Ginkgoes

The ginkgo is the last species of a once widespread family of trees. They are called "living fossils" because virtually no ginkgoes are now known to live in the wild. Ginkgoes have unique fan-shaped leaves. Pollen is produced in catkins, or small conelike structures that dangle from the tree branches. Only male ginkgoes produce catkins. After fertilization, female trees produce fleshy seeds that look like pale berries.

Gnetales

The order Gnetales includes trees and woody vines that have traits of gymnosperms, but some species have reproductive structures like those found in angiosperms. One species produces an edible plumlike fruit. Another, Welwitschia, grows only in the deserts of southern Africa.

Angiosperms

Angiosperms are flowering plants. They produce seeds enclosed in fruits, as opposed to the uncovered seeds of the gymnosperms. To botanists, the history of angiosperms is still a mystery, because the flowering plants appeared so suddenly in the fossil record about 130 million years ago. Of the more than 250,000 species of vascular plants, about 235,000 are angiosperms. They include most green plants. Oaks, birches, vegetables and grasses are all angiosperms.

Physical structure

Angiosperms are classified according to the number of cotyledons in their seeds. Plants with one cotyledon are called monocots; those with two are called dicots. Monocots include about 89,000 species; dicots have about 1700,000 (Table 4.1).

Angiosperms are also commonly classified by the characteristics of their stem tissues. In woody plants, the xylem and phloem produce cumulative layer of new plant tissue that increase the width of the stem and make it strong and hard. This cumulative growth is called secondary growth. Woody plants often live for many years and tend to produce relatively few seeds. Most woody plants are dicots, such as maple, walnut and chestnut trees. Almost all monocots are herbaceous plants. Their stems are usually green and lack secondary growth. Herbaceous plants typically have shorter lives and produce more seeds than woody plants. Corn and orchids are examples of herbaceous monocot plants. Tomatoes and spinach are herbaceous dicots.

Date: 2014-12-22; view: 1064