22. The Plants Kingdom


22.11. The Coevolution of Plants and Animals


The first terrestrial organisms were plants. Shortly after the plants became established on land, animals, such as insects and amphibians, arrived. Thus, terrestrial plants and animals have a long history of interaction, which has had an influence on the evolution of each group. There are many examples of their coevolution.

Most flowering plants are pollinated by insects or other animals. Insect-pollinated plants produce flowers that are showy, have nectar, and produce odors. Many flowers that are pollinated by birds are red and produce much nectar. Some flowers bloom only at night and are pollinated by moths or bats.

Grasses and grazers have coevolved. Grasses have silica in their cell walls. This is a very hard material, and it tends to wear down the teeth of grazers. Most grazing animals have very long teeth, which can accommodate a lifetime of wear. Grasses also differ from most other plants in that their leaves and stems grow from the base of the plant rather than from the tip. Thus, they can withstand regularly having the tips of their leaves chewed off.

Many kinds of flowering plants produce large, nutritious fruits, which animals use for food, in the process distributing the seeds. There are even seeds that will not germinate unless they have passed through the gut of an animal. Birds eat small fruits and their seeds, which are dispersed when the birds defecate. In tropical forests, many trees have very large fruits, which are eaten by monkeys. They eat the fleshy part of the fruit and drop the seeds.

Browsers are animals that eat the leaves and small twigs of woody plants. Many kinds of woody plants have thorns, making this task more difficult, but browsers have techniques or structures that allow them to put up with the prickly deterrents.

Plants produce a variety of chemicals. Some of these chemicals are toxic or irritating and deter certain animals from eating portions of the plant. Others chemicals produce odors that help animals locate flowers or fruits. These attractive odors aid the plant by assuring that pollination and seed dispersal will take place. Humans use many of these plants and the chemicals they produce as spices and flavorings (Outlooks 22.2).



Spices and Flavorings

Think about all the plant materials we use to season our foods. Black pepper comes from the hard, dried berries of a tropical plant. Piper nigrum. Cayenne pepper is made from the ground-up fruits of Capsicum annuum, and the hot, spicy chemical in the fruit and seeds is known as capsaicin. The seeds of the dill plant, Anethum graveolens, are used to flavor pickles and many other foods. The dried or fresh leaves of many herbs such as thyme, rosemary, chives, and parsley, are also used as flavorings. Many kitchen cabinets also contain cinnamon from the bark of a tree found in India; cloves, which are the dried flower buds of a tropical tree; ginger from the root of a tropical plant of Africa and China; nutmeg from the seed of a tropical tree of Asia; and saffron from the dried, fragrant stigmas of the crocus flower (Crocus sativus) from Spain. Saffron's small size and difficulty in harvesting the stigmas make it the most expensive spice on Earth.

Centuries ago, spices such as these were so highly prized that fortunes were made in the spice trade. Beginning in the early 1600s, ships from Europe regularly visited the tropical regions of Asia and Africa, returning with cargoes of spices and other rare commodities that could be sold at great profit. Consequently, India has been greatly influenced by Britain, Indonesia has been greatly influenced by the Netherlands, and the development of various portions of Africa has been influenced by Britain and France.




26. Describe how the coevolution of grasses and grazing animals affected both kinds of organisms.

27. In what ways do some flowering plants encourage insects to visit them?



The plant kingdom is composed of eukaryotic, multicellular organisms that can manufacture their own food through photosynthesis. All plants have alternation of generations with a haploid, gametophyte generation and a diploid, sporophyte generation. The gametophyte produces sex cells (eggs and sperm) by mitosis and the sporophyte produces haploid spores by meiosis. There is a general trend in the evolution of plants for less dependence on a moist environment and an increase in the importance of the sporophyte generation. The nonvascular plants (mosses and related organisms) lack vascular tissue, have the gametophyte generation as the dominant generation, and have swimming sperm. The seedless vascular plants (ferns and related organisms) have vascular tissue, a dominant sporophyte generation, and swimming sperm. Seed-producing vascular plants (cone-bearing and flowering plants) have vascular tissue, a dominant sporophyte generation, the development of pollen, and seeds.

Vascular tissue consists of xylem, which carries water and inorganic nutrients from the roots to the leaves; and phloem, which carries organic molecules from the leaves to places where they are needed. The presence of vascular tissue is associated with the specialization of body parts into roots, stems, and leaves. Roots anchor the plant and absorb water and nutrients from the soil. Leaves are generally thin, flat structures specialized for photosynthesis. Stems connect the roots to the leaves and position the leaves to receive sunlight.

Pollen is the male gametophyte plant and is either carried by wind or animals from one plant to another. Wind-pollinated plants produce huge amounts of pollen. Some flowering plants are pollinated by animals and produce showy flowers and nectar to attract animals.

Plants respond to their environment in complex and interesting ways. They respond to the position of the sun, day length, contact with other objects, and injury.


Basic Review

1. The sporophyte generation

a. is diploid.

b. produces spores.

c. is the dominant stage in angiosperms.

d. All of the above are correct.

2. Gymnosperms produce fruits. (T/F)

3. All of the following are vascular plants except

a. ferns.

b. mosses.

c. horsetails.

d. gymnosperms.

4. There are two kinds of vascular tissue: xylem and _____.

5. The central part of a woody stem consists of

a. xylem.

b. phloem.

c. silica.

d. bark.

6. Nearly all gymnosperms are woody perennials. (T/F)

7. Plants are able to sense changes in their environment. (T/F)

8. From a plant’s point of view, a fruit

a. is used to feed seeds.

b. is used to disperse seeds.

c. requires little energy to produce.

d. All of the above are correct.

9. The pollination of plants by insects is an example of _____.

10. Which one of the following plant groups has the largest number of species?

a. conifers

b. ferns

c. angiosperms

d. mosses

11. Which of the following does not have swimming sperm?

a. moss

b. fern

c. gymnosperm

d. horsetail

12. Phototropism is a response to _____.

13. Phloem

a. cells are hollow.

b. is found in the center of a tree.

c. has companion cells.

d. is found in mosses.

14. Fossils of mosses that are trees have been found. (T/F)

15. All plants have a gametophyte generation. (T/F)



1. d 2. F 3. b 4. phloem 5. a 6. T 7. T 8. b 9. coevolution 10. c 11. c 12. light 13. c 14. F 15. T


Thinking Critically

Potato Facts

Some people say the ordinary “Irish” potato is poisonous when the skin is green, and they are at least partly correct. A potato develops a green skin if the potato tuber grows so close to the surface of the soil that it is exposed to light. An alkaloid called solanine develops under this condition and may be present in toxic amounts. Eating such a potato raw can be dangerous. However, cooking breaks down the sola- nine molecules and makes the potato as edible and tasty as any other. The so-called Irish potato is of interest historically. Its country of origin is only part of the story. Check your local library to find out about this potato and its relatives. Are all related organisms edible? Where did this group of plants develop? Why is it called the Irish potato?