Chapter 33 Summary
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Core Concepts Summary
33.1 Angiosperms 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. page 690
The other 10% of plant species is distributed among the other six major groups of plants. page 690
Angiosperms first appear in the fossil record about 140 million years ago, more than 300 million years after plants first moved onto land. page 690
The evolution of angiosperms resulted in a rapid and dramatic increase in total plant diversity. page 690
As angiosperm diversity increased, other plant groups, such as gymnosperms and ferns, declined in diversity. page 690
Moist tropical rain forests dominated by angiosperms provided new types of habitat into which other plants could evolve. page 690
33.2 Bryophytes form persistent, photosynthetic gametophytes and small, unbranched sporophytes; today, they grow in environments where the ability to pull water from the soil does not provide an advantage.
Bryophytes constitute a paraphyletic group consisting of three groups of plants: mosses, liverworts, and hornworts. page 690
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. page 691
The persistent component of the life cycle is the gametophyte. Sporophytes range from tiny and non-
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. page 692
Sphagnum moss is the dominant plant of peat bogs. page 692
Sphagnum moss produces water-
33.3 Spore-
Only two groups of seedless vascular plants have living relatives today: lycophytes, and ferns and horsetails. page 693
Three hundred million years ago, lycophytes included large trees that dominated swamp forests. Today, lycophytes are small plants that either grow in the forest understory as epiphytes or occur in shallow ponds. page 694
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. page 697
Although ferns have a long history, most present-
33.4 Gymnosperms produce seeds and woody stems and are most common in seasonally cool 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. page 699
Gymnosperms are composed of four groups of woody plants: cycads, ginkgos, conifers, and gnetophytes. page 699
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-
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. page 700
Conifers include the tallest and longest-
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 having xylem formed entirely of tracheids. page 701
The gnetophytes are a small group, containing only three genera and few species, that has independently evolved xylem vessels. page 702
33.5 Angiosperms are distinguished by flowers, fruits, double fertilization, and xylem vessels; their diversity is the result of traits that increase the efficiency of completing their life cycle and building their bodies.
Xylem vessels make it possible for angiosperms to have a diversity of form and to grow toward light. page 704
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Angiosperms reproduce quickly and with less use of resources as a result of pollination and double fertilization, allowing them to diversify into ephemeral habitats. page 704
Angiosperm diversity may result as much from low rates of extinction as from high rates of speciation. page 704
Plants having flowers can reproduce even if they are far apart, allowing rare species to persist and reproduce. page 704
The angiosperm phylogeny indicates a major split between two diverse groups, monocots and eudicots. page 704
Monocots include grasses, coconut palms, bananas, ginger, and orchids. page 704
Monocots have a single cotyledon, or embryonic seed leaf, and they do not form a vascular cambium. page 705
Eudicots are characterized by pollen grains with three openings through which the pollen tube can grow. page 707
Eudicots are diverse, and include many familiar plants such as legumes, roses, cabbage, pumpkin, coffee, tea, and cacao. page 708
Modern agriculture is based on just a few plant species with low genetic diversity, making crops vulnerable to pests and pathogens. page 709
Self-Assessment
Using information in Fig. 33.2, draw a pie chart that depicts proportional plant diversity today.
Self-Assessment 1 Answer
Imagine a world in which mosses, liverworts, and hornworts form a monophyletic group. How would your ability to infer what the first land plants looked like be affected?
Self-Assessment 2 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 group. 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.
Describe three environments that allow bryophytes to coexist with vascular plants.
Self-Assessment 3 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.
Describe the habitats in which lycophytes are found today.
Self-Assessment 4 Answer
Lycophytes can be found in diverse habitats ranging from the tropical rain forest to the Arctic tundra. Some live as epiphytes in tropical forests or in seasonally dry climates. Quillworts live along the margins of lakes and slow-
moving streams. List three ways that ferns, which lack secondary growth, are able to elevate their leaves and thus access more sunlight.
Self-Assessment 5 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 to a slender, but upright stem; (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.
Describe how fern diversity has been affected by the evolution of the angiosperms.
Self-Assessment 6 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.
Contrast the ways in which the evolution of angiosperms has affected the distribution of cycads and of conifers.
Self-Assessment 7 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, occur in small fragmented populations.
Explain how xylem produced by conifers differs from that of angiosperms and how that difference may have influenced the present-
day distribution of conifers. Self-Assessment 8 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 have a competitive advantage over conifers in many environments. In cold and dry climates, the size of angiosperm channels is constrained by the risk of cavitation, thereby leveling the playing field for the conifers. In some environments, conifers may even have a competitive edge. Compare the movement of pollen from 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. Self-Assessment 9 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. Name two features that have contributed to the diversity and success of angiosperms and discuss possible advantages of these features.
Self-Assessment 10 Answer
One feature that contributes to the diversity and success of angiosperms is flowers adapted for animal pollination. Animal pollination allows angiosperms to reproduce even if they are far apart. As a result, rare species can persist even at a low population density. Animal pollination can also contribute to reproductive isolation and thus the formation of new species.
Another feature contributing to the diversity and success of angiosperms is their differentiated wood, consisting of xylem vessels to transport water and fibers to provide mechanical support. Xylem vessels allow angiosperms to produce stems that are highly conductive of water with relatively little investment in structural costs. This efficiency in wood construction is thought to underlie the diversity of form exhibited by angiosperms, which in turn allows more species to coexist by reducing competition for the same resources.