Chapter 31 Self-Assessment Questions

How has the evolution of angiosperms affected the distribution of cycads and of conifers?

Cycad and conifer distribution changed dramatically with the appearance of angiosperms. Conifers are now commonly found in northern latitudes, whereas cycads, which were once widely distributed, occur in small fragmented populations.

Explain how features found in angiosperms may have allowed the evolution of herbaceous plants, including annual plants.

Efficient xylem allows the production of inexpensive stems.

If the stomata of bryophytes and of vascular plants are not the result of convergent evolution, what is an alternative explanation for their appearance in these groups?

An alternative explanation is that stomata evolved once, in the common ancestor of mosses, hornworts, and vascular plants.

Which three species make up more than two-thirds of human food consumption?

Corn, rice, and wheat make up the majority of human food consumption.

Which factor is thought to limit the stature and productivity of the gametophyte generation?

The stature and productivity of the gametophyte generation are limited by the reliance on surface moisture for both fertilization and hydration.

Where are lycophytes found today?

Lycophytes can be found in diverse habitats ranging from the tropical rain forest to the Arctic tundra. Many live as epiphytes in tropical forests; quillworts live along the margins of lakes and slow-moving streams.

What are two features (and their possible advantages) that have contributed to the diversity and success of angiosperms?

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, in turn, to the formation of new species. Another feature contributing to the diversity and success of angiosperms is their differentiated wood, which includes both 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.

How has fern diversity been affected by the evolution of the angiosperms?

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.

How did the addition of a sporophyte generation contribute to the success of plants on land?

The addition of a second generation meant that natural selection could lead to traits (such as an erect form) that enhanced dispersal, without simultaneously decreasing the chances of fertilization.

How was plant diversity affected by the evolution of angiosperms?

The diversity of non-angiosperm groups decreased, but the total number of plant species increased.

Double fertilization evolved independently in gnetophytes and in flowering plants. What are two other features usually associated with angiosperms that evolved independently in gymnosperms?

Xylem vessels evolved convergently in gnetophytes and angiosperms. Insect pollination evolved convergently in cycads and angiosperms. Fleshy tissues to assist seed dispersal evolved convergently in early seed plants (reflected today in ginkgo and cycad seeds), conifers such as juniper and yew, and flowering plants.

Are there any habitats where a bryophyte might be more productive than a vascular plant?

A bryophyte might be more productive than a vascular plant in any place without soil, such as on top of rocks.

How does the xylem produced by conifers differ from that of angiosperms? How could this difference have influenced the present-day distribution of conifers?

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.

What are three ways that ferns, which lack secondary growth, are able to elevate their leaves and thus access more sunlight?

Ferns are able to elevate their leaves in three ways: (1) by producing upright stems with 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 growing as epiphytes.

How does the movement of pollen from an animal-pollinated angiosperm compare with that from a wind-pollinated conifer?

For wind pollination of conifers to be effective, a large number of individuals of the same species must be present 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. In this case, 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.

What are some of the distinctive features of monocots?

Monocots produce only a single cotyledon; monocot roots are produced continuously directly from the stem; many monocots produce horizontal stems that run along, or slightly under, the surface of the ground; and the strap-shaped leaves of many monocots elongate from a zone of cell division located at the base of the leaf, which surrounds the stem to form a sheath. Monocots do not form a vascular cambium. In most monocot stems, the vascular bundles are distributed throughout the cross section, as opposed to being arranged in a ring.

Which features of the sporophyte generation enhance the likelihood of successful spore dispersal?

Two factors of the sporophyte generation that enhance the likelihood of spore dispersal are an erect morphology and the ability to dry out before releasing spores.

Why did xylem and phloem evolve only in the sporophyte generation?

Xylem and phloem help overcome size limitations associated with relying solely on diffusion for the movement of materials. Increased size and stature tend to increase dispersal success, so these characteristics would have been selected for in the sporophyte generation. In contrast, there would not have been strong selection for increased size in the gametophyte generation because size and stature tend to decrease the likelihood of fertilization that relies on releasing swimming sperm into the environment.