Gymnosperms are the most ancient seed plants of our planet. They played an important role in the development of wildlife and continue to occupy a significant position in the life of the Earth. Well-known spruce, pine, fir, thuja, yew or larch and little-known welvichia, saga or ginkgo are all representatives of a group called gymnosperms. The structure and reproduction of them we will consider further in the article.
Origin and age
The age of gymnosperms is 350 million years. They appeared in the Upper Devonian (Paleozoic) and by the beginning of the Mesozoic era had already reached their peak. Presumably, their origin is associated with the gradual evolution of spore - ferns. Compared with them, the new plant species already had ovules and pollen grains directly involved in the formation of the seed. There were a number of advantages that gymnosperms possessed:
- reproduction occurred without the participation of water;
- seed peel and supply of nutrients ensured the preservation of the seedling.
For these reasons, after a short time, the green cover of the planet was mostly gymnosperms, the structure and reproduction of which contributed to the successful development of new territories and ecological niches.
Structural features
Basically, these plants grow in the form of trees and shrubs, more often - evergreens, and rarely deciduous. Sometimes they can reach huge sizes (sequoia, cedar). The leaves in the vast majority of them are needle-shaped or have the appearance of scales. They are called needles and contain resin passages.
Wood makes up most of the trunk. It contains hollow dead cells with perforated walls - tracheids . Their presence is a characteristic specific to this group of plants. It is through them that an upward flow of water from the root to the leaves is carried out.
In addition to tracheids, gymnospermous wood also contains resin passages. Therefore, representatives of plants of this group can be easily recognized by the fragrant coniferous smell. The resin impregnates the wood, which prevents its decay. Due to this, among coniferous trees there are a lot of centenarians. For example, the age of some sequoias totals about 3 thousand years.
The most important evolutionary advantage possessed by gymnosperms is reproduction without the participation of droplet moisture and the presence of formations that play the role of reproductive organs.
Reproductive organs of gymnosperms
The lack of colors is a key feature that gymnosperms possess. Reproduction of these plants occurs with the participation of cones, or strobils . These formations are easy to recognize on pine, spruce, fir, larch and other conifers. They can be called organs of sexual reproduction.
Usually cones are heterosexual - they can be male and female and can be located either on the same plant (monoecious) or on different plants (dioecious), differing in size and color.
Male cones are called microstrobils , and female cones are called megastrobiles . The strobila is a shortened and modified shoot, on which sporophylls are located - modified leaves. Pollen ripens in microstrobes. In megastrobils - ovules.
Gymnosperms reproduction features
Usually, the process of propagation of gymnosperms is considered on the example of Scots pine. This is a monoecious plant, i.e., male and female cones develop on one tree. The first ones are larger, reddish in color. The second - small, greenish or bluish.
The propagation scheme of gymnosperms is quite simple. When pollen ripens in microstrobes, it spills out, spreads by the wind, and can travel great distances. To do this, each pollen grain is equipped with special devices - pollen bags. Dust particles fall on ovules developing on megastrobils. This is pollination.
It usually occurs in late spring or early summer. Pollinated female cones close together, their scales stick together with resin. Fertilization occurs in the ovules inside the closed cones.
How does the fertilization process
Inside the ovule is a female gametophyte, or germinal sac, which previously goes through several stages of maturation. The pollen enters the ovum, it begins to germinate into the pollen tube toward the archegony (from the Greek arche - “beginning”, gone - “mother’s womb”). This formation contains an egg. In total, two develop in the ovule, but only one undergoes fertilization.
By that time, two sperm had already matured in the pollen grain and began to advance along the growing tube. When one of the sperm reaches the egg, fertilization occurs. The second sperm at the same time dies. After the fusion of two germ cells, a seed embryo, a zygote, is formed. The ovule itself is transformed into a seed. Germinating, it will use the supply of nutrients.
This is how the multiplication of gymnosperms occurs. A diagram of this process is shown in the photograph.
Seed ripening and distribution
After fertilization has occurred, seed maturation begins. In ordinary pine, both of these processes last for 2 years. Ripening, cones lignify and change color. Gradually, their scales open, and seeds spill out of them.
Gymnosperms do not form fruits. But in order to spread over long distances, the seeds have special devices - pterygoid membranous outgrowths, easily tolerated by the wind.
If external conditions do not favor germination, the seed can rest for a long time until the optimum temperature regime. It is these features that allowed gymnosperms to achieve a high number of species and spread over large areas.
The number of species and representatives
The described group has about 600-700 species. Not all of them have survived to the present day, some representatives exist only in fossil form. For instance:
- cordaite;
- bennetitic;
- seed ferns.
The remaining representatives are distributed throughout the globe and grow in all climatic zones: from southern tropical (sago palm trees) to cold northern latitudes (cedar, larch, spruce, pine).
The oldest representative of gymnosperms, which exists to this day, is Ginkgo Biloba (bilobate). This plant is called a living fossil, since it has been growing on the planet since the Mesozoic era, as evidenced by numerous prints in the sediments of ancient rocks.
Evolutionary development
The first gymnosperms that arose in the Paleozoic era had a number of advantages that allowed them at that time to take a dominant position on the planet. These evolutionarily new traits are called aromorphoses and allow reaching a new level in the development of new territories and ecological niches. There are several such features that gymnosperms possess:
- Reproduction is carried out without the participation of the aquatic environment. This made it possible to populate large areas on land.
- Fertilization occurs inside the ovule and is protected from external influences - such features of the multiplication of gymnosperms provide greater safety for the future plant.
- The appearance of the seed made it possible to provide the future embryo with protective covers (seed coat) and a supply of nutrition (endosperm), which, in turn, increased the number of plants in this group.
It was these traits that distinguished them from their fern-like ancestors. The next, and today the last, stage of the evolutionary development of the plant world is the appearance of angiosperms.
The value of gymnosperms
The role of gymnosperms is difficult to overestimate. It is they who fill about a third of the forests of the world involved in air purification and the constant renewal of oxygen. In addition, the appearance and wide distribution on the planet of plants that produce a large amount of oxygen, has led to the appearance of other organisms that can use this gas in the process of their breathing.
Conifers are the main suppliers of wood for various spheres of human life. These raw materials are actively used in furniture manufacturing, construction, ship manufacturing and in other strategically important areas of economic activity. Spruce wood fibers are especially valuable in the manufacture of high quality paper grades.
Gymnosperms are widely used in medicine. Special substances - volatile, secreted by almost all coniferous trees, are able to purify the air from pathogenic microbes. For this reason, most resorts and health resorts are built on the territory of coniferous forests. Such therapy can significantly alleviate or eliminate many pulmonary diseases.