4.3.3. 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.

4.3.4. 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.

4.4. 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.

4.4.1. 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.

Kinds of plant tissues

One of the chief differences between plants and animals is the location of cell division in the organism. Many kinds of the cells in an animal’s body may undergo division. In plants, cells divide only in specific areas called meristems. The meristems located near the tips of roots and stems are called apical meristems. The apical meristems produce the most rapid growth.

As the apical meristems produce new cells, the root or stem grows longer. The lengthening of roots and stems is called primary growth. Young plants show primary growth as they send up green shoots. Mature plants also show primary growth in the lengthening of their roots and stems.

The cells produced by meristems become specialized and carry out particular plant functions. Differentiation is the name for this speciation. Plant cells differentiate into three basic kinds of tissue: the epidermis, vascular tissue, and ground tissue.

Roots

Through variations in their roots, plants haven adapted too many different kinds of soil. Some roots split rocks as they grow through cracks in the rock. Others push through dense clay, or anchor plants in shifting sand. In any environment, roots carry out three basic functions. Roots anchor the plant, absorb water and minerals from the soil, and store food produced in leaves and the green stem.