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 to us spruce, pine, fir, thuja, yew or larch and little-known velvichia, saga or ginkgo - all these are representatives of the group called "Gymnosperms". We will consider their structure and reproduction later in the article.
Origin and age
The gymnosperms are 350 million years old. 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 spores - ferns. Compared to them, the new plant species already had ovules and pollen grains that were directly involved in the formation of the seed. There were a number of advantages that gymnosperms had:
- reproduction took place withoutwater participation;
- the seed coat and supply of nutrients ensured the safety of the seedling.
For these reasons, after a short time, the green cover of the planet was already mostly gymnosperms, the structure and reproduction of which contributed to the successful development of new territories and ecological niches.
Building features
Mostly named plants grow in the form of trees and shrubs, more often - evergreen, and rarely - deciduous. Sometimes they can reach huge sizes (sequoia, cedar). The vast majority of their leaves 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 sign specific to this group of plants. It is through them that the upward flow of water from the root to the leaves is carried out.
In addition to tracheids, the wood of gymnosperms also contains resin ducts. Therefore, representatives of plants of this group are easily recognized by the fragrant coniferous smell. The resin impregnates the wood, which prevents it from rotting. Due to this, there are a lot of centenarians among coniferous trees. For example, some sequoias are about 3,000 years old.
The most important evolutionary advantage that gymnosperms have is reproduction without the participation of drip moisture and the presence of formations that act as reproductive organs.
Reproductive organs of gymnosperms
The absence of flowers is a key feature possessed by gymnosperms. The reproduction of these plants occurs with the participation of cones, or strobilus. These formations are easy to recognize on pine, spruce, fir, larch and other representatives of conifers. They can be called organs of sexual reproduction.
Usually the cones are of different sexes - they can be male and female and located either on the same plant (monoecious) or on different ones (dioecious), differing in size and color.
Male buds are called microstrobils, and female buds are called megastrobils. Strobila is a shortened and modified shoot, on which sporophylls are located - modified leaves. Pollen ripens in microstrobiles. In megastrobils - ovules.
Features of reproduction of gymnosperms
Usually, the process of reproduction of gymnosperms is considered on the example of Scotch pine. This is a monoecious plant, i.e. male and female cones develop on the same tree. The first of them are larger, reddish in color. The second ones are small, greenish or bluish.
The breeding scheme of gymnosperms is quite simple. When pollen matures in microstrobiles, it spills out, spreads by wind and can travel long distances. To do this, each pollen grain is equipped with special devices - pollen bags. Dust particles fall on the ovules that develop on megastrobiles. This is how pollination occurs.
Usually it falls at the end of spring or the beginning of summer. pollinatedfemale cones close, their scales stick together with resin. Fertilization occurs in the ovules inside the closed cones.
How fertilization happens
Inside the ovule is the female gametophyte, or embryo sac, which goes through several stages of maturation. Pollen enters the pollen entrance of the ovule, it begins to germinate into the pollen tube towards the archegonium (from the Greek arche - "beginning", gone - "mother's womb"). This formation contains an egg. In total, two of them develop in the ovule, but only one undergoes fertilization.
By that moment, two spermatozoa had already matured in the pollen grain and began to move along the growing tube. When one of the sperm reaches the egg, fertilization occurs. The second sperm dies. After the fusion of two germ cells, a seed embryo is formed - a zygote. The ovule itself is converted into a seed. Germinating, it will use the supply of nutrients.
This is how gymnosperms reproduce. The scheme of this process is shown in the photo.
Ripation and seed dispersal
After fertilization has taken place, the maturation of the seed begins. In Scots pine, both of these processes last for 2 years. When ripe, the cones become woody and change color. Gradually, their scales open, and seeds spill out of them.
Gymnosperms do not form fruits. But in order to spread to largedistance, the seeds have special adaptations - pterygoid membranous outgrowths, easily carried by the wind.
If external conditions are not favorable for germination, the seed can rest for a long time before the optimum temperature conditions. It was these features that allowed the gymnosperms to reach 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 this day, some representatives exist only in fossil form. For example:
- cordaite;
- bennetite;
- seed ferns.
The rest of the representatives are distributed throughout the globe and grow in all climatic zones: from the southern tropical (sago palms) to the cold northern latitudes (cedar, larch, spruce, pine).
The most ancient representative of the gymnosperms, which exists to this day, is Ginkgo Biloba (biloba). This plant is called a living fossil, as it has been growing on the planet since the Mesozoic era, as evidenced by numerous imprints in the deposits of ancient rocks.
Evolutionary development
The first gymnosperms that arose in the Paleozoic era had a number of advantages that allowed them to take a dominant position on the planet at that time. These evolutionarily new features are called aromorphoses and allow you to reacha new level in the development of new territories and ecological niches. There are several such features that gymnosperms have:
- 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 reproduction of gymnosperms ensure greater safety of the future plant.
- The appearance of the seed made it possible to provide the future embryo with protective covers (seed peel) and a supply of food (endosperm), which, in turn, increased the number of plants in this group.
It was these features that distinguished them from their ancestors - ferns. The next, and today the last, stage of the evolutionary development of the plant world is the appearance of angiosperms.
Meaning of gymnosperms
The role of gymnosperms can hardly be overestimated. It is they who fill about a third of the world's forests involved in air purification and constant oxygen renewal. In addition, the emergence and wide distribution of oxygen-producing plants on the planet has led to the emergence of other organisms that can use this gas in their respiration.
Conifers are the main suppliers of wood for various spheres of human life. This raw material is actively used in the manufacture of furniture, construction, ship production and other strategically important areas of economic activity. Spruce wood fibers are especially valuable in the manufacture of high-quality papers.
Gymnosperms are widely used in medicine. Special substances - phytoncides, secreted by almost all coniferous trees, are able to purify the air from pathogenic microbes. For this reason, most sanatoriums and he alth resorts are built on the territory of coniferous forests. Such therapy can significantly alleviate or eliminate many lung diseases.
