CHAPTER SUMMARY
33.1 PLANT DIVERSITY IS DOMINATED BY ANGIOSPERMS, WHICH MAKE UP APPROXIMATELY 90% OF ALL PLANT SPECIES FOUND TODAY.
- There are thought to be nearly 400,000 species of plants living today. Of these, approximately 90% are angiosperms.
- The other 10% of plant species is distributed among the other six major groups of plants.
- Angiosperms first appear in the fossil record about 140 million years ago, more than 300 million years after plants first moved onto land.
- The evolution of angiosperms resulted in a rapid and dramatic increase in total plant diversity.
- As angiosperm diversity increased, other plant groups, such as gymnosperms and ferns, declined in diversity.
- Moist tropical rain forests dominated by angiosperms provided new types of habitat into which other plants could evolve.
33.2 BRYOPHYTES DIVERGED BEFORE THE EVOLUTION OF VASCULAR PLANTS, AND THEY GROW IN ENVIRONMENTS WHERE THE ABILITY TO OBTAIN WATER FROM THE SOIL DOES NOT PROVIDE AN ADVANTAGE.
- Bryophytes constitute a paraphyletic group consisting of three groups of plants: mosses, liverworts, and hornworts.
- Bryophytes are small plants that produce one of two morphological types: a flattened thallus or an upright leafy type. They do not form roots, but instead absorb water through their surfaces.
- The persistent component of the life cycle is the gametophyte. Sporophytes range from tiny and non-photosynthetic in some liverworts to relatively long-lived and photosynthetic in hornworts.
- There are several examples of convergent evolution between bryophytes and vascular plants, including insect dispersal of spores in some mosses and the evolution of internal transport cells in some mosses and liverworts.
- Sphagnum moss is the dominant plant of peat bogs.
- Sphagnum moss produces water-holding cells that allow it to soak up water, and it acidifies the environment. Both characteristics help slow decomposition, so large amounts of organic carbon build up year after year.
33.3 SPORE-DISPERSING VASCULAR PLANTS ARE SMALL, OFTEN EPIPHYTIC PLANTS THAT USUALLY GROW IN MOIST ENVIRONMENTS.
- Only two groups of seedless vascular plants have living relatives today: lycophytes, and ferns and horsetails.
- Three hundred million years ago, lycophytes included large trees that dominated swamp forests. Today, lycophytes are small plants that grow either in the forest understory as epiphytes, or occur in shallow ponds.
- Ferns and horsetails are morphologically diverse. Ferns produce large leaves that uncoil as they grow; horsetails have tiny leaves and whisk ferns with no leaves at all.
- Although ferns have a long history, most present-day species are the result of a radiation that occurred after the rise of the angiosperms.
33.4 GYMNOSPERMS PRODUCE SEEDS AND WOODY STEMS AND ARE MOST COMMON IN COLD OR DRY REGIONS..
- Of the many groups of seed plants that have evolved, only two can be found today: the gymnosperms, with fewer than 1000 species, and the angiosperms, with more than 380,000 species.
- Gymnosperms are composed of four groups of woody plants: cycads, ginkgos, conifers, and the gnetophytes.
- Cycads produce large leaves on stout, unbranched stems. Although they once were widely distributed, they now occur in small, fragmented populations, primarily in the tropics and subtropics. Many cycads are insect pollinated, and all form symbiotic associations with nitrogen-fixing bacteria.
- Ginkgo is the single living species of a group that was distributed globally before the evolution of the angiosperms. Ginkgo is wind pollinated and produces tall, branched trees.
- Conifers include the tallest and longest-lived trees on Earth. Wind-pollinated and largely evergreen, they are found primarily in cool to cold environments.
- Before the angiosperms appeared, conifers were widespread. Their almost complete absence in the tropics and persistence in temperate regions may be due to their dependence upon wind pollination and xylem formed entirely of tracheids.
- The gnetophytes are a small group, containing only three genera and few species, that has independently evolved xylem vessels.
33.5 ANGIOSPERM DIVERSITY IS PARTLY EXPLAINED BY ANIMAL POLLINATION AND XYLEM VESSELS.
- Angiosperm diversity may result as much from low rates of extinction as from high rates of species formation.
- Plants having flowers can reproduce even if they are far apart, allowing rare species to persist and reproduce.
- Xylem vessels make it possible for angiosperms to have a diversity of form and to grow toward light.
- The angiosperm phylogeny indicates a major split between two diverse groups, monocots and eudicots.
- Monocots include grasses, coconut palms, bananas, ginger, and orchids.
- Monocots have a single cotyledon, or embryonic seed leaf, and they do not form a vascular cambium.
- Eudicots are diverse, including many familiar plants such as legumes, roses, cabbage, pumpkin, coffee, tea, and cacao, which yields chocolate.
- Eudicots are characterized by pollen grains with three openings through which the pollen tube can grow.
- Modern agriculture is based on just a few plant species with low genetic diversity, making it vulnerable to pests and pathogens.
Self-Assessment Question 1
Using information in Fig. 33.2, draw a pie chart that depicts proportional plant diversity just before the appearance of the angiosperms.
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Model Answer:
Self-Assessment Question 2
Imagine a world in which mosses, liverworts, and hornworts formed a monophyletic group. How would your ability to infer what the first land plants looked like be affected?
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Model Answer:
If bryophytes formed a monophyletic group, then it would be more difficult to infer whether traits shared by all bryophytes but not by the most closely related group of green algae (e.g., desiccation tolerance) evolved before the last common ancestor between bryophytes and vascular plants or evolved after the split between a monophyletic bryophyte lineage and the vascular plants.
Contrast this scenario with the phylogeny of land plants as we understand it today, with liverworts, mosses, and hornworts each forming an independent lineage. In this case, it is more likely that traits shared by mosses, liverworts, and hornworts were also present in their common ancestor, as otherwise we would have to hypothesize that they evolved independently in liverworts, mosses, and hornworts.
Self-Assessment Question 3
Describe three environments that allow bryophytes to coexist with vascular plants.
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Model Answer:
Bryophytes can be found in almost any environment, including the Antarctic continent. Often bryophytes are epiphytes that can live on other plants because they are not dependent on the soil as a source of water. Bryophytes are a main component of peat bogs, wetlands in which dead organic matter accumulates. Bryophytes can be found in swamps and deserts and every environment in between.
Self-Assessment Question 4
Describe the habitats in which lycophytes are found today.
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Model Answer:
Lycophytes can be found in diverse habitats ranging from the tropical rain forest to the Arctic tundra. Selaginella live in tropical forests or in seasonally dry climates. Quillworts live along the margins of lakes and slow moving streams.
Self-Assessment Question 5
List three ways that ferns, which lack secondary growth, are able to elevate their leaves and thus access more sunlight.
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Model Answer:
Ferns are able to elevate their leaves in three ways: (1) by producing thick roots that descend from the leaves to the ground, adding both vascular capacity and mechanical support; (2) by producing leaves that twine around the stems of other plants, using them as support; and (3) by floating on the surface of water, where they have little competition for sunlight.
Self-Assessment Question 6
Describe how fern diversity has been affected by the evolution of the angiosperms.
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Model Answer:
Overall the diversity of ferns decreased as angiosperms became the dominant species in the ecosystems. However, new environments and habitats created by the angiosperms allowed a subset of ferns to adapt and diversify as they inhabited the new niches.
Self-Assessment Question 7
Contrast the ways in which the evolution of angiosperms has affected the distribution of cycads and of conifers.
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Model Answer:
Cycad and conifer distribution changed dramatically with the appearance of angiosperms. Conifers are now commonly found in northern latitudes, while cycads, once widely distributed, now occur in small fragmented populations.
Self-Assessment Question 8
Explain how xylem produced by conifers differs from that of angiosperms and how that difference may have influenced the present-day distribution of conifers.
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Model Answer:
Angiosperms transport water in wider multicellular xylem channels, whereas conifers transport water in smaller single-celled tracheids. Because angiosperms are able to take up water more efficiently, they can perform a variety of cellular functions at an increased rate. Thus they had a competitive advantage over conifers in many environments. In cold and dry climates, the size of angiosperm channels is constrained by the dangers of cavitation, thereby leveling the playing field for the conifers. In some environments, conifers may even have a competitive edge.
Self-Assessment Question 9
Compare the movement of pollen in an animal-pollinated angiosperm and a wind-pollinated conifer, noting what features of angiosperm reproduction increase the efficiency (or lower the costs) of pollen transfer.
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Model Answer:
For wind pollination of conifers to be effective, there has to be a large number of the conifers in the general area so that the probability of the pollen landing on the correct species is high. Animal-pollinated angiosperms can be less densely populated because the animals will seek out the specific plant. Thus the probability that the pollen will get to the right species is high. Therefore, angiosperms do not have to make as much pollen as conifers because they have a more direct route from plant to plant.
Self-Assessment Question 10
Name several features that might account for the diversity and success of angiosperms and discuss possible advantages of these features.
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Model Answer:
One feature that might account for the diversity and success of angiosperms is that their flowers and fruits are typically specific for that plant’s pollinator of choice. By having animals pollinate and spread seeds, angiosperms can reproduce even if they are far apart. As a result, rare species can persist even at a low population density.
Another feature accounting for diversity and success are angiosperm xylem vessels. These vessels have two cell types, allowing them to separate the functions of water transport and mechanical support. Xylem vessels give angiosperms a greater flexibility of form. Angiosperms may thus have been able to adopt a diversity of new forms, such as vines and sprawling shrubs, that better allowed them to grow toward the light.