Supplemental Lecture (97/05/19 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Evolution of Plants
    1. A list of vocabulary words is found toward the end of this document
    2. When it comes to organisms that harvest energy from sun, in terms of terrestrial eucaryotes, this means plants. In other words, the vast majority of terrestrial producers are plants (the others are the unicellular symbiotic algae and cyanobacteria which exist in lichens). At the bottom of the majority of terrestrial food chain can be found plants, etc. Plants in the strictest sense are thus photosynthetic, multicellular eucaryotes which live on land.
    3. The requirement for land distinguishes plants from their ancestors and relatives which live in the water. These are algae. While many algae are nonetheless lumped phylogenetically together with terrestrial plants, terrestrial plants nevertheless appear to form a unique lineage unto themselves (i.e., algae did not evolve from terrestrial plants, quite the reverse).
    4. The plant lineages have deep roots (pardon) dating to the original dispersion of organisms onto land some 400 to 450 million years ago (presumably plants, the producers, led the way in this multi-organismal assault). These original plants were still tied somewhat to water, particularly for replication, just as amphibians were and are today. However, just as the reptilian lineage arose from the amphibians, truly terrestrial plants also developed from these early, water-tied (therefor quasi-) terrestrial plants. Also, just as with the reptilian lineage, the truly great innovation was the development of methods by which sex and development were accomplished in the absence of standing water (pollen, seeds, and then flowers).
    5. This lecture considers the evolution of land plants in some detail and in the process gives an overview of the major plant taxonomic groupings. We will consider reproduction of angiosperm (e.g., flowering plants) in a second lecture.
  2. Plants
    1. Characteristics:
      1. Plants differ from algea in that they:
        1. are multicellular
        2. develop from embryos
      2. Both plants and algae:
        1. are eucaryotes
        2. are photosynthetic
        3. contain cells which contain chloroplasts1.
      3. 1lack of chloroplasts among plants is a derived trait).
      4. Plants as well as some algae:
        1. use starch as their carbohydrate storage material
        2. have cellulose-based cell walls
        3. contain chloroplasts featuring chlorophylls a and b and carotenoids
      5. See "early evolution", below, for various distinctions particularly between plants and algae.
    2. Monophyly:
      1. Plants are considered to be a truely monophyletic taxon, i.e., one which evolved from a single algal lineaege.
      2. Plants plus the green algae also likely form a monoplyletic taxon.
      3. In constrast, the various algae, with the plant-like algae (green algae) and the plants, together constitute a polyphyletic taxon, each apparently independently (red algae, brown algae, green algae) arising from protist ancestors (implying that endosymbiotic chloroplasts are an example of convergent evolution---i.e., arose more than once during evolutionary history).
  3. Embryo
    1. Embryos are multicellular young which are supported by maternal cells.
  4. Alternation of generations
    1. Diploid to haploid to diploid:
      1. Most plants display alternations of generations, a life cycle that passes alternately through diploid and haploid stages.
      2. The haploid stage is generated by meiosis.
      3. The diploid stage is generated by fertilization.
    2. Both haploid and diploid mitosis:
      1. In plants, both the diploid and haploid stages are capable of undergoing mitosis.
      2. These stages are called the sporophyte and gametophyte, respectively.
    3. Separate generations:
      1. A semantic consequence of both the diploid and haploid stages undergoing mitosis is that they are considered separate generations.
      2. Thus, one speaks of alternating sporophyte and gametophyte generations, or, for short, simply alternation of generations.
  5. Gametophyte ["gamete plant"]
    1. The gametophyte is the plant haploid generation.
    2. In ancestral and primitive plants (e.g., green algae and mosses, respectively) the gametophyte generation is an actual, free living, haploid plant which produces gametes by mitosis.
    3. Conspicuous plant:
      1. In fact, the gametophyte generation in primitive plants is actually the dominant plant observed ("the conspicuous organism").
      2. This contrasts greatly with the dominant, less primitive plants found today in which the diploid (sporophyte) generation is dominant.
      3. The gametophyte generation in these (less primative) plants consists of only a few cells protected and nourished by the sporophyte generation (i.e., the gametophyte are not free living in gymnosperms and angiosperms).
      4. In the sophisticated seed-bearing plants the "haploid spores divide by mitosis and produce tiny haploid plants that will form gametes, and hence, these little individuals are called gametophytes, or gamete-producing plants. The miniature female gametophyte consists of just seven cells, which are totally dependent on the much larger diploid sporophyte for support. One of these cells is the egg, the female gamete. The male gametophyte is the pollen grain; it contains just three cells when mature, including two sperm, the male gametes." (p. 798, Postlethwait and Hopson, 1995)
  6. Sporophyte ["spore-plant", spore]
    1. Diploid generation:
      1. The sporophyte is the plant diploid generation.
      2. In all plants it is the sporophyte generation which generates the haploid generation (spores) via meiosis.
    2. Conspicuous plant:
      1. In some primitive plants both the gametophyte and the sporophyte can serve as conspicuous organisms.
      2. In modern land plants (vascular plants) the sporophyte is the conspicuous organism.
      3. In other words, the land plants we see around us represent the diploid, sporophyte generation of these plants.
      4. In primitive plants (bryophytes particularly) the sporophyte often is not free living and must be nourished by the gametophyte. This is essentially the opposite situation from that observed among the gymno- and angiosperms.
    3. Spores give rise to gametes:
      1. The haploid products of sporophyte cell meiosis are considered spores because they are capable of undergoing and, indeed, do undergo mitosis prior to the occurrence of fertilization.
      2. It is only the final products of these mitotic divisions which are considered the plant gametes.
      3. Hence, it is the haploid gametophyte generation which, technically, gives rise to the gametes, i.e., the cells which do the actual fusing to effect fertilization.
  7. Evolutionary history of plants
    1. Overview:
      1. Plants evolved from primitive green algae and likely began their terrestrial invasion approximately 450 million years ago.
      2. Like animals, the steps most pertinant to the invasion of land appear to have involved the evolution of desiccation resistance.
      3. With a terrestrial existence came further modifications which resulted in significant differentiation of plant tissue as well as innovative mechanisms of gamete and progeny dispersal such as flowers and seeds.
      4. Thus, besides their living on land, plants display a more sophisticated and differentiated multicellular anatomy than do even the most sophisticated algae.
    2. Kinds of plants:
      1. Based on the presence of these various, sophisticated adaptations, plants may be categorized as follows:
      2. nonvascular plants versus vascular plants
      3. vascular non-seed bearing and vascular seed bearing
      4. seed bearing non-flowering and seed bearing flowering
      5. The most advanced plants are considered to be the flowering vascular plants while the most primitive plants are considered to be the nonvascular plants.
      6. There exists approximately 275,000 individual species of flowering plant out of about 325,000 species of plants. Clearly, especially given their tendency toward extremely macroscopic bodies, flowering plants are the dominant terrestrial producers.
  8. Plant divisions
    1. Division = phylum:
      1. The plant equivalent of the term phyla (i.e., that major hierarchical classification taxon found just below kingdom) in some systems of classification is division.
      2. Note, however, that the term phyla is increasingly being used to describe the plant taxonic division in the stead of the word division.
    2. 12 extant divisions:
      1. Though classification schemes can differ, one scheme posits 12 extant divisions:
        1. three associated with the nonvascular plants (a.k.a., bryophytes)
        2. four seedless vascular plants
        3. four seed-bearing plants which do not flower
        4. one division of flowering plants
      2. These divisions include:

    division

    description

    examples

    comments

    Bryophyta

    nonvascular plants

    mosses

     

    Hepaticophyta

    nonvascular plants

    liverworts

     

    Anthocerophyta

    nonvascular plants

    hornworts

     

    Psilophyta

    seedless vascular plants

    whisk ferns

     

    Lycophyta

    seedless vascular plants

    club mosses

     

    Sphenophyta

    seedless vascular plants

    Horsetails

     

    Pterophyta

    seedless vascular plants

    ferns

     

    Cycadophyta

    non-flowering, seed-bearing, vascular plants

    Cycads (a.k.a., sago palms)

     

    Ginkgophyta

    non-flowering, seed-bearing, vascular plants

    ginkgo

     

    Gnetophyta

    non-flowering, seed-bearing, vascular plants

    Mormon tea

    division Gnetophyta may form a monophyletic clade with members of division Anthophyta

    Coniferophyta

    non-flowering, seed-bearing, vascular plants

    conifers

     

    Anthophyta

    flowering, seed-bearing vascular plants

    angiosperms

     

  9. Early evolution
    1. Land monophyly:
      1. Plants along with fungi and the insects appear to be the only major groups of organisms which evolved on land (as opposed to evolving in water and then finding their way onto land as did the fish and the mollusks).
      2. What does that mean? Basically it is a suggestion that some creature living in the sea evolved into something significantly different and new upon its transition to terrestrial living: Plants.
    2. Major innovations:
      1. In plants, the major innovations which differentiate them from their green algae ancestors were, not surprisingly, adaptations to terrestrial living. These include:
      2. the invention of the cuticle (to keep water in), algae live in water
      3. the invention of stoma (to allow gasses such as CO2 and O2 in and out through the cuticle), algae don't have cuticles
      4. differentiation of parts into distinct tissues such as leaves and stems (a feature of mosses and vascular plants but to a much lesser extent the liverworts and hornworts), algae get most what they need from the surrounding medium
      5. symbiotic relationships with fungi (mycorrhizae which aid in the uptake of nutrients from moist soil, as opposed to flooded soil, standing water, or flowing water), algae in contact with dissolved minerals; fungi are land organisms
      6. invention of vascularization (to efficiently transport water and nutrients to tissue lying above the water), algae in contact with water and photosynthesize throughout their thalli (sp?)
      7. differentiation of above and below ground tissues (e.g., the invention of roots specialized for nutrient transport as well as support), algae see a similar environment top to bottom
      8. invention of secondary growth (to allow the formation of thick structures as opposed to simply long, thin ones which grow only at either end), algae live in a bouyant medium
      9. diminishment of the conspicuousness of the gametophyte associated with increased conspicuousness of the sporophyte, algae disseminate sperm in water so there is no potential for embryos to dry out
      10. invention of reproductive strategies which did not depend on the existence of standing water (e.g., pollen, seeds, flowers), algae disseminate sperm in water so there is no potential for embryos to dry out
      11. Note that not all of the above listed innovations are found on all plants. In fact, you should consider the above to be a list of trends occurring among plants with adaptations associated with increased evolutionary success more or less read from top to bottom.
  10. Cuticle [cutin]
    1. Desiccation resistor:
      1. The cuticle is a waxy covering (of cutin) which prevents water found inside the plant body from being lost to the air.
      2. The cuticle inhibits desiccation thereby allowing extended exposure to the air.
  11. Stoma [sing., stomata, pl., transpiration]
    1. The stoma are valves (regulated holes) found on the surface of leaves and some stems which allow the passage of gasses (such as CO2 and O2) into and out of plants otherwise blocked by cuticle.
    2. Transpiration:
      1. The down side of this movement of gasses is that water vapor is included among them.
      2. This loss of water through stoma is called transpiration.
      3. Transpiration gradually desiccates the plants and therefore necessitates a fairly constant replenishment of water.
    3. Cost of terrestrial photosynthesis:
      1. Transpiration is a cost of photosynthesis born by terrestrial living.
      2. This cost does not occur, due to their immersion, among algae living in aquatic environments.
      3. It forces plants to have efficient vascularization or short distance between ground and leaves.
  12. Primary growth
    1. Elongation, not thickening:
      1. Primary growth is a consequence of cell division which occurs only at the tip of plant bodies
      2. Primary growth results in elongation but not thickening.
    2. Limiting growth strategy:
      1. Plants capable of only primary growth are not capable of thickening over time.
      2. Plants not capable of thickening over time are extrememly limited in how tall they may grow.
      3. Note that primary growth contrasts with secondary growth.
  13. Bryophytes [mosses, liverworts, hornworts]
    1. Nonvascular plants:
      1. The bryophytes are a phenotypic grouping of plants consisting only of those lacking in vascularization.
      2. The bryophytes represent approximately 10% of all plants.
      3. These include:
        1. the mosses
        2. the liverworts
        3. the hornworts
    2. Conspicuous gametophyte:
      1. The bryophytes are plants which, like many algae, display the gametophyte generation as the conspicuous organism.
      2. In moss the fuzzy part is the gametophyte while the little "sticks," which resemble street lamps, are the sporophytes.
      3. The sporophyte in nonvascular plants may be nutritionally dependent on the gametophyte, but the gametophyte is not nutritionally dependent on the sporophyte.
      4. Note that this contrasts with the vascular plants in which the evolutionary trend is for the gametophyte to become profoundly nutritionally dependent on the sporophyte.
    3. Avoid confusing the term bryophyte with "division Bryophyta." The latter refers to the mosses only.
  14. Nonvascular plants
    1. Amphibious plants:
      1. These primitive plants tend to grow close to the ground and require liquid water to effect reproduction.
      2. They are essentially the amphibians of the plant world---existing at an air-water interface but requiring a return to the water (or, at least, a highly moist environment) to reproduce.
      3. As do many algae, the nonvascular plants employ sperm, the male gamete and water requiring progenitor of the pollen produced by seed bearing vascular plants.
      4. Not surprisingly, nonvascular plants tend to occupy the frequently (or cyclically) very-wet-ground niche, especially where height is not necessarily advantageous (the arctic tundra, for example).
      5. Despite their reproductive requirement for standing water, the nonvascular plants can be quite resistant to desiccation and can consequently occupy harsh environments.
      6. The one harshness they are not resistant to is air pollution. Nonvascular plants, like vertebrate amphibians, consequently tend to be associated with polluted environments much less than with the less adulterated environments far from man.
    2. Limited nutrient transport:
      1. Small size and consequent proximity of the plant to surface water allows these plants to forgo vascularization.
      2. Instead, movement of water and other materials through the plant occurs through minimally differentiated cells, in a manner analogous to that employed by fungal hyphae.
      3. Despite their primitiveness (or, in fact, a consequence of their ability to exploit the above noted niche), the nonvascular plants today are far more successful than the similar though more advanced seedless vascular plants.
    3. No roots:
      1. Nonvascular plants lack roots in the sense of nutrient transport organs, but instead employ analogous structures, called rhizoids, which serve primarily as holdfasts.
      2. Indeed, nonvascular plants display no anatomical differentiation between the above and below ground tissue (except for the existence of rhizoids below ground).
      3. Exceptionally, the mosses appear to be the most sophisticated of the nonvascular plants. They sport innovations such as significant differentiation into distinct tissues including leaves and even sport primitive vascularization (despite the category of plants in which they are classified).
  15. Vascularization [sieve elements, phloem, tracheary elements, xylem]
    1. Movement within cells:
      1. Dissolved nutrients are conducted through vascular plants within specialized cells.
      2. Two types of vascularization exist, phloem and xylem.
    2. Phloem:
      1. Phloem or sieve elements represent vascularization which conducts carbohydrate away from sites of manufacture (e.g., leaves) to sites where they are not manufactured (e.g., roots).
      2. Remember, "Phlo to the Snow," to keep track of how this vascularization generally conducts nutrients toward the center of the earth.
    3. Xylem:
      1. Xylem or tracheary elements represent vascularization which conducts water and dissolved minerals away from sites of capture (e.g., roots) to sites where they are not acquirable (e.g., leaves).
      2. Remember, "Xy to the Sky," to keep track of how this vascularization generally conducts nutrients away from the center of the earth.
  16. Seedless vascular plants [ferns, horsetails, club mosses]
    1. Selection for height:
      1. Plants derive a large fraction of their nourishment from the sun (it is their energy source).
      2. Interspecific competition between plants therefore tends to involve competition for access to the sun (i.e., light).
      3. All else being equal, under crowded conditions it is the taller plants which monopolize the limited sunlight striking the earth at the expense of shorter plants.
      4. Consequently, a dominant evolutionary trend among plants is the development of ever taller, leaf-bearing structures.
    2. Height requires vascularization:
      1. The development of vascularization was an important early step toward the evolution of taller structures.
      2. Without communication between nourishment found at ground level (generally, at the base of the plant) and those captured from leaves found well above the ground, plants cease to exist as viable entities.
      3. It is vascularization, a series of tubes, which provides this communication.
    3. Vascularization came before seeds:
      1. The evolution of vascularization and, hence, height, appears to have predated the development of the plant equivalent of the "reptilian" stage.
      2. There exist numerous plants which on the one hand display vascularization and height, but on the other hand are as tied to the same highly aquatic (at least highly moist environment) mode of reproduction as are nonvascular plants.
      3. These plants are the seedless vascular plants.
      4. Though clearly an advancement over the nonvascular plants, the seedless vascular plants are neither the descendants of the nonvascular plants nor nearly as successful (today, when measured in terms of numbers of species) as are the nonvascular plants.
    4. Ferns:
      1. In terms of shear numbers as well as diversity, the seedless vascular plants are dominated by the ferns.
      2. Additional seedless vascular plants are the horsetails and club mosses.
    5. Overgrown non-vasculars:
      1. Despite their height (and size and complexity), the seedless vascular plants are still reproduce a lot like the nonvascular plants, particularly the mosses (complete with swimming sperm).
      2. In fact, you might consider seedless vascular plants, such as ferns, to be essentially greatly enlarged, nutritionally independent, specialized, and highly differentiated moss sporophytes complete with an independently living, moss-size gametophyte generation.
    6. Once dominant amphibious plant:
      1. Once, in a warmer age long ago, the seedless vascular plants were dominant on land, sufficient in number, size (height), and mass that the remains of the enormous amount of carbon they captured we know today as coal.
      2. Perhaps not coincidentally, the rein of the seedless vascular plants coincided with the rein of the amphibians as the dominant vertebrate.
      3. In a world where only swamps had abundant plant life there probably was little incentive for the evolution of a fully terrestrial vertebrate.
  17. Mycorrhizae [my co rye' zay, endomycorrhizae]
    1. Dealing with sterile soil:
      1. As plants began to explore habitats well above the water table, or flood levels, they were ultimately forced to make a transition from environments in which dissolved nutrients were reasonably available (bodies of water) to one in which nutrients were relatively unavailable (essentially sterile mineral soil).
      2. In fact, it is entirely plausible that this dilemma was first faced by lichens which even to this day dominate the growth-on-sterile-minerals niche.
      3. The algae (or cyanobacteria) associated with lichen solve the problem of lack of nutrient availability by partitioning nutrient and energy gathering between the fungi and the symbiotic algae, respectively.
    2. Fungi as roots:
      1. Early plants solved this problem similarly, though unlike in lichens in this case it was the energy transducer (the plants) which were the conspicuous organism.
      2. Particularly, fungi are found living in conjunction with plant roots, either external to these roots (mycorrhizae) and/or within the root cells themselves (endomycorrhizae).
      3. The majority of the plants today (80%) as well as numerous fossil plants display mycorrhizae.
  18. Seeds
    1. Specialized embryo delivering vehicles:
      1. Seeds are protected plant embryos deliverable in specialized vehicles.
      2. The role of seeds in the plant life cycle is four-fold:
        1. protection
        2. nourishment
        3. dispersion
        4. dormancy
    2. Protection:
      1. Seeds protect the embryonic sporophyte generation.
      2. This role is equivalent to the shell of the amniotic egg.
    3. Nourishment:
      1. Seeds nourish the embryonic sporophyte generation allowing them to get a good, strong start.
      2. This role is equivalent to the yolk of the amniotic egg.
    4. Dispersion:
      1. Seeds and the tissue which surrounds them display numerous adaptations to dispersion.
      2. This role of seeds as vehicles has no analogy among animal reproduction except to the extent that juvenile forms often are the dispersers especially among sesile animals.
      3. The reason for this parallel particularly with sesile animals is no doubt attributable to the lack of motility associated with non-embryonic plants
      4. In other words, seeds supply a means for plant dispersion, one which is ordinarily achieved instead by the non-embryonic stage of most animals.
    5. "Seeds are able to bypass unfavorable conditions, such as drought, in a dormant condition, and germinate when favorable circumstances return. The seed is a crucial adaptation to life on land because it protects the embryonic plant from drying out during its most vulnerable stage." (p. 730, Raven and Johnson, 1996)
    6. "The egg and sperm, the two haploid gametes, unite via fertilization, and the result is a single diploid cell, the zygote. The zygote divides by mitosis and gives rise to the embryo inside the seed. When the seed is planted, the diploid embryo grows." (p. 799, Postlethwait and Hopson, 1995)
    7. "A seed consists of an embryo, the young sporophyte whose embryonic development has been temporarily arrested, and of a tough protective covering, the seed coat. In seeds, all of the products of the mature megagametophyte (although they may have been altered greatly during the seed's maturation), together with the young plant of the next generation, are included together in a compact, drought-resistant package. Seeds are the means by which plants, being rooted in the ground, are dispersed to new places. Many seeds have devices, like the wings on the seeds of pines or the fruits of maples, or the plumes on the fruits of a dandelion, that aid in their dispersal. In addition, seeds are able to bypass unfavorable conditions, such as drought, in a dormant condition, and germinate when favorable circumstances return. The seed is a crucial adaptation to life on land because it protects the embryonic plant from drying out during its most vulnerable stage. The seed coat also assists in protecting the embryo and its stored food material from being eaten by predators, attacked by fungi, or otherwise destroyed. Most kinds of seeds have abundant food stored in them, either inside the embryo or in specialized storage tissue. This food, playing the same role as the yolk of an egg, is used as a ready source of energy by the rapidly growing young plant. The evolution of the seed clearly was a critical step in the domination of the land by plants." (p. 730, Raven and Johnson, 1996)
  19. Pollen [microgametophyte; pollen tube]
    1. Gamete dispersion:
      1. Pollen is the means by which plants achieve the motility necessary to effect outcrossing.
      2. Instead of possessing tails and swimming (as do sperm), pollen either is dispersed on the wind (in both gymnosperms and angiosperms).
      3. Or the pollen may be serendipitously carried by animals lured from flower to flower by tasty rewards supplied by flowers just for that purpose (angiosperms only).
    2. Microgametophyte:
      1. The pollen consists of more than just a single, elaborate cell.
      2. Instead it actually consists of three cells, two of which are sperm, and one of which is a supportive, dispersive structural cell which contains the sperm cells, called a tube cell.
      3. "The microgametophyte (pollen) is the male gametophyte, containing (the) 2 sperm cells involved in fertilization and a tube cell involved in growth of the pollen tube (all of which occurs) within the style of the pistil. The sperm are completely enclosed by the tube cell." (B.G. Abedon, pers. comm.)
  20. Seed-bearing vascular plants [gymnosperms, cycads, ginkgos]
    1. The reptiles among plants:
      1. The invasion of the land in the reptilian sense required the evolution of seeds, the plant equivalent of reptilian amniote egg.
      2. That is, a reproductive strategy and development program which is not dependent on the occurrence of standing water.
      3. First achieved among plants by the gymnosperms includes:
        1. invention of the seed
        2. abandonment of the standing water requirement for reproduction
        3. the invention of pollen
        4. an overwhelming dominance by the sporophyte (diploid) generation
      4. This invention of the seed no doubt allowed a significant expansion of the range of plants, as well as that of vertebrates as the reptiles followed the gymnosperms away from the water.
      5. "The reproductive innovations of the gymnosperms include the pollen grain and seed, as well as a further shifting toward dominance by the diploid generation. Pollen grains are immature male gametophytes composed of just two to five non-motile cells plus a dry outer coat. Pollen grains are produced in huge numbers by male cones borne on the familiar conspicuous plant, the sporophyte, and are disseminated by the wind. In effect, the pollen grain 'airlifts' the sperm cells to the egg cells harbored inside special chambers in female cones, also borne on a mature sporophyte. Both male and female haploid gametophytes are reduced to small nonphotosynthetic structures housed entirely by the diploid sporophyte generation; thus, they embody the trend toward a dominant diploid phase. In a sense, water needed for fertilization is provided by moisture in the apparent plants' tissues, rather than by splashing raindrops of standing water." (p. 486, Postlethwait and Hopson, 1995)
    2. Gymnosperm characteristics:
      1. The gymnosperms are a group which includes the:
        1. cycads
        2. ginkgos
        3. extinct seed-bearing ferns
        4. the conifers
      2. All of these plants bear seeds.
      3. Note that in none of these plants are the seeds surrounded (or completely surrounded) by maternal tissue.
  21. Conifers
    1. Cone-bearing trees:
      1. The conifers are the dominant gymnosperms on earth today so are worth special mention.
      2. The conifers:
        1. are cone-bearing (like many gymnosperms) trees
        2. have narrow, desiccation resistant leaves (needles)
      3. The conifers include:
        1. cedars
        2. firs
        3. hemlocks
        4. junipers
        5. larches
        6. pines
        7. sequoias
        8. spruces
        9. yews
      4. Altogether there exist 600 extant conifers.
    2. "Conifers were able to colonize higher and drier reaches of the continents during the Mesozoic because of their numerous evolutionary advances: drought-resistant leaves; protective seed coats; greatly reduced male gametophytes that depend on the sporophyte and give rise to airborne pollen grains; equally reduced female gametophytes that give rise to eggs protected inside the ovule; a well-developed vascular system that produces wood and stiffens the trunk, branches, and roots; the tendency to form mycorrhizal associations with fungi; and the ability to survive for many centuries." (p. 488, Postlethwait and Hopson, 1995)
  22. Flowering plants [Anthophyta, Magnoliophyta, angiosperms]
    1. The mammals among plants:
      1. Oddly enough, just as the amphibians followed the amphibious plants onto land, the reptiles followed the reptile-like plants (conifers) away from standing water, the age of the mammals coincided with the rise of "mammal-like" plants, those which exhibit essentially internal development, i.e., enclosed structures capable of both protecting and nourishing embryos.
        1. in mammals these structures are the womb and placenta
        2. in plants it is the ovary (this contrasts with the gymnosperms which, by definition, do not completely enclose their embryos in maternal tissue)
      2. Both mammals and angiosperms came into prominence starting 65 million years ago. Angiosperms like mammals, however, appear to have come into being much earlier than this, perhaps 130 million years ago or earlier.
    2. More efficient fertilization:
      1. Per their name, these flowering plants also developed more efficient means of pollination/fertilization (flowers).
      2. Often this pollination is accomplished by the actions of animals, many of them specialized to the task (a huge number of particularly wonderful examples of coevolution).
      3. Note that angiosperm reproduction is covered in a subsequent lecture (reproduction by seed).
    3. Flowering plants also evolved a rich variety of mechanisms of seed dispersal ranging from edible fruit to various spurs which often are dispersed by animals.
    4. Flowering plants have broad leaves which allow them to more efficiently capture light, though often at the cost of reduced resistance to desiccation.
  23. Megagametophyte
    1. "A gametophyte is a gamete bearing spore in plants. The megagametophyte is an 8 celled spore that contains the egg (1 cell) and 2 polar cells. The other 5 cells are normally not involved with formation of the seed. The pollen tube contains 2 generative sperm cells, one of which fertilizes the egg to form the 2n zygote and the other of which fuses with the polar cells to form the 3n endosperm (this is known as double fertilization)." (B.G. Abedon, pers. comm.)
  24. Endosperm
    1. "The endosperm both nourishes the developing embryo and supplies energy to the developing seedling upon" its germination (in the species that retain an endosperm at seed maturity). (B.G. Abedon, pers. comm.)
  25. Vocabulary
    1. Alteration of generations
    2. Angiosperms
    3. Anthophyta
    4. Bryophytes
    5. Club mosses
    6. Conifers
    7. Cuticle
    8. Cutin
    9. Cycads
    10. Division Anthocerophyta
    11. Division Anthophyta
    12. Division Bryophyta
    13. Division Coniferophyta
    14. Division Cycadophyta
    15. Division Ginkgophyta
    16. Division Gnetophyta
    17. Division Hepaticophyta
    18. Division Lycophyta
    19. Division Psilophyta
    20. Division Pterophyta
    21. Division Sphenophyta
    22. Early evolution
    23. Embryo
    24. Endomycorrhizae
    25. Endosperm
    26. Ferns
    27. Flowering plants
    28. Gamete-plant
    29. Gametophyte
    30. Ginkgo
    31. Gymnosperms
    32. Hornworts
    33. Horsetails
    34. Liverworts
    35. Magnoliophyta
    36. Megagametophyte
    37. Microgametophyte
    38. Mosses
    39. Mycorrhizae
    40. Nonvascular plants
    41. Phloem
    42. Plant divisions
    43. Plants
    44. Pollen
    45. Pollen tube
    46. Primary growth
    47. Rhizoids
    48. Secondary growth
    49. Seeds
    50. Seed bearing vascular plants
    51. Seedless vascular plants
    52. Sieve elements
    53. Spore
    54. Spore-plant
    55. Sporophyte
    56. Stoma
    57. Stomata
    58. Terrestrial plants
    59. Tracheary elements
    60. Xylem
  26. Practice questions
    1. In terms of ploidy number, distinguish gametophyte from sporophyte. [PEEK]
    2. What are the Anthophyta? [PEEK]
    3. In terms of nutrient acquisition, contrast the gametophyte and sporophyte generations in nonvascular plants. [PEEK]
    4. Specifically, what kind of organisms are found in phylum Bryophyta? [PEEK]
  27. Practice question answers
    1. Gametophytes are haploid while sporophytes are diploid.
    2. the flowering plants, the angiosperms
    3. the sporophytes tend to be nutritionally dependent on the gametophytes
    4. the mosses
  28. References
    1. Holden, C. (1996). The world's oldest flower. Science 271:1237.
    2. Postlethwaite, J.H., Hopson, J.L. (1995). The Nature of Life. Third Edition. McGraw-Hill, Inc., New York. pp. 478-493, 798, 799.
    3. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 656-682.
    4. Raven, P. H., Johnson, G. B. (1996). Biology. Fourth Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 722-740.