Evolutionary trends of land plants. 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. Increasing independence from water: Land plants became increasingly independent of liquid water by various adaptations to avoid water loss, etc.
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. The haploid phase is dominant in simpler plants; the diploid phase is dominant in higher plants. Simpler plants are very dependent on liquid water; higher plants are less dependent on liquid water.
Higher plants produce seeds a life stage adapted to dispersal and flowers efficient reproduction. 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. 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. 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. 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 seed plants protected the embryonic sporophyte from drying up by encasing it in a tough waterproof seed coat. The evolution of the seed is as profound a step as the evolution of the shelled egg in reptiles. Just as the evolution of the amniotic egg enabled reptiles to become the first truly terrestrial vertebrates, to break that final link with their aquatic heritage, so did the evolution of the seed allow plants to escape the limitation of growing in very moist environments.
These gymnosperms soon became the dominant plants. The Mesozoic is sometimes called the Age of Cycads. But their success was short-lived. During the mid to late Mesozoic, the first flowering plants or angiosperms appeared. They rapidly dominated the more primitive gymnosperms, and are the dominant plants on Earth today.
These waves of competition are typical of the history of life. The survivors are relegated to scattered populations in restricted habitats, where they live in the shadows of their successful competitors. Among the gymnosperms, only the conifers are major competitors with flowering plants. Having evolved in a dryer, cooler climate, conifers are better adapted to dry or cool habitats, and dominate forests in northern latitudes, at high elevations, and on sandy soils.
Today we will examine both gymnosperms and angiosperms, and compare their complex life cycles. The trend toward a dominant sporophyte stage is now complete. The gametophytes of seed plants are microscopic. The female gametophyte consists of a handful of cells buried in the tissues of the sporophyte. The male gametophyte, the pollen grain , has a brief free-living stage while it is carried from plant to plant by wind, water, or animals.
No longer relying on flagellated sperm, and with their developing embryos protected from desiccation, seed plants break the last link with their aquatic ancestors. The first seed plants evolved relatively early on, in the late Devonian. By the end of the Paleozoic they were competitive enough to replace the club mosses, horsetails, and whisk ferns, and become the dominant vegetation of the Mesozoic, the era of the dinosaurs.
By the end of the Mesozoic, they too would be swept aside by the newly evolved angiosperms, the flowering plants. There are only living species of gymnosperms, a pale remnant of a once diverse and dominant race. Living gymnosperms are a diverse group of plants, most of which bear their sporangia in large, prominent strobili or cones. These strobili are similar to those of lycopsids and horsetails.
Strobili consist of a shortened stem with several modified leaves sporophylls that bear sporangia. Like all seed plants, gymnosperms are heterosporous. The sporangia that generate the male microspores and female megaspores are usually borne on separate cones. Male cones staminate cones are typically much smaller than female cones ovulate cones. Sporophylls that bear microsporangia are called microsporophylls. Sporophylls that bear macrosporangia are called macrosporophylls.
The pine life cycle is typical of gymnosperms, and is described in detail below. Cycads have very thick leaves, that look like very tough versions of fern fronds. These palm-like plants have unbranched stems, with a terminal crown of leaves. These leaves are incredibly well defended with sharp tips and with complex secondary compounds, including potent neurotoxins and carcinogenic compounds.
They reached their peak during the Mesozoic, with species reaching from feet. A giant cycad today might reach feet max. They are unisexual or dioecious, having separate male and female plants. Dioecious means two houses, vs. Only one genus of cycad Zamia is native to North America. The Seminoles ate the starchy roots of Zamia pumila , found in southern Florida. In India, Japan, and Sri Lanka, sago flour is often made from cycad stems it is also made from real palms, which are angiosperms.
Cycads are widely grown as ornamental landscape plants. Cycads also enrich the fertility of barren soil, because they are symbiotic with nitrogen-fixing cyanobacteria.
Cycads are extremely slow growing, and can live 1, years or more. They are wind pollinated, a strategy which requires immense amounts of airborne pollen. A few may have been pollinated by beetles attracted to the edible pollen grains. This may be the humble beginnings of the complex animal pollination developed by flowering plants. The pollen sacs and ovules are born on scalelike sporophylls in compact cones. Unlike pine cones, the cones of cycads are often very large in relation to the plant.
Ginkgo trees are commonly seen in cities today. They are attractive shade trees, reaching feet or more, with beautiful yellow foliage in the Fall. They are very resistant to air pollution and insects. That the sole remaining species did not join its brethren in extinction we owe to the ancient Chinese and Japanese, who cultivated it in their temple gardens for centuries. Their may no longer be a single living wild tree.
It is a popular tree for bonsai, because the leaves will readily miniaturize, and the branches are easy to shape. The species name biloba comes from the two distinct lobes of its fan-shaped leaves, very different from the straplike or needle shaped leaves of other gymnosperms.
The common name maidenhair tree comes from the similarity of ginkgo leaves to fronds of the maidenhair fern. Ginkgos and cycads show a transitional stage between the primitive ferns and the more advanced conifers and flowering plants.
They have flagellated sperm, but the male gametophyte grows a pollen tube, a long filament through which the sperm can safely swim to the egg. The pollen grains of other seed plants grow similar tubes. The megasporangia, which contains the eggs, form tiny female strobili on the tips of special branches on the female tree. The microsporangia, which produce the pollen grains, are in male strobili that hang down like little pine cones on the male tree. The seed that forms on the female trees is covered with a thick fleshy coat which makes the seed look like a little fruit which it is technically not.
So be very careful if you plant one of these wonderful trees and select a male tree!! Although in fairness to the female tree, its seed is prized in China as a source of medicinal drugs. This odd little group of gymnosperms are mainly xerophytes, plants that are adapted to dry conditions. They share a close common ancestor with flowering plants. Each genera has some species that produce nectar, and attract insects. It was recently discovered that double fertilization, a trait we thought was unique to flowering plants, also occurs in Ephedra , one of the three surviving genera of gnetophytes.
Ephedra , incidentally is the natural source of the alkaloid ephedrin, used to treat hay fever, sinus headaches, and asthma. Its medicinal properties have been known for at least 5, years! Most gnetophytes are stem plants, like Ephedra, branched photosynthetic stems with no leaves.
Gnetum has leaves like those of modern flowers. But the third genus, Welwitschia , is one of the strangest plants on earth. Welwitschia really looks like something out a science fiction novel. It grows in the deserts of southwestern Africa. Most of the plant is deep underground, with a root stretching down to the water table. The top appears above the soil as a squat cup- shaped stem with two strap-shaped leaves.
These are the only leaves the plant will ever grow, and they may live a hundred years or more and reach several meters, usually torn into strips. Male or female strobili grow from the margins of the upper stem. Division Coniferophyta - sp. The conifers are the largest and most successful group of living gymnosperms. Many of our familiar forest trees are conifers, including pines, spruces, firs, hemlocks, yews, redwoods and cypress trees.
They are an ancient group, dating back mya. They evolved during the Permian, toward the end of the Paleozoic, at a time when the climate was very cool and dry. Their special water conducting cells, called tracheids, allowed them to thrive in these climates and these same adaptations let them continue to dominate in colder and dryer environments today, such as northern latitudes, mountain slopes, and sandy soils.
Because they are superior competitors in such habitats even today, they are the only Division of gymnosperms to successfully compete with the flowering plants. Most conifers are evergreens, with the larch and the bald cypress being notable exceptions.
Their needle-shaped leaves are also an adaptation to conserve water. Needles usually occur in small bundles, each bundle emerging from a base that is actually a greatly truncated branch. Conifers have tremendous economic importance, as a source of timber and for byproducts such as pitch, tar, turpentine, and amber and other resins. Millions are sold each year as Christmas trees. All conifers produce cone shaped strobili, both male cones often called pollen cones and female cones often called seed cones or ovulate cones.
Both male and female cones are usually produced on the same tree, but not at the same time, so the trees do not fertilize themselves. Female cones are large and conspicuous, with thick woody scales. Seed cones can persist on the tree for several years after fertilization. A few species, like junipers and the locally common podocarpus front of Richardson , have seeds that are covered with a fleshy coating, and resemble small berries.
They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent phylum in the coniferous biome or taiga , where the evergreen conifers have a selective advantage in cold and dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to take advantage of the first sunny days of spring.
One disadvantage is that conifers are more susceptible than deciduous trees to leaf infestations because most conifers do not lose their leaves all at once. They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring. The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which reduced male and female gametophytes reside. All gymnosperms are heterosporous. The male and female reproductive organs can form in cones or strobili.
The life cycle of a conifer will serve as our example of reproduction in gymnosperms. Therefore, they are monoecious plants.
Like all gymnosperms, pines are heterosporous and generate two different types of spores male microspores and female megaspores. Male and female spores develop in different strobili, with small male cones and larger female cones. Each pollen grain consists of just a few haploid cells enclosed in a tough wall reinforced with sporopollenin. In the spring, large amounts of yellow pollen are released and carried by the wind.
Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly, and the generative cell in the pollen grain produces two haploid sperm or generative nuclei by mitosis. At fertilization, one of the haploid sperm nuclei will unite with the haploid nucleus of an egg cell. Female cones, or ovulate cones , contain two ovules per scale.
Each ovule has a narrow passage that opens near the base of the sporophyll. This passage is the micropyle, through which a pollen tube will later grow.
One megaspore mother cell, or megasporocyte , undergoes meiosis in each ovule. Three of the four cells break down; only a single surviving cell will develop into a female multicellular gametophyte, which encloses archegonia an archegonium is a reproductive organ that contains a single large egg.
As the female gametophyte begins to develop, a sticky pollination drop traps windblown pollen grains near the opening of the micropyle. A pollen tube is formed and grows toward the developing gametophyte. One of the generative or sperm nuclei from the pollen tube will enter the egg and fuse with the egg nucleus as the egg matures. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant.
Although several eggs may be formed and even fertilized, there is usually a single surviving embryo in each ovule. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination.
The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte tissue that will provide nutrients, and the embryo itself. Figure illustrates the life cycle of a conifer. The sporophyte 2 n phase is the longest phase in the life of a gymnosperm. The gametophytes 1 n —produced by microspores and megaspores—are reduced in size. It may take more than a year between pollination and fertilization while the pollen tube grows towards the growing female gametophyte 1 n , which develops from a single megaspore.
The slow growth of the pollen tube allows the female gametophyte time to produce eggs 1 n. The diploid zygote forms after the pollen tube has finished forming, so that the male generative nuclei can fuse with the female gametophyte.
Modern gymnosperms are classified into four phyla. Coniferophyta, Cycadophyta, and Ginkgophyta are similar in their pattern of seed development and also in their production of secondary cambium cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation.
However, the three phyla are not closely related phylogenetically to each other. Gnetophyta are considered the closest group to angiosperms because they produce true xylem tissue, with vessels as well as the tracheids found in the rest of the gymnosperms.
It is possible that vessel elements arose independently in the two groups.
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