4.2.1. Physical structure

Some ferns are delicate plants scarcely 3 mm (0.04 in.) tall. In contrast, huge tree can reach 28 mm (93 ft.). Few plant phyla show such wide variation.

Generally ferns are supported by underground rhizomes that produce roots. Each fern leaf, called a frond, has two parts. The stipe is the stemlike structure that attaches the leaf to the rhizome. The blade is the broad, green part of the leaf that carries on photosynthesis. Fronds spread out over a wide area. In this way they catch the dim light that reaches the forest floor.

The vascular plants have true roots, stems, and leaves that contain tubelike tissues. These tubelike tissues transport water and food throughout the plant. Ferns can survive outside the water and may grow much larger than mosses and algae because of these specialized tissues. However, like mosses and liverworts, fern must live in a moist environment because water is necessary for their reproduction.

Some tropical rare tree ferns grow to a height of 28 meters. The large feathery leaf of a fern is a frond. The fern’s stem usually grows just beneath the surface of the ground. Small roots grow out from the stem.

4.2.2. Life cycle of ferns

The ferns life cycle is typical of seedless vascular plants. Gametes form the gametophyte generation fuse to form the zygotes. The sporophyte generation grows from the zygote. As the sporophyte matures, it forms a tightly curled leaf (fiddlehead). When the fiddlehead is exposed to sunlight, it gradually opens into a new frond. Haploid fern spores develop in sporangia on the underside of fronds. Sporangia usually occur in clusters called sori. Mature spores are released from the sporangia and dispersed by the wind.

When a spore falls on damp, shaded soil, it begins to grow. It grows into a tiny, heart-shaped plant. This plant is called a prothallus. The prothallus is only about one centimeter across. This plant lives only a few weeks. Since the prothallus is so small, chances are that you have never seen one.

In the life of a fern, the prothallus is the gamete-forming stage, or gametophyte. Eggs and sperm form on the underside of the prothallus. Archegonia and antheridia develop on the underside of the prothallus. Like mosses, ferns must have water to reproduce. When water is present, sperm can swim to the eggs and fertilize them.

Ferns can be cloned by cutting a section containing rhizome (the underground stem) from a mature fern and replanting it in soil. Cloned ferns are genetically identical.

4.3. Gymnosperms

As winter approaches in areas that have cold winter, people notice the plants that stay green. These plants, called evergreens, may be the only sign of plant life in a winter landscape. Other kinds of plants may loose their leaves in the fall. But evergreens stay green because they do not loose all their leaves at the same time. They loose them throughout the year. Many evergreens have leaves that stay on the plant for several years.

At one time there were many more kinds of gymnosperms living on the earth than there are today. Scientists have found the shapes of their leaves, stems, roots, and seeds preserved in rocks.

Four groups of gymnosperms have living representatives.

There are about 700 different kinds of gymnosperms living today. More than 500 of these are Conifers. The second largest group of gymnosperms is the Cycads with about 100 different kinds and the order Gnetales has approximately 70 species. One interesting species belongs to this group, welwitschia. Ginkgoes, the rarest of gymnosperms, have only one species, Ginkgo biloba.

Like conifers the other kinds of gymnosperms have woody stems. All but the ginkgo form their seeds in cones.

Gymnosperms are seed plants. But the seeds of gymnosperms are not covered. That is, their seeds are not protected by any kind of fruit. Instead, the seeds of many gymnosperms grow on cones or conelike parts. So many gymnosperms are also called cone-bearing plants.