Chapter 28

RECAP 28.1

  1. Pollen grains are highly resistant to dehydration and can be carried by wind or pollinators. Once a pollen grain lands on a stigma of a flower, it can grow a pollen tube, from which it releases sperm directly to the megagametophyte.

  2. Seeds protect the embryo from excessive drying and from predators. Seeds can remain in a resting state in soil, often for many years, until conditions become favorable for growth. Many seeds also aid in dispersal by wind, water, or animals. When growth begins, the seed provides the developing sporophyte with the food reserves it needs.

  3. Wood provided structural stability for the stems of plants and allowed them to reach great heights. The forests of the world are able to exist because of this evolutionary innovation.

RECAP 28.2

  1. Some plants require fire for seeds to be released. For example, fire is needed to open the tightly closed cones and release the seeds of some pines, which are then able to germinate.

  2. The megagametophyte is the female gametophyte, or multicellular haploid tissue contained within the ovary of all seed plants. The pollen grain is the multicellular male gametophyte, which produces the sperm that fertilize the megagametophyte.

  3. These fleshy cones serve to attract animals, much as the fruits of flowering plants do. The animals eat the fleshy cones and the seeds they contain, and the seeds pass through the digestive tracts of the animals and are dispersed by their feces.

RECAP 28.3

  1. One sperm produces the diploid zygote, and another produces the (usually) triploid endosperm that will provide nourishment for the developing embryo.

  2. Pollination refers to the arrival of pollen on a stigma. After the pollen produces a pollen tube, it releases sperm to the megagametophyte, resulting in fertilization.

  3. Fruits that attract animals to a food resource often have a fleshy, edible ovary. Animals gather or eat the fruit, and the seeds may pass through the digestive tract alive and germinate after they are deposited in the animal’s stool. In some cases, such as tree nuts, the seeds are themselves edible, and the fruit serves as a hard, protective covering. Animals such as squirrels gather the nuts, eat some, and store others for later. Some of the stored nuts then germinate into new plants. In contrast, some seeds have sticky surfaces that adhere to animals that brush by the fruiting plant. If you have ever had to pull burrs off your clothes or your pet, you have pulled out fruits of plants that are being dispersed in this manner.

  4. Flowers are the site of pollination and fertilization and, in many species, serve to attract pollinators. Fruits may provide protection or means of dispersal for the seeds; often, the fruits are attractive to an animal that then disperses the seeds in its feces. The seeds protect the developing embryos until conditions are favorable for germination.

RECAP 28.4

  1. There are many possible answers, some of which are shown below.

    Product Plant Source Medical Application
    Atropine Belladonna Dilate pupils for eye examination
    Bromelain Pineapple stem Control tissue inflammation
    Digitalin Foxglove Strengthen heart muscle contraction
    Ephedrine Ephedra Ease nasal congestion
    Menthol Japanese mint Relieve coughing
    Morphine Opium poppy Relieve pain
    Quinine Cinchona bark Treat malaria
    Taxol Pacific yew Treat ovarian and breast cancers
    Tubocurarine Curare plant Muscle relaxant (used in surgery)
    Vincristine Periwinkle Treat leukemia and lymphoma

  2. Among the possible answers: Plants produce oxygen and remove carbon dioxide from the atmosphere, and they play important roles in forming soils and renewing soil fertility. Plant roots help hold soil in place, providing protection against erosion by wind and water. Plants also moderate the local climate in various ways, such as by increasing humidity, providing shade, and blocking wind.

  3. Grasses produce the grain crops (including rice, wheat, and corn) that account for a large percentage of the calories in human diets. Grasses are also the basis for feeding grazing animals, such as cattle, which provide humans with meat and dairy products.

WORK WITH THE DATA, P. 596

  1. image

  2. One approach to this problem is to calculate a linear trend line for survivorship of Verbascum blattaria seeds by calculating a linear regression line (see Appendix B) and then projecting it forward in time to the point at which it intersects zero percent survival. The resulting regression equation is y = 102.09 – 0.62x. The graph plotted in answering Question 1 uses this approach and predicts that the last Verbascum blattaria seeds should germinate in about year 165 of the experiment (set y to 0, and solve for x using the regression equation; the result is x = 164.7 years). This approach assumes a linear decline in viability of the seeds. It may be more reasonable to assume an exponential decay in seed viability (similar to radioactive decay; see Figure 24.1). If seeds decay exponentially, then we would expect some low level of survivorship of Verbascum blattaria seeds well beyond year 165.

    A-31

  3. At least four factors are related to seed survivorship:

    1. Size of the seed: larger seeds have more food reserves (endosperm).

    2. Thickness of the seed coat: thicker seed coats provide better protection of the seed.

    3. Density of the seed coat: tougher seed coats provide better protection of the seed.

    4. Level of dormancy of the embryos: deeper dormancy results in longer survivorship.

FIGURE QUESTIONS

Figure 28.2 The haploid gametophyte stage of the moss is the most prominent stage, and the stage we recognize most readily as a moss. The haploid stage of ferns, in contrast, is very inconspicuous, so we usually see the larger diploid sporophyte. In flowering plants, the haploid gametophytes are rarely seen, except as pollen.

Figure 28.17 Although rodents destroy the seeds of nut-bearing plants when they eat them, these plants produce far more nuts than rodents can immediately eat. Rodents gather extra nuts and stash them for later consumption in or around their nests, often burying them in soil in the process. Many of these extra nuts then germinate before they are consumed by the rodents.

APPLY WHAT YOU’VE LEARNED

  1. The bright red color of M. cardinalis flowers is consistent with of the expectations of a bird-pollinated species. The nectar guides (stripes on the petals) of M. lewisii flowers are consistent with the expectations of a bee-pollinated species.

  2. The corolla of M. cardinalis is narrow and tubelike, while that of M. lewisii is broader and flatter. The narrow tubular corolla of M. cardinalis is an adaptation that accommodates the long beaks and tongues of its hummingbird pollinators, while restricting pollination by other species.

  3. Variation at the yup locus did not significantly affect visitation by hummingbirds, but bees were much less likely to visit the CC genotype (red) flowers.

  4. Although birds showed a slightly higher preference for red flowers over pink flowers, the difference shown here is not statistically significant. However, bees visit red flowers at a significantly lower rate than pink flowers. It may be that the primary function of red flowers is to discourage visitations by bees so that pollination is restricted to hummingbirds.