BCOR 012 Study Guide Chapter 21 PDF

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Don Stratton...

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The Evolution of Plants: Seedless Plants (Chapter 21.1 - 21.3) What do we mean by “alternation of generations?” -

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A universal feature of the life cycles of land plants is alternation of generations. Recall the two hallmarks of alternation of generations: o The life cycle includes both a multicellular diploid stage and a multicellular haploid stage. o Gametes are produced by mitosis, not by meiosis. Meiosis produces spores that develop into multicellular haploid organisms. If we begin looking at the land plant life cycle at the single-cell stage—the diploid zygote—then the first phase of the cycle is the formation, by mitosis and cytokinesis, of a multicellular embryo, which eventually grows into a mature diploid plant. This multicellular diploid plant is called the sporophyte (“spore plant”).

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Cells contained within specialized reproductive organs of the sporophyte, called sporangia (singular sporangium), undergo meiosis to produce haploid, unicellular spores. By mitosis and cytokinesis, a spore develops into a haploid plant. This multicellular haploid plant, called the gametophyte (“gamete plant”), produces haploid gametes by mitosis. The fusion of two gametes (fertilization) forms a single diploid cell—the zygote—and the cycle is repeated

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The sporophyte generation extends from the zygote through the adult multicellular diploid plant and sporangium formation. In contrast, the gametophyte generation extends from the spore through the adult multicellular haploid plant to the gametes. The transitions between the generations are accomplished by fertilization and by meiosis. In all land plants, the sporophyte and the gametophyte differ genetically: the sporophyte has diploid cells, and the gametophyte has haploid cells.

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There is a trend toward reduction of the gametophyte generation in plant evolution. In the nonvascular land plants, the gametophyte is larger, longer-lived, and more self-sufficient than the sporophyte. In

those groups that appeared later in plant evolution, however, the sporophyte is the larger, more conspicuous, longer-lived, and more self-sufficient generation. What is the phylogenetic relationship between plants and algae? How do we know? -

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More than a billion years ago, when a cyanobacterium was first engulfed by an early eukaryote, the history of life was altered radically. The chloroplasts that resulted from primary endosymbiosis of this cyanobacterium were obviously important for the evolution of plants and other photosynthetic eukaryotes, but they were also critical to the evolution of all life on land. Until photosynthetic plants were able to move onto land, there was very little on land to support multicellular animals or fungi, and almost all life was restricted to the oceans and fresh waters. Primary endosymbiosis is a shared derived trait—a synapomorphy—of the group known as Plantae. Although Plantae is Latin for “plants,” in everyday language—and throughout this book—the unmodified common name “plants” is usually used to refer only to the land plants. However, the first several clades to branch off the tree of life after primary endosymbiosis are all aquatic. Most aquatic photosynthetic eukaryotes (other than those secondarily derived from land plants) are known by the common name algae. This name, however, is just a convenient way to refer to all such groups, which are not all closely related.

What are some essential adaptations for life on land? -

The first plants possessing vascular tissues did not arise until tens of millions of years after the earliest plants had colonized the land. But once vascular tissues arose, their ability to transport water and food throughout the plant body allowed vascular plants to spread to new terrestrial environments and to diversify rapidly.

Vascular tissues transport water and dissolved materials -

Vascular plants differ from the other land plants in crucial ways, one of which is the possession of a well-developed vascular system consisting of tissues that are specialized for the transport of materials from one part of the plant to another. One type of vascular tissue, the xylem, conducts water and minerals from the soil to aerial parts of the plant. Because some of its cell walls contain a stiffening substance called lignin, xylem also provides support against gravity in the terrestrial environment. The other type of vascular tissue, the phloem, conducts the products of photosynthesis from sites where they are produced or released to sites where they are used or stored.

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Although the vascular plants are an extraordinarily large and diverse group, a particular event was critical to their evolution. Sometime during the Paleozoic era, probably in the mid-Silurian (430 mya), a new cell type—the tracheid—evolved in sporophytes of the earliest vascular plants. The tracheid is the principal water-conducting element of the xylem in all vascular plants except the angiosperms

(flowering plants) and one small group of gymnosperms—and tracheids persist even in these groups, along with a more specialized and efficient system derived from them. -

The evolution of tracheids set the stage for the complete and permanent invasion of land by plants. First, these cells provided a pathway for the transport of water and mineral nutrients from a source of supply to regions of need in the plant body. And second, the cell walls of tracheids, stiffened by lignin, provided rigid structural support. This support is a crucial factor in a terrestrial environment because it allows plants to grow upward and thus compete for sunlight. A taller plant can intercept more direct sunlight (and thus conduct photosynthesis more readily) than a shorter plant, which may be shaded by the taller one. Increased height also improves the dispersal of spores.

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The vascular plants featured another evolutionary novelty: a branching, independent sporophyte. A branching sporophyte body can produce more spores than an unbranched body, and it can develop in complex ways. The sporophyte of a vascular plant is nutritionally independent of the gametophyte at maturity. Among the vascular plants, the sporophyte is the large and obvious plant that one normally pays attention to in nature, in contrast to the relatively small, dependent sporophytes typical of most nonvascular land plants.

The diversification of vascular plants made land more suitable for animals -

The initial absence of herbivores (plant-eating animals) on land helped make the first vascular plants successful. By the late Silurian period (about 425 mya), the proliferation of land plants made the terrestrial environment more hospitable to animals. Arthropods, vertebrates, and other animals moved onto land only after vascular plants became established there.

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Trees of various kinds appeared in the Devonian period and dominated the landscape of the Carboniferous period (359–299 mya). Forests of lycophytes (club mosses) up to 40 meters tall, along with horsetails and tree ferns, flourished in the tropical swamps of what would become North America and Europe. Plant parts from those forests sank into the swamps and were gradually covered by layers of sediment. Over millions of years, as the buried plant material was subjected to intense pressure and elevated temperatures, it was transformed into coal. Today that coal provides more than half of our electricity. The world’s coal deposits, although huge, are not infinite, and humans are burning coal deposits at a far faster rate than they were produced.

What were the first land plants? When did they arise? The earliest vascular plants lacked roots -

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The earliest known vascular plants belonged to a now-extinct group called the rhyniophytes. The rhyniophytes were one of a very few types of vascular plants in the Silurian period. The landscape at that time probably consisted mostly of bare ground, with stands of rhyniophytes in low-lying moist areas. Early versions of the structural features of all the other vascular plant groups appeared in the rhyniophytes of that time. These shared features strengthen the case for the origin of all vascular plants from a common nonvascular land plant ancestor. Rhyniophytes did not have roots. Like most modern ferns and lycophytes, they were apparently anchored in the soil by horizontal portions of stem, called rhizomes, which bore water-absorbing unicellular filaments called rhizoids. These plants also bore aerial branches, and sporangia— homologous to the sporangia of mosses—were found at the tips of those branches. Their branching pattern was dichotomous, meaning the apex (tip) of the shoot divided to produce two equivalent new branches, each pair diverging at approximately the same angle from the original stem.

What is the life cycle of a bryophyte (mosses, etc)? You should know the haploid and diploid phases and how fertilization occurs.

A Moss Life Cycle The life cycles of nonvascular land plants, exemplified here by that of a moss, are dependent on an external source of liquid water. The visible green structure of such plants is the gametophyte. How does that compare to the life cycle of a fern? Life Cycle of a Fern The most conspicuous stage in the fern life cycle is the mature diploid sporophyte, shown at the bottom of this diagram. The inset shows sori on the underside of a fern leaf. Each sorus contains many spore-producing sporangia.

What are some of the similarities and differences between ferns, horsetails, and lycophytes? The lycophytes are sister to the other vascular plants -

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The club mosses and their relatives, the spike mosses and quillworts, are collectively called lycophytes. The lycophytes are the sister group to the remaining vascular plants). There are relatively few (just over 1,200) surviving species of lycophytes. The lycophytes have true roots that branch dichotomously. The arrangement of vascular tissue in their stems is simpler than that in other vascular plants. They bear simple leaflike structures called microphylls, which are arranged spirally on the stem. Growth in lycophytes comes entirely from apical cell division. Branching in the stems, which is also dichotomous, occurs by division of an apical cluster

of dividing cells. -

The sporangia of many club mosses are aggregated in cone-like structures called strobili). The strobilus of a club moss is a cluster of spore-bearing leaves inserted on an axis (a linear supporting structure). Other club mosses lack strobili and bear their sporangia on (or adjacent to) the upper surfaces of specialized leaves.

Horsetails and ferns constitute a clade -

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The horsetails and ferns were once thought to be only distantly related. As a result of genomic analyses, we now know that they form a clade: the monilophytes. In the monilophytes—as in the seed plants, to which they are the sister group—there is differentiation between a main stem and side branches. This pattern contrasts with the dichotomous branching characteristic of the lycophytes and rhyniophytes, in which each split gives rise to two branches of similar size. Today there are only about 15 species of horsetails, all in the genus Equisetum. The horsetails have reduced true leaves that form in distinct whorls (circles) around the stem. Horsetails are sometimes called “scouring rushes” because rough silica deposits found in their cell walls once made them useful for cleaning. They have true roots that branch irregularly. Horsetails have a large sporophyte and a small gametophyte, each independent of the other. The first ferns appeared during the Devonian period. Today this group comprises more than 12,000 species. Analyses of gene sequences indicate that a few species traditionally allied with ferns may in fact be more closely related to horsetails than to ferns. Nonetheless, the majority of ferns form a monophyletic group. Although most ferns are terrestrial, a few species live in shallow fresh water. Terrestrial ferns are characterized by large leaves with branching vascular strands. Some fern leaves become climbing organs and may grow to be as long as 30 meters.

The vascular plants branched out -

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Several features that were new to the vascular plants evolved in lycophytes and monilophytes. Roots probably had their evolutionary origins as a branch, either of a rhizome or of the aboveground portion of a stem. That branch presumably penetrated the soil and branched further. The underground portion could anchor the plant firmly, and even in this primitive condition, it could absorb water and minerals. The microphylls of lycophytes were probably the first leaflike structures to evolve among the vascular plants. Microphylls are usually small and only rarely have more than a single vascular strand, at least in existing species. Some biologists believe that microphylls had their evolutionary origins as sterile sporangia. The principal characteristic of a microphyll is a vascular strand that departs from the vascular system of the stem in such a way that the structure of the stem’s vascular system is scarcely disturbed. This pattern was evident even in the lycophyte trees of the Carboniferous period, many of which had microphylls many centimeters long.

RANDOM LECTURE NOTES: -

Gain of chloroplast (>1 billion years ago)

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Derived traits of land plants o Some key traits appear in nearly all land plants but are absent in the charophytes:

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Alternation of generations

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Walled spores produced in sporangia

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Multicellular gametangia

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Waxy cuticle

All land plants alternate generations, though extensively modified

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Water-borne spores  protected spores

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Protective organs for gamete production o Archegonium - eggs o Antheridium – sperm

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Cuticle: A waxy layer on the outer body surface that retards water loss

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Bryophytes: o Liverworts o Hornwarts o Mosses

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The empty niche – vertical growth o Key innovation: Vascular tissue for movement of nutrients against gravity

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Xylem

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Phloem

Vascular systems permitted evolution of new specialized plant organs o Roots o Leaves

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Sori: Grouped sporangia on sporophylls

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Dominant sporophyte grows out of gametophyte

Concept 21.1: Primary endosymbiosis produced the first photosynthetic eukaryotes - Primary endosymbiosis gave rise to chloroplasts and the subsequent diversification of the Plantae. The descendants of the first photo-synthetic eukaryote include glaucophytes, red algae, several groups of green algae, and land plants, all of which contain chlorophyll a. - Streptophytes include the land plants and two groups of green algae. Green plants, which include the streptophytes and the remaining green algae, are characterized by the presence of chlorophyll b (in addition to chlorophyll a). - Land plants, also known as embryophytes, arose from an aquatic green algal ancestor related to today’s stoneworts. Land plants develop from embryos that are protected by parental tissue. Concept 21.2: - The acquisition of a cuticle, stomata, gametangia, a protected embryo, protective pigments, thick spore walls with a protective polymer, and a mutualistic association with a fungus were all adaptations of land plants to terrestrial life.

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All land plant life cycles feature alternation of generations, in which a multicellular diploid sporophyte alternates with a multicellular haploid gametophyte. The nonvascular land plants comprise the liverworts, hornworts, and mosses. These groups lack specialized vascular tissues for the conduction of water or nutrients through the plant body. The life cycles of nonvascular land plants depend on liquid water. The sporophyte is usually smaller than the gametophyte and depends on it for water and nutrition. In many land plants, spores form in structures called sporangia and gametes form in structures called gametangia. Female and male gametangia are, respectively, an archegonium and an antheridium.

Concept 21.3: Vascular tissues led to rapid diversification of land plants - The vascular plants have a vascular system consisting of xylem and phloem that conducts water, minerals, and products of photosynthesis through the plant body. The vascular system includes cells called tracheids. - The rhyniophytes, the earliest known vascular plants, are known to us only in fossil form. They lacked true roots and leaves but apparently possessed rhizomes and rhizoids. - Among living vascular plant groups, the lycophytes (club mosses and relatives) have only small, simple leaflike structures (microphylls). True leaves (megaphylls) are found in monilophytes (which include horsetails and ferns). The monilophytes and the seed plants are collectively called euphyllophytes. - Roots may have evolved either from rhizomes or from stems. Microphylls probably evolved from sterile sporangia, and megaphylls may have resulted from the flattening and reduction of a portion of a stem system with overtopping growth. - The earliest-diverging groups of vascular plants are homosporous, but heterospory—the production of distinct megaspores and microspores—has evolved several times. Megaspores develop into female megagametophytes; microspores develop into male microgametophytes....