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The Green Algal Ancestor of Plants

Land plants evolved from a common ancestor with multicellular, freshwater green algae about 450 MYA. During the evolution of plants, protecting the embryo, apical growth, vascular tissue for transporting water and organic nutrients, possession of megaphylls, using seeds to disperse offspring, and having flowers were all adaptations to a land existence.

All plants have a life cycle that includes an alternation of generations. In this life cycle, a haploid gametophyte alternates with a diploid sporophyte. During the evolution of plants, the sporophyte gained in dominance, while the gametophyte became microscopic and dependent on the sporophyte.

Evolution of Bryophytes: Colonization of Land

Ancient bryophytes were the first plants to colonize land. Today, the bryophytes consist of liverworts, hornworts, and mosses that lack well-developed vascular tissue. Sporophytes of bryophytes are nutritionally dependent on the gametophyte, which is more conspicuous and photosynthetic. The life cycle of the moss (see Fig. 23.8) demonstrates reproductive strategies such as flagellated sperm and dispersal by means of windblown spores. The bryophytes are not a monophyletic group.

Evolution of Lycophytes: Vascular Tissue

Vascular plants, such as the rhyniophytes, evolved during the Silurian period. The sporophyte has two kinds of well-defined conducting tissues. Xylem is specialized to conduct water and dissolve minerals, and phloem is specialized to conduct organic nutrients. The lycophytes are descended from these first plants and they have vascular tissue. Ancient lycophytes also had the first leaves, which were microphylls; their life cycle is similar to that of the fern.

Evolution of Pteridophytes: Megaphylls

In pteridophytes (ferns and their allies, horsetails and whisk ferns), and also lycophytes, the sporophyte is dominant and is separate from the tiny gametophyte, which produces flagellated sperm. Windblown spores are dispersal agents. Today's ferns have obvious megaphylls—horsetails and whisk ferns have reduced megaphylls.

Seedless vascular plant is a description that applies to lycophytes, ferns, and fern allies that grew to enormous sizes during the Carboniferous period when the climate was warm and wet. Today, seedless vascular plants that live in the temperate zone use asexual propagation to spread into environments that are not favorable to a water-dependent gametophyte generation.

Evolution of Seed Plants: Full Adaptation to Land

Seed plants also have an alternation of generations, but they are heterosporous, producing both microspores and megaspores. Gametophytes are so reduced that the female gametophyte is retained within an ovule. Microspores become the windblown or animal-transported male gametophytes—the pollen grains. Pollen grains carry sperm to the egg-bearing female gametophyte. Following fertilization, the ovule becomes the seed. A seed contains a sporophyte embryo, and therefore seeds disperse the sporophyte generation. Fertilization no longer uses external water, and sexual reproduction is fully adapted to the terrestrial environment.

The gymnosperms (cone-bearing plants) and also possibly angiosperms (flowering plants) evolved from woody seed ferns during the Devonian period. The conifers, represented by the pine tree, exemplify the traits of these plants. Gymnosperms have “naked seeds” because the seeds are not enclosed by fruit, as are those of flowering plants.

A woody shrub, Amborella trichopoda, has been identified as most closely related to the common ancestor for the angiosperms. In angiosperms, the reproductive organs are found in flowers; the ovules, which become seeds, are located in the ovary, which becomes the fruit. Therefore, angiosperms have “covered seeds.” In many angiosperms, pollen is transported from flower to flower by insects and other animals. Both flowers and fruits are found only in angiosperms and may account for the extensive colonization of terrestrial environments by the flowering plants. (See also Chapter 27.)

understanding the terms

Match the terms to these definitions:

  1. Diploid generation of the alternation of generations life cycle of a plant; meiosis produces haploid spores that develop into the gametophyte.

  2. Flowering plant group; members have one embryonic leaf, parallel-veined leaves, scattered vascular bundles, and other characteristics.

  3. Male gametophyte in seed plants.

  4. Rootlike hair that anchors a nonvascular plant and absorbs minerals and water from the soil.

reviewing this chapter
  1. Refer to Figure 23.1, and trace the evolutionary history of land plants. What traits do charophytes have that are shared by land plants? 41011

  2. What is meant when it is said that a plant alternates generations? Distinguish between a sporophyte and a gametophyte. 41213

  3. Describe the various types of bryophytes and the life cycle of mosses. Discuss the ecological and commercial importance of mosses. 41315

  4. When do vascular plants appear in the fossil record, and why do lycophytes perhaps resemble the first vascular plants? Mention the importance of branching. 41618

  5. Draw a diagram to describe the life cycle of a fern, pointing out significant features. What are the human uses of ferns? 419

  6. What features do all seed plants have in common? When do seed plants appear in the fossil record? 420

  7. List and describe the four phyla of gymnosperms. What are the human uses of gymnosperms? 42022

  8. Use a diagram of the pine life cycle to point out significant features, including those that distinguish a seed plant's life cycle from that of a seedless vascular plant. 421

  9. What is known about the ancestry of flowering plants? 424

  10. How do monocots and eudicots differ? What are the parts of a flower? 42425

  11. Use a diagram to explain and point out significant features of the flowering plant life cycle. 42627

  12. Offer an explanation as to why flowering plants are the dominant plants today. 427

testing yourself

Choose the best answer for each question.

  1. Which of these are characteristics of land plants?

    1. multicellular with specialized tissues and organs

    2. photosynthetic and contain chlorophylls a and b

    3. protect the developing embryo from desiccation

    4. have an alternation of generations life cycle

    5. All of these are correct.

  2. In bryophytes, sperm usually move from the antheridium to the archegonium by

    1. swimming.

    2. flying.

    3. insect pollination.

    4. worm pollination.

    5. bird pollination.

  3. Ferns have

    1. a dominant gametophyte generation.

    2. vascular tissue.

    3. seeds.

    4. Both a and b are correct.

    5. Choices a, b, and c are correct.

  4. The spore-bearing structure that gives rise to a female gametophyte in seed plants is called a

    1. microphyll.

    2. spore.

    3. megasporangium.

    4. microsporangium.

    5. sporophyll.

  5. A small, upright plant that resembles a tiny upright pine tree with club-shaped strobilii and microphylls is a

    1. whisk fern.

    2. lycophyte.

    3. conifer.

    4. horsetail.

    5. fern.

  6. Trends in the evolution of plants include all of the following except

    1. from homospory to heterospory.

    2. from less to more reliance on water for life cycle.

    3. from nonvascular to vascular.

    4. from nonwoody to woody.

  7. Gymnosperms

    1. have flowers.

    2. are eudicots.

    3. are monocots.

    4. do not have spores in their life cycle.

    5. reproduce by seeds.

  8. In the moss life cycle, the sporophyte

    1. consists of leafy green shoots.

    2. is the heart-shaped prothallus.

    3. consists of a foot, a stalk, and a capsule.

    4. is the dominant generation.

    5. All of these are correct.

  9. Microphylls

    1. have a single strand of vascular tissue.

    2. evolved before megaphylls.

    3. evolved as extensions of the stem.

    4. are found in lycophytes.

    5. All of these are correct.

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  11. How are ferns different from mosses?

    1. Only ferns produce spores as dispersal agents.

    2. Ferns have vascular tissue.

    3. In the fern life cycle, the gametophyte and sporophyte are both independent.

    4. Ferns do not have flagellated sperm.

    5. Both b and c are correct.

  12. Which of these pairs is mismatched?

    1. pollen grain—male gametophyte

    2. ovule—female gametophyte

    3. seed—immature sporophyte

    4. pollen tube—spores

    5. tree—mature sporophyte

  13. In the life cycle of the pine tree, the ovules are found on

    1. needle-like leaves.

    2. seed cones.

    3. pollen cones.

    4. root hairs.

    5. All of these are correct.

  14. Monocotyledonous plants often have

    1. parallel leaf venation.

    2. flower parts in units of four or five.

    3. leaves with petioles only.

    4. flowers with stipules.

    5. Choices b, c, and d are correct.

  15. Which of these pairs is mismatched?

    1. anther—produces microspores

    2. carpel—produces pollen

    3. ovule—becomes seed

    4. ovary—becomes fruit

    5. flower—reproductive structure

  16. Which of these plants contributed the most to our present-day supply of coal?

    1. bryophytes

    2. seedless vascular plants

    3. gymnosperms

    4. angiosperms

    5. Both b and c are correct.

  17. Which of these is found in seed plants?

    1. complex vascular tissue

    2. pollen grains that are not flagellated

    3. retention of female gametophyte within the ovule

    4. roots, stems, and leaves

    5. All of these are correct.

  18. Which of these is a seedless vascular plant?

    1. gymnosperm

    2. angiosperm

    3. fern

    4. monocot

    5. eudicot

  19. Label this diagram of alternation of generations life cycle.

thinking scientifically
  1. Using as many terms as necessary (from both X and Y axes), fill in the proposed phylogenetic tree for vascular plants.

  2. Using Figure 23.1, distinguish between the (a) microphyll and the (b) megaphyll clade.

bioethical issue
Saving Plant Species

Pollinator populations have been decimated by pollution, pesticide use, and destruction or fragmentation of natural areas. Belatedly, we have come to realize that various types of bees are responsible for pollinating such cash crops as blueberries, cranberries, and squash and are partly responsible for pollinating apple, almond, and cherry trees.

Why are we so shortsighted when it comes to protecting the environment and living creatures like pollinators? Because pollinators are a resource held in common. The term commons originally meant a piece of land where all members of a village were allowed to graze their cattle. The farmer who thought only of himself and grazed more cattle than his neighbor was better off. The difficulty is, of course, that eventually the resource is depleted, and everyone loses.

So, a farmer or property owner who uses pesticides is only thinking of his or her field or lawn and not the good of the whole. The commons can only be protected if citizens have the foresight to enact rules and regulations by which all abide. DDT was outlawed in this country in part because it led to the decline of birds of prey. Similarly, we may need legislation to protect pollinators from factors that kill them off. Legislation to protect pollinators would protect the food supply for all of us.

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