)
The zygote develops as a multicellular sporophyte, but remains dependent on the female gametophyte.
Meiosis takes place in a specialized structure on the sporophyte (the sporangium), producing spores (not gametes!).
The spores divide and produce male or female gametophytes.
The sporophyte of mosses: a dependent stage.
The sporophyte phase of mosses remains attached to the female gametophyte.
In contrast to higher plants, the sporophyte of mosses has a brief existence (as part of sexual reproduction).
The most conspicuous phase of mosses is the gametophyte phase.
Adaptation to life on land.
Mosses are able to grow on land, not in the water (algae grow only in the water).
Adaptations to land include:
1. A waxy cuticle that reduces evaporation.
2. A cellular jacket around the cells that produce sperm and egg (protecting them).
3. A large gametophyte (haploid) stage that supports the sporophyte stage.
What keeps mosses tied to damp habitats?
Mosses produce sperm that are dependent on liquid water. With out the presence of liquid water, the sperm are unable to reach the egg.
Because of this feature of the life cycle, mosses are tied to habitats in which there is some liquid water at least part of the time. (Rain drops or water film are sufficient.)
Seedless vascular plants.
The seedless vascular plants (Ferns and their relatives) have an independent long-lived sporophyte stage. The gametophyte stage is smaller, but independent.
The sporophyte possess a complex of vascular tissue: vascular tissue permits larger size because water can be translocated from the soil to the plant.
Vascular tissue.
Higher plants have two types of vascular tissue: xylem and phloem. (see p380 ff)
Xylem: Xylem cells are tubes that conduct water from the soil to other parts of the plant. Most xylem cells are actually dead, but their structure continues to function.
Phloem: Phloem cells transport sugars and other foodstuff throughout the plant. Phloem cells remain alive.
Seedless vascular plants: a diverse array of plant species.
Ferns: the best known seedless vascular plants.
Lycophytes: Living species are small and inconspicuous, but their ancestors were the dominant plants of the Carboniferous Period.
Horsetails: Only a few surviving species, but like the Lycophytes, they were once dominant land plants.
Life cycle of a fern (see figure 19.8)
The life cycle of a fern includes a free-living gametophyte stage. It is small and inconspicuous, and lacks vascular tissue.
The zygote begins life attached to the gametophyte, but soon develops into a large and independent sporophyte. The sporophyte has vascular tissue and may attain a very large size.
The gametophyte of ferns.
The gametophyte is haploid.
Both sperm and eggs are produced on the same plant (by mitosis!)
The gametophyte begins life with the germination of a haploid spore. The spores are an effective dispersal phase of ferns.
The sporophyte of ferns.
The large and familiar phase of ferns is the sporophyte. It is the diploid phase.
The sporophyte has vascular tissue, and can conduct water from the soil to other parts of the plant.
The sporophyte produces haploid spores by meiosis. ( Spores are the dusty brown material on the underside of the "leaf".)
Mitosis and meiosis in the life cycle of a fern.
A haploid spore germinates and begins to divide by mitosis to form the small multicellular gametophyte stage.
The gametophyte stage produces gametes (by mitosis) which fuse to form a zygote.
The zygote divides by mitosis to form the large multicellular sporophyte stage.
Mitosis and meiosis in Ferns (continued).
The sporophyte produces haploid spores by meiosis. The spores are an effective dispersal phase in the life cycle of the fern.
Note that meiosis produces spores, not gametes. A spores germinates and grows into an independent gametophyte stage. (Meiosis does not produce gametes in these plants!)
Gymnosperms: Conifers and their relatives.
Conifers are woody trees and shrubs with needlelike 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.
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.
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