The pollen is magnified 1, times. Bryophyte and fern spores are haploid cells dependent on moisture for rapid development of multicellular gametophytes. In the seed plants, the female gametophyte consists of just a few cells: the egg and some supportive cells, including the endosperm-producing cell that will support the growth of the embryo. After fertilization of the egg, the diploid zygote produces an embryo that will grow into the sporophyte when the seed germinates. Storage tissue to sustain growth of the embryo and a protective coat give seeds their superior evolutionary advantage.
Several layers of hardened tissue prevent desiccation, and free the embryo from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy—induced by desiccation and the hormone abscisic acid —until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.
Pollen grains Figure 3 are male gametophytes containing just a few cells and are distributed by wind, water, or an animal pollinator. The whole structure is protected from desiccation and can reach the female organs without depending on water.
After reaching a female gametophyte, the pollen grain grows a tube that will deliver a male nucleus to the egg cell. The sperm of modern gymnosperms and all angiosperms lack flagella, but in cycads, Ginkgo , and other primitive gymnosperms, the sperm are still motile, and use flagella to swim to the female gamete; however, they are delivered to the female gametophyte enclosed in a pollen grain. The pollen grows or is taken into a fertilization chamber, where the motile sperm are released and swim a short distance to an egg.
The roughly million years between the appearance of the gymnosperms and the flowering plants gives us some appreciation for the evolutionary experimentation that ultimately produced flowers and fruit. Fossil evidence Figure 4 indicates that flowering plants first appeared about million years ago in the Lower Cretaceous late in the Mesozoic era , and were rapidly diversifying by about million years ago in the Middle Cretaceous. Earlier traces of angiosperms are scarce.
Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, suggesting that pollinating insects played a key role in the evolution of flowering plants.
Figure 4. This leaf imprint shows a Ficus speciosissima , an angiosperm that flourished during the Cretaceous period.
Lee, USGS. New data in comparative genomics and paleobotany the study of ancient plants have shed some light on the evolution of angiosperms. Although the angiosperms appeared after the gymnosperms, they are probably not derived from gymnosperm ancestors.
Instead, the angiosperms form a sister clade a species and its descendents that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. There is no current consensus on the origin of the angiosperms. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were related to the ancestors of tropical grasses.
Both views draw support from cladistics, and the so-called woody magnoliid hypothesis —which proposes that the early ancestors of angiosperms were shrubs like modern magnolia—also offers molecular biological evidence. The most primitive living angiosperm is considered to be Amborella trichopoda , a small plant native to the rainforest of New Caledonia, an island in the South Pacific.
Analysis of the genome of A. The nuclear genome shows evidence of an ancient whole-genome duplication. The mitochondrial genome is large and multichromosomal, containing elements from the mitochondrial genomes of several other species, including algae and a moss. A few other angiosperm groups, called basal angiosperms, are viewed as having ancestral traits because they branched off early from the phylogenetic tree.
Most modern angiosperms are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more ancestral in nature because they share morphological traits with both monocots and eudicots.
Angiosperms produce their gametes in separate organs, which are usually housed in a flower. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations.
With about , species, flowering plants are the most diverse phylum on Earth after insects, which number about 1,, species. Flowers come in a bewildering array of sizes, shapes, colors, smells, and arrangements. Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between pollinator and flower characteristics is one of the great examples of coevolution. Following fertilization of the egg, the ovule grows into a seed.
The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, green peppers, corn, and avocados are all examples of fruits. Along with pollen and seeds, fruits also act as agents of dispersal. 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 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 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 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 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.
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.
All plants undergo a life cycle that takes them through both haploid and diploid generations. The multicellular diploid plant structure is called the sporophyte, which produces spores through meiotic asexual division.
The multicellular haploid plant structure is called the gametophyte, which is formed from the spore and give rise to the haploid gametes. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations.
The way in which the alternation of generations occurs in plants depends on the type of plant. In bryophytes mosses and liverworts , the dominant generation is haploid, so that the gametophyte comprises what we think of as the main plant. The opposite is true for tracheophytes vascular plants , in which the diploid generation is dominant and the sporophyte comprises the main plant. Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival see Plant Classification, Bryophytes.
Like all plants, the bryophyte life cycle goes through both haploid gametophyte and diploid sporophyte stages.
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