Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 37 (p. 752)

1. Though the ancestry of land plants is uncertain, the ancestors are believed to be fresh water green algae. Green algae similar to the ancestor are found in fresh water lakes today. To become successful, land plants had to master three problems in order to photosynthesize. They had to take up water and conserve it against forces of desiccation, they had to take in carbon dioxide, and they had to find a place in the sun.

2. In plants, alternation of generations refers to an alternation of a diploid generation with a haploid generation. Sporophytes are diploid, spore-producing reproductive structures; gametophytes are haploid, gamete-producing reproductive structures. The sporophyte is comparable to an adult animal.

3. Male gametophytes produce antheridia, and female gametophytes produce archegonia. The haploid spores that give rise to these are microspores and megaspores, respectively.

4. Mosses rely upon swimming sperm for reproduction, and swimming sperm require water for locomotion. Sufficient water for swimming is uncommon on tree trunks. Sperm have little stored energy for a long swim up a tree.

5. Gametophytes of seedless plants are the larger of the two generations, are autotrophic and usually longer-lived than the sporophyte generation. Gametophytes of seed plants are the smaller of the two generations by far and are entirely dependent upon the sporophyte for nutrition. They are shorter-lived than the sporophyte.

6. Both the gametophyte and sporophyte are nutritionally independent.

7. A seed is a developing sporophyte in a state of arrested embryonic development surrounded by a protective coat. The seed is crucial because it protects the embryo from drying out or being eaten and provides a source of energy for the growing plant. In angiosperms, the ovule is completely surrounded by the tissue of the sporophyte; in gymnosperms the ovule is partially or totally exposed at pollination. In both plant types, male and female gametophytes develop within the sporophyte and are completely dependent upon it for nutrients and water.

8. Angiosperms differ because they contain ovules enclosed within carpels, the parent sporophytic tissue.

9. Mosses are dispersed by wind-borne spores that are released from a small height above the ground. Pines are dispersed by twirling seeds that are released from high in the tree. Angiosperms have diverse means of dispersal. They are dispersed in seeds or fruit by wind, water, or animals. Fruits and seeds are modified in many ways to promote such dispersal.