Ferns - one of the oldest and most primitive plants

If you often go out, you must have seen some plants with feathery leaves on the roadside, on the mountain wall or under the big tree. They are light and green, but you seem to have never seen them bloom and bear fruit. These special plants are ferns. Ferns are one of the oldest and most primitive plants in the plant kingdom. Ferns have no flowers, fruits or seeds. This plant reproduces by spores. Although it grows under the shadow of seed plants, it still finds a way out of life and becomes one of the most common plants in the world.

Most ferns grow in the soil, often under big trees or in the shade. But some ferns are different. Unlike the general shade-loving ferns, oak ferns often grow high up in trees, and look like clumps of feathers from a distance. Oak ferns like light very much. On the tall tree trunks, oak ferns can receive more sunlight because they are less blocked by branches and leaves. Oak ferns are epiphytes, and the bark of the tree trunks they climb on is often mottled and rough. The texture on the bark makes it easier for the root hairs on the rhizomes to climb and fix. The name of oak ferns is related to its special leaf type. Its leaves are divided into sporophylls and nutrient leaves. The leaf shapes of its nutrient leaves and sporophylls are completely different, and this condition is called heteromorphic leaves. Among them, the verdant and upright feathery leaves are sporophylls. They are similar to the leaves of ordinary ferns, growing obliquely upward and standing upright on the tree trunks. Another type of leaf-like leaves that are firmly attached to the tree trunk are nutrient leaves. Generally, we see them as hard dry films or hard leathers. The new nutrient leaves are light green and will soon turn to dry brown. The yellow nutrient leaves below are stacked layer by layer, and the young leaves are slightly green. The two colors complement each other, like blooming flowers. The main function of the nutrient leaves is to collect and transport nutrients, and at the same time, they play a role in assisting the fixation of plants and moisturizing. Because the shape of the nutrient leaves is quite similar to the leaves of oaks, and it is a fern plant, it is named oak fern.

In the cool and humid places beside the gurgling streams in the mountains, there are patches of straight, hollow and segmented plants. This strange plant can also be seen in some parks. This is not bamboo. Their plant height is only about one meter, which is much shorter than the common bamboo. This patch of plants looks like a brush inserted into the ground. This is the fern plant that looks very different from ordinary ferns and more like bamboo or grass, horsetail. Horsetail seems to have no leaves, but from a macro perspective, you can see that its leaves are very small, have shrunk and degenerated into fused scales, and form a toothed collar-shaped sheath on the node. As a fern, horsetail naturally reproduces by spores. From June to August every year, a sporangium spike will grow on the top of the horsetail stem, and the tip looks like a small black tip. The main body of the spike is a hexagonal shield-shaped sporangium leaf. Spores drift out from the mature sporangium spike, and horsetail can also reproduce.

We can't help but wonder, why is such a good plant called "thief"? In fact, this is related to the special stems of horsetail: Li Shizhen said that this grass is segmented and has a rough surface. It can be used to polish wood, which can make the surface of the wood smooth and clean, just like a wood-eating insect. In ancient times, pests that feed on seedlings were called thieves, so the name "horsetail" came into being.

Observing from a macro perspective, there are vertical ridges on the stems of horsetail, usually 18 to 30 in number. There are many small, shiny, warty protrusions on the ridges, making the surface quite rough. In addition, it is rich in silica, so it is quite hard, and the entire surface becomes like sandpaper, which can be used to polish objects. We cut the horsetail and saw that its cross section is hollow. From a microscopic perspective, we can see that the stem is a network structure of many small cavities, just like the corrugated paper of a courier box. This makes the stems of horsetail highly resistant to compression, which can provide good support for the hollow stems.

Horsetail has nodes but it is not called knotweed. Knotweed is another plant of the genus Equisetum. The main difference between the two plants is in the branching. Horsetail generally does not branch, and even if it branches, it is only a small amount, while knotweed branches in whorls, growing many branches horizontally like a tree with branches and leaves. Knotweed has a good adsorption effect on heavy metals such as cadmium, copper, and zinc, so it has a very positive improvement effect on land contaminated by heavy metals. It can generally be used as a super-accumulator plant for the restoration of heavy metal contaminated land.

In a canyon in southwest China, there are large areas of "living fossils" of terrestrial plants, and also the only woody fern plant today, Alsophila spinulosa. Compared with most dwarf ferns, Alsophila spinulosa is a giant among ferns, ranging in height from a few meters to more than ten meters, like a big tree, so it is also called tree fern. Alsophila spinulosa has a beautiful appearance, tall and straight, and its crown is like a giant umbrella, which has a high ornamental value. Although Alsophila spinulosa looks like a palm tree, it does not have the wooden trunk of a tree. Through the naturally generated cross-section, we can get a glimpse of its structure: its main trunk is an upright rhizome, and a large number of dense roots and fibers are wrapped outside to form a protective outer structure, which supports the huge body of Alsophila spinulosa and maintains an upright posture. Alsophila spinulosa likes warm and humid conditions and grows by valley streams. The small tentacles on the main trunk allow Alsophila spinulosa to absorb moisture from the air to the greatest extent. The Alsophila species are common plants in the Mesozoic Era, but due to changes in the environment and climate, their distribution area is shrinking. In my country, all species of the Alsophila species are national second-class protected plants, and every Alsophila species is worth our protection.

From the dark surface to the tall tree canopy, in the long time of nearly 400 million years, ferns have preserved a huge amount of genes, most of which have many unique survival mechanisms. They can be used as food and medicine, or can be used for landscaping, helping humans purify soil, air and water quality. Each fern contains great value and is worthy of our protection and continuous research.

Author: Small Vision

Reviewer: Wang Kang, Director of the National Botanical Gardens Science Museum, Professor-level Senior Engineer

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.