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CHAPTER 29 and 30 - Questions and Answers

Identify three derived traits that distinguish plants from charophyceans and facilitate life on land. Explain.

Spore walls toughened by sporopollenin; multicellular, dependent embryos; cuticle. Such traits help prevent drying out.

Identify each structure as either haploid or diploid: (a) sporophyte; (b) spore; (c) gametophyte; (d) zygote; (e) sperm; (f) egg

a. diploid, b. haploid; c. haploid; d. diploid; e. haploid; f. haploid

Describe the evidence linking plants to a charophycean ancestry.

Land plants share some key traits only with charophyceans: rosette cellulose-producing complexes, presence of peroxisome enzymes, similarity in sperm structure, and similarity in cell division (the formation of a phragmoplast). Comparisons of nuclear and chloroplast genes also point to a common ancestry.

How do bryophytes differ from other plants?

Bryophytes are described as nonvascular plants because they do not have an extensive transport system. Another difference is that their life cycle is dominated by gametophytes rather than sporophytes.

Give three examples of how structure fits function in bryophytes.

Large surface area of protonema enhances absorption of water and minerals; the vase-shaped archegonia protect eggs during fertilization and transport nutrients to the embryos via placental transfer cells; the stalk-like seta conducts nutrients from the gametophyte to the capsule where sperm are produced; the peristome enables gradual spore discharge; stomata enable CO2/O2 exchange while minimizing water loss. Lightweight spores are wind-dispersed; mosses can lose water without drying and rehydrate when moisture is available.

NOTE: Fossils dating to about 420 million years ago exhibits dichotomous (Y- shaped) branching and terminal sporangia. These traits characterize living vascular plants but are lacking in bryophytes (nonvascular plants).

NOTE ON LYCOPHYTES: Many lycophytes grow on tropical trees as epiphytes, plants that use other plants as a substrate but are not parasites. Other species grow on temperate forest floors. In some species, the tiny gametophytes live aboveground and are photosynthetic. Others live underground, nurtured by symbiotic fungi.

Sporophytes have upright stems with many small leaves, as well as ground-hugging stems that produce dichotomously branching roots. In club mosses, sporophylls are clustered into club-shaped cones (strobili). Spike mosses are usually smaller and often grow horizontally. Quillworts, named for their leaf shape, are a single genus that lives in marshy areas. Club mosses are homosporous, whereas spike mosses and quillworts are heterosporous. Spores are released in clouds and are so rich in oil that magicians and photographers once ignited them to create smoke or flashes of light.

What are a few key differences between seedless vascular plants and bryophytes?

Some characteristics that distinguish seedless vascular plants from bryophytes are a sporophyte-dominant life cycle, the presence of xylem and phloem, and the evolution of true roots and leaves.

What is the major difference between most lycophytes and most ferns and their relatives?

Most lycophytes have microphylls, whereas ferns and most fern relatives have megaphylls.

Contrast sperm delivery in seedless plants with sperm delivery in seed plants.

To have any chance of reaching the eggs, the flagellated sperm of seedless vascular plants must rely on swimming through a film of water, usually limited to a range of less than a few centimeters. In contrast, the sperm of seed plants are produced within durable pollen grains that can be carried long distances by wind or by animal pollinators. Although flagellated in some species, the sperm of most seed plants do not require water because pollen tubes convey them directly to the eggs.

What additional features of seed plants, not present in seedless plants, contributed to their enormous success on land?

The reduced gametophytes of seed plants are nurtured by sporophytes and protected from stress, such as drought conditions and UV radiation. Pollen grains have tough protective coats and can be carried long distances, facilitating widespread sperm transfer without reliance on water. Seeds are more resilient than spores, enabling better resistance to environmental stresses and wider distribution.

Based on Figure 30.4, explain why the various types of gymnosperms can be described as being similar yet distinctive.

Although gymnosperms are similar in not having their seeds enclosed in ovaries and fruits, their seed-bearing structures vary greatly. For instance, cycads have large cones, whereas some gymnosperms, such as Ginkgo and Gnetum, have small cones that look somewhat like berries, even though they are not fruits. Leaf shape also varies greatly, from the needles of many conifers to the palmlike leaves of cycads to Gnetum leaves that look like those of flowering plants.

Explain how the pine life cycle (see Figure 30.6) reflects basic characteristics of seed plants.

The life cycle illustrates heterospory, as ovulate cones produce megaspores and pollen cones produce microspores. The reduced gametophytes are evident in the form of the microscopic pollen grains and the microscopic female gametophyte within the megaspore. The egg is shown developing within an ovule, and the pollen tube is shown conveying the sperm. The figure also shows the protective and nutritive features of a seed.
It has been said that an oak is an acorn is way of making more acorns. Write an explanation that includes these terms: sporophyte, gametophyte, ovule, seed, ovary, and fruit.

In the oak’s life cycle, the tree (the sporophyte) produces flowers, which contain gametophytes in pollen grains and ovules; the eggs in ovules are fertilized; the mature ovaries develop into dry fruits called acorns; and the acorn seeds germinate, resulting in embryos giving rise to seedlings and finally to mature trees, which produce flowers and then acorns.

Compare and contrast a pine cone and a flower in terms of structure and function.

Pine cones and flowers both have sporophylls, modified leaves that produce spores. Pine trees have separate pollen cones (with pollen grains) and ovulate cones (with ovules inside cone scales). In flowers, pollen grains are produced by the anthers of stamens, and ovules are within the ovaries of carpels. Unlike pine cones, many flowers produce both pollen and ovules.

Explain the use of the terms monocot, dicot, and eudicot.

Traditionally, angiosperms have been classified as either monocots or dicots, based on certain traits, such as the number of cotyledons. However, recent molecular evidence reveals that while monocots are a clade, dicots are not. Based on phylogenetic relationships, most dicots form a clade, now known as eudicots.

Explain why it is accurate to consider plant diversity to be a nonrenewable resource.

Because extinction is irreversible, it decreases the total diversity of plants, many of which may have brought important benefits to humans.
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