UT Arlington BIOL BIOL 3427 - ch23 (15 pages)

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ch23



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ch23

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Pages:
15
School:
University of Texas at Arlington
Course:
Biol Biol 3427 - Plant Science
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Chapter 23 SEED PLANTS ANGIOSPERMS All flowering plants are classified in a single Division Magnoliophyta Angiosperms are commonly known as flowering plants This division contains the greatest number of living species about 257 000 They are characterized by producing flowers a synapomorphy that unifies them First appeared in the Cretaceous about 130 million years ago The angiosperms have developed numerous methods of interaction with animals that are beneficial to both They have developed many antiherbivore compounds that defend them against plant eating animals and parasites CHARACTERISTICS OF ANGIOSPERMS Woody or herbaceous Presence of vessels Axial parenchyma parenchyma cells mixed with vessels and tracheids Produce flowers Pollinated by wind or animals Double fertilization egg sperm embryo and 2 polar nuclei sperm endosperm Seeds enclosed in a fruit EVOLUTION OF THE FLOWER Angiosperm stamens and carpels developed from the gymnosperm sporophylls Gymnosperm sporophylls are flat and spirally arranged and microsporophylls never occur with megasporophylls Flowering plants with flat spirally arranged flower parts are thought to be most ancient Basal angiosperms have flat stamens without distinct filament and anther portions arranged in spirals and their sporogenous tissues microspore mother cells form relatively large prominent internal masses Amborella Nymphaeales Austrobaileyales and Magnoliales In most angiosperm carpels the edges of sporophyll grow shut against one another sometimes leaving a suture sometimes not These are called closed carpels Closed carpels develop into fruits that enclose the embryos as they develop into seeds Carpels in the basal angiosperms are leaf like resembling young leaves whose blades have not yet opened Basal angiosperms lack stigma and style instead the ovary edges have rows of secretory hairs that secrete a thick liquid that both seals the seam and functions as a stigmatic surface The stigmatic surface acts as a selector of pollen grains eliminating pollen from another species or preventing self fertilization THE FLOWER The flower is a reproductive shoot or branch It has four parts arranged in whorls or circles on a stalk or peduncle The parts of the flower are the sepals calyx petals corolla stamens and carpels Stamens consist of a filament and an anther Carpels are also referred to as pistils They consist of an ovary a style and a stigma Flowers may be borne singly or in clusters called inflorescence Flower parts are considered modified leaves The ability to produce bisexual flowers must have occurred very early All flowering plant clades including basal angiosperms have the megasporophylls located above the microsporophylls on a single axis The fusion of carpels into a single structure pistil and the fusion of petals into one corolla sympetaly and floral parts that are bilaterally symmetrical rather than radially symmetrical zygomorphy DOUBLE FERTILIZATION It is characteristic of flowering plants and universally present Double fertilization results in the formation of a diploid zygote and a triploid endosperm The female gametophyte or embryo sac has an egg nucleus and two polar nuclei One sperm fertilizes the egg nucleus and forms the zygote 2n Another sperm joins the two polar nuclei forming the triploid 3n nutritive tissue called the endosperm Double fertilization also occurs in gnetophytes but there is a disagreement if it is a case of a shared ancestral feature symplesiomorphy or convergent evolution homoplasy OTHER ANGIOSPERM APOMORPHIES Study Table 23 1 on page 526 of your textbook 1 Vessel elements possibly evolved after the flowers because two basal species Amborella and Drimys lack them Lack of vessels was thought to be a primitive characteristic of some basal angiosperms Other features however were inconsistent with the idea that these were primitive plants The current hypothesis is that Amborella is primitive primitively vesselless and never had vessels while others arose after vessels had developed and then lost them These are referred to as secondarily vesselless 2 Sieve tubes probably originated next Several species still have sieve cells in their phloem 3 All gymnosperms seed ferns and basal angiosperms are or were woody plants The wood of basal angiosperms has many gymnosperms characteristics Ancestral flowering plants were woody perennial in all probability 4 The herbaceous habit resulted from the loss of vascular cambium and this has occurred in several clades 5 The annual habit is a unique angiosperm characteristic Some groups of angiosperms like grasses bromeliads and orchids are still changing and evolving CHANGING CONCEPTS ABOUT EARLY ANGIOSPERMS C E Bessey developed the hypothesis of the ranalean flower about 100 years ago This generalized type of flower has 1 All parts sepals petals stamens and carpels 2 These are arranged spirally 3 The carpels are superior that is above all other flower parts This is the Magnolia type of flower Most botanists consider the angiosperms to be monophyletic The transition from gymnosperms to angiosperm it is believe to have happened in the Jurassic 200 145 m y a and Lower Cretaceous 135 m y a periods of the Mesozoic Era The earliest fossils to be considered those of angiosperms are from the Lower Cretaceous 130 m y a They represent both dicot and monocot leaves These fossils are rare making about 2 of the plant fossils In the Upper Cretaceous ending about 65 m y a angiosperm fossils are more common outnumbering the fossils of gymnosperms and ferns Upper Cretaceous and Tertiary flora contains fossils that resemble modern genera but are not identical The oldest angiosperm wood comes the Aptian Epoch of Japan 125 m y a Flowers and fruits occurred in the Lower Cretaceous CLASSIFICATION OF FLOWERING PLANTS Flowering plants used to be divided into two groups monocots and dicots This division has been largely abandoned because of new information that has become available mainly through molecular and cytological studies In general monocots can be recognized by the following traits MONOCOTS have floral parts in multiples of three and the seed contains one cotyledon The endosperm provides the food for the embryo Venation is usually parallel there are exceptions Their vascular bundles are scattered throughout the ground tissue The root system is fibrous DICOTS have floral parts in multiples of four or five and their seeds contain two cotyledons The cotyledons usually absorb the food from the


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