400 million years ago, a huge part of the earth's surface of our planet was occupied by seas and oceans. The first living organisms arose in the aquatic environment. They were pieces of slime. After several million years, these primitive microorganisms developed a green color. By appearance they began to look like algae.
Plants in the Carboniferous
Climatic conditions favorably influenced the growth and reproduction of algae. Over time, the surface of the earth and the bottom of the oceans were subjected to changes. New continents arose, while the old ones disappeared under water. The earth's crust was actively changing. These processes led to the fact that water appeared on the site of the earth's surface.
Retreating, sea water fell into crevices, depressions. They then dried up, then again filled with water. As a result, those algae that were on seabed, gradually moved to the earth's surface. But since the drying process was very slow, during this time they adapted to the new living conditions on earth. This process has been going on for millions of years.
The climate at that time was very humid and warm. He contributed to the transition of plants from marine to terrestrial life. Evolution led to the complication of the structure various plants, ancient algae also changed. They gave rise to the development of new terrestrial plants - psilophytes. In appearance, they resembled small plants that were located near the banks of river lakes. They had a stem that was covered with small bristles. But, like algae, psilophytes did not have a root system.
Plants in a new climate
Ferns originated from psilophytes. The psilophytes themselves ceased to exist 300 million years ago.
humid climate and a large number of waters led to the rapid spread of various plants - ferns, horsetails, club mosses. The end of the Carboniferous period was marked by a change in climate: it became drier and colder. Huge ferns began to die out. The remains of dead plants rotted and turned into coal, with which people then heated their homes.
Ferns had seeds on their leaves, which were called gymnosperms. Modern pines, spruces, firs, which are called gymnosperms, originated from giant ferns.
With climate change, ancient ferns have disappeared. The cold climate destroyed their tender shoots. They were replaced by seed ferns, which are called the first gymnosperms. These plants have perfectly adapted to the new conditions of a dry and cold climate. In this plant species, the reproduction process did not depend on the water that is in the external environment.
130 million years ago, various shrubs and herbs arose on Earth, the seeds of which were in the surface of the fruit. They were called angiosperms. For 60 million years, angiosperms have lived on our planet. These plants have remained virtually unchanged from then to the present day.
400 million years ago, a huge part of the earth's surface of our planet was occupied by seas and oceans. The first living organisms arose in the aquatic environment. They were pieces of slime. After several million years, these primitive microorganisms developed a green color. In appearance, they began to resemble algae.
Climatic conditions favorably influenced the growth and reproduction of algae.
Over time, the surface of the earth and the bottom of the oceans were subjected to changes. New continents arose, while the old ones disappeared under water. The earth's crust was actively changing. These processes led to the fact that water appeared on the site of the earth's surface.
Retreating, sea water fell into crevices, depressions. They then dried up, then again filled with water. As a result, those algae that were on the seabed gradually moved to the earth's surface. But since the drying process was very slow, during this time they adapted to the new living conditions on earth. This process has been going on for millions of years.
The climate at that time was very humid and warm. He contributed to the transition of plants from marine to terrestrial life. Evolution led to the complication of the structure of various plants, and ancient algae also changed. They gave rise to the development of new terrestrial plants - psilophytes. In appearance, they resembled small plants that were located near the banks of river lakes. They had a stem that was covered with small bristles. But, like algae, psilophytes did not have a root system.
Plants in a new climate
Ferns originated from psilophytes. The psilophytes themselves ceased to exist 300 million years ago.
Humid climate and a large amount of water led to the rapid spread of various plants - ferns, horsetails, club mosses. The end of the Carboniferous period was marked by a change in climate: it became drier and colder. Huge ferns began to die out. The remains of dead plants rotted and turned into coal, with which people then heated their homes.
Ferns had seeds on their leaves, which were called gymnosperms. Modern pines, spruces, firs, which are called gymnosperms, originated from giant ferns.
With climate change, ancient ferns have disappeared.
The cold climate destroyed their tender shoots. They were replaced by seed ferns, which are called the first gymnosperms. These plants have perfectly adapted to the new conditions of a dry and cold climate. In this plant species, the reproduction process did not depend on the water that is in the external environment.
130 million years ago, various shrubs and herbs arose on Earth, the seeds of which were in the surface of the fruit. They were called angiosperms. For 60 million years, angiosperms have lived on our planet. These plants have remained virtually unchanged from then to the present day.
Without plants, our planet would be a lifeless desert. And the leaves of the trees are small factories or chemical laboratories, where, under the influence sunlight and heat is the transformation of substances. Trees not only improve the composition of the air and soften its temperature. The forest has medicinal value, it also provides most of our needs for food, as well as for materials such as wood and cotton; they are also raw materials for the production of medicines.
I. What were the very first plants on earth?
Life on Earth began in the sea. Plants were the first to appear on our planet. Many of them got out on land and became completely different. But those that remained at sea remained almost unchanged. They are the most ancient ones, everything started with them. Without plants, life on Earth would not be possible. Only plants can absorb carbon dioxide and release oxygen. To do this, they use the sun's rays. Algae were among the first plants on earth.
More than 20,000 species of algae are known. They can anchor themselves to rocks or to the seafloor with a foot-like 'bracket' that transitions into a leafy branch. Brown algae grow in cold waters and reach enormous sizes. Red algae are characteristic of warm seas. Green and blue-green algae can be found in both warm and cold waters. From brown algae get a lot useful substances used in the manufacture of plastics, varnishes, paints, paper and even explosives. They are used to make medicines, fertilizer, and feed for livestock. Among the peoples of Southeast Asia, algae are the basis of many dishes.
Algae "Floating forest".
In the old days, there were legends about the Sargasso Sea, where ships died, stuck in algae. But still, in some places the thickets of algae are so dense that they can delay a light boat. These are brown algae-sargasso, after which the sea itself is named. Sargassums look like bushes dotted with "berries" - air bubbles that allow the plant to stay on the surface of the water. Unlike other large algae, sargasso do not attach to the bottom and travel in huge clusters along the waves and form a floating forest. A myriad of molluscs, worms, and bryozoans attach themselves to the leaves of the sargassum, and crabs, shrimps, and fish hide in its thickets. Almost all the "residents" are brownish-yellow in color, in the tone of sargasso, and their bodies often copy the shape of the "leaves" of this algae. Some are hiding so as not to frighten off the victim. So this whole community swims, never touching the shore.
II. They feed, clothe, delight.
1. Trees that provide food.
Coffee is one of the most popular drinks in the world.
Who and how gave us this wonderful drink? According to an old Arab legend, we are obliged to find coffee. goats. One Ethiopian shepherd, the legend says, noticed that his goats, having eaten some berries from a bush, continued to graze all night, not thinking of resting. The shepherd told the wise old man about this, and he, having tasted these berries, discovered their miraculous power and invented the coffee drink.
The Ethiopians liked coffee so much that later one of the tribes, having moved to the Arabian Peninsula, took its grains with them. This was the beginning of the first coffee plantations. And it happened, as is known from ancient manuscripts, in the 9th century. Coffee for a long time was known only to the Arabs, but the Turks, who conquered in the XV-XVI centuries. part of the Arab territories, also appreciated the taste and wonderful properties of the drink. This is how the famous method of making Turkish coffee appeared: coffee is brewed on hot sand in special copper vessels with a handle - “Turks”.
Europeans were first introduced to coffee by an Italian who returned from Turkey. A doctor by profession, he recommended that his patients drink coffee for medicinal purposes. Venice was the first to import coffee to Europe. And in 1652 the first coffee house was opened in England. Turkey was the monopoly supplier of coffee to Europe, but the cunning Dutch, having stolen seedlings of coffee trees from the Turks, transported them to Indonesia, where the climate was quite suitable for growing coffee.
Brazil is now the world leader in coffee production.
Coffee came to Russia thanks to Peter the Great.
Coffee drink is brewed from processed seeds coffee tree. This evergreen from the madder family. White lush inflorescences of the coffee tree, located in the axils of the leaves, after pollination by insects, turn into fruits - red berries remove the pulp from them, the seeds are polished in special drums and packed in bags. Before brewing coffee beans fry.
The birthplace of coffee is Africa. The Arabian species is considered the highest quality and most delicious. Brazilian coffee (this is not a species, but only a place where coffee is grown), which fills all the markets of the world, is much worse in quality than coffee grown in other countries.
2. Noble friends.
Cedruses are real cedars. Phoenicia, Egypt, Assyria were powerful powers of antiquity. But the territories they occupied were deserted, there were almost no forests there. And wood is needed for the construction of housing, and for ships. The wood is strong and does not rot. The cedar that the ancients loved is not the cedar that grows in the taiga and is famous for its delicious nuts. Siberian pines are "namesakes" of real cedars - cedrus.
The Phoenicians cut Cedruses into ships, the Egyptians into sarcophagi for the funeral ceremonies of their nobles, the Greeks and Romans used cedar to build temples and make furniture. Later, the crusaders began to cut down the cedrus. And during the First World War, the most valuable cedars with their pink wood, for lack of other fuel, were burned in locomotive fireboxes. So there are only 4 groves of Lebanese cedars left. True, other types of cedrus - Atlas, Cypriot and Himalayan, although very rare trees, but, unlike the Lebanese cedar, are still not disappearing.
Lebanese cedars are majestic trees with powerful horizontal branches. Their needles are bluish, collected in tassels. Cones the size of a fist, dense, almost smooth, like barrels. When the seeds ripen in them, the cones do not open, but crumble, and the earth is covered with a layer of scales. The wind blows off the winged seeds from them and spreads them around. If the goats, which are bred in abundance by the locals, do not eat the young shoots, a new generation of handsome cedars may grow out of them. The fame of the beauty of the Lebanese cedars has also reached Russia. Therefore, when the Russian pioneers saw Siberian pines, tall, majestic, with large cones, they called them cedars.
Siberian cedar is an amazing pine. The main wealth of the cedar is its nuts. They contain fats, proteins, starch, vitamins B and D, and the needles contain many healing substances. Nuts contain more than 60% oil, which in many ways surpasses animal fats and is not inferior in nutritional value to meat and eggs. Under Ivan the Terrible, these nuts were exported abroad, and under Peter I, they began to prepare a healing and strengthening remedy in Russia - nut milk.
Pine nuts play a huge role in the life of animals. “Where there is no cedar,” the hunters say, “there is no sable.” Bears and chipmunks, squirrels and various birds eat nuts.
Healing and cedar resin - resin. During the years of the Great Patriotic War cedar balm saved from wounds and burns. Resin is a necessary raw material for obtaining such a valuable medicine as camphor. Resin is also needed in optical technology.
Cedar wood is also valuable - pencil sticks are made from it, musical instruments, make furniture. Turpentine and other useful products are obtained from sawdust.
III. The study of the bark of a tree.
Norway maple
The maple I was watching is young. It has a tree trunk, which thickens every year, side branches form a crown, which consists of smaller branches, leaves. The tree is held in the soil by roots that absorb moisture and minerals dissolved in it. Therefore, the bottom of the tree trunk is wider.
If you smell the bark, then the smell is bitter, astringent. In spring, the smell of the bark intensifies and becomes sweetish.
There is no hollow in my tree. But I met trees with a hollow. Various birds make their homes in the hollow.
There are no lichens, mosses and mushrooms on the maple that I observe. Sometimes fungi form mushroom roots on the roots, supplying the trees with nitrogen and minerals.
On the bark of my tree there are traces left by a man: peeled bark and scratches from a knife, which over time it could heal.
IV. Why is my friend the best.
Norway maple - branch with fruits
Maple is one of the most elegant trees growing in our forests. In the spring, when the branches of the trees are not yet covered with leaves, the maple blossoms. Its yellow-green flowers, collected in an inflorescence, delight the eye. Maple is no less elegant in summer, when its crown becomes “curly”. Autumn outfit will not yield in beauty to any other plant. The tree seems to be on fire, striking with the richness of shades of crimson and green, orange and yellow. Each leaf has its own color, and each leaf is beautiful in its own way. And everyone has the same shape: rounded with 5-7 sharp protrusions, hence the name Norway maple. Maple is a good honey plant. Up to 10 kg of honey is obtained from one tree. Maple juice is very tasty. In Russia, kvass and various soft drinks were prepared from it.
The flag of Canada features a leaf of the Sugar Maple. Its sweet juice was used to make maple syrups, molasses, and even maple beer, which was very popular in the 19th century. Canada was the leader in the production of juice products. The maple leaf has become the national symbol of this country.
Maple wood was used to make musical instruments strong and light. Sports equipment is also made from maple. Pharmacists and chemists use the leaves and bark. Maple has one more interesting property A: He can predict the weather. From the petioles of the leaves, at the very branch, sometimes “tears” flow drop by drop - the maple seems to be crying. This is the property of maple to get rid of excess moisture. And the “tears” of maple depend on whether the air is dry or humid. The drier the air, the stronger the evaporation and vice versa. The air becomes humid when it rains. If they appeared on maple leaves“tears, it means that in a few hours it will rain.
V. Fossil trees that have remained on the earth.
Ancient, ancient ginkgo tree! It appeared on earth as early as the time of the dinosaurs - 125 million years ago.
years ago. And since then, this plant has not changed much. Ginkgo is a beautiful tree up to 30 m high, with large fan-like leaves. The appearance of ginkgo resembles our ordinary aspen. But it was not there! Ginkgo is a gymnosperm, more closely related to spruce than to aspen, a flowering plant. In spring, “earrings” appear on the branches along with foliage. By autumn, large seeds resembling plums hang on the branches. The pulp of the seed, which looks like a fruit, is actually just the seed coat. It is edible and tastes salty. One problem - it smells like rotten meat. This is a way to attract seed dispersal animals. Ginkgo, although it survived the dinosaurs, did not survive in the wild. This tree has become a garden tree. In Japan and China, it is considered sacred - it is grown near temples. Now ginkgo appears on the streets of European cities. Ginkgo easily resists air pollution, diseases, and insects. Ginkgo leaves and wood contain substances that repel insects. Bookmarks from dried ginkgo leaves will protect old manuscripts from bookworms. And the walls covered with ginkgo shingles will not let cockroaches or bedbugs into the house.
CONCLUSION.
What can I do for all trees?
Coming to the forest, I will not kindle fires.
This can lead to fires.
I won't destroy bird nests. Birds eat insects that damage trees. I will not break branches from trees and shrubs. I will plant new seedlings in the yard and take care of them in the future.
Acid rain also causes irreparable damage: the death of crops, flora and fauna, the destruction of buildings.
The first land plants
Life originated in water. Here the first plants appeared - algae. However, at some point, land appeared, which had to be populated. Pioneers among animals were lobe-finned fish. And among plants?
What did the first plants look like?
Once upon a time, our planet was inhabited by plants that had only a stem. They were attached to the ground with special outgrowths - rhizoids. These were the first plants to reach land.
Scientists call them psilophytes. This is a Latin word. In translation, it means "naked plants." The psilophytes did look "naked". They had only branching stems with ball-shaped outgrowths in which spores were stored. They are very similar to the "alien plants" that are depicted in illustrations for fantastic stories.
Psilophytes became the first land plants, but they lived only in swampy areas, since they did not have a root, and they could not extract water and nutrients in the soil. Scientists believe that once these plants created huge carpets over the bare surface of the planet. There were both tiny plants and very large ones, taller than human growth.
How did scientists discover the first plants?
The fact that such plants once existed on our planet, scientists learned only at the beginning of the last century, in 1912, thanks to a Scottish rural doctor who was fond of geology. Exploring the soil, he discovered the remains of hitherto unknown plants, which he later called rhinia, after the name of the village in which it was first found. It is believed that it was the first terrestrial plant, from which other psilophytes originated.
Ancient plants dominated the planet for millions of years, but died out long before the appearance of man. But they left their "descendants" - they were horsetails, club mosses and ferns. Some scientists believe that the lower psilophytes became the progenitors of modern mosses.
Question 1. When did the first land plants?
At the beginning of the Paleozoic era, plants inhabited mainly the seas, but in the Ordovician - Silurian, the first terrestrial plants appeared - psilophytes (Fig. 1).
Rice. 1. The first land plant
They were small plants intermediate between algae and terrestrial vascular plants. Psilophytes already had a conducting (vascular) system, the first weakly differentiated tissues could become stronger in the soil, although the roots (like other vegetative organs) were still absent. The further evolution of plants on land was aimed at the differentiation of the body into vegetative organs and tissues, and the improvement of the vascular system (ensuring the rapid rise of water to great heights).
Question 2. In what direction was the evolution of plants on land?
After the appearance of psilophytes, the evolution of plants on land went in the direction of dividing the body into vegetative organs and tissues, improving the vascular system (ensuring the rapid movement of water to a great height). Already in the arid Devonian, horsetails, club mosses, and ferns are widely distributed. Ground vegetation reaches even greater development in the Carboniferous period (Carboniferous), characterized by a humid and warm climate throughout the year. Gymnosperms appear, descended from seed ferns. The transition to seed propagation gave many advantages: the embryo in the seeds is protected from favorable conditions shells and provided with food, has a diploid number of chromosomes. In some gymnosperms (conifers), the process of sexual reproduction is no longer associated with water. Pollination in gymnosperms is carried out by the wind, and the seeds have adaptations for distribution by animals. These and other advantages contributed to the widespread distribution of seed plants. Large spore plants die out in the Permian due to the drying up of the climate.
Question 3. Describe the evolution of animals in the Paleozoic era.
The animal world in the Paleozoic era developed extremely rapidly and was represented by a large number of diverse forms. Life flourishes in the seas. In the Cambrian period, all the main types of animals already exist, except for chordates. Sponges, corals, echinoderms, various mollusks, huge predatory crustaceans - this is an incomplete list of the inhabitants of the Cambrian seas.
In the Ordovician, the improvement and specialization of the main types continues. For the first time, the remains of animals with an internal axial skeleton are found - jawless vertebrates, the distant descendants of which are modern lampreys and hagfishes. The mouth of these peculiar organisms was a simple opening leading to the digestive tract. The anterior part of the digestive tube was pierced by gill slits, between which were located supporting cartilaginous gill arches. Jawless ate organisms living in the muddy bottom of rivers and lakes, and detritus (organic residues), sucking food in their mouths. In some jawless animals, the division of the gill arches occurred, which made it possible to change the lumen of the pharynx with the help of the gill muscles and, consequently, to keep the mobile prey that got into the digestive tube.
The appearance of the prehensile oral apparatus- large aromorphosis - caused a restructuring of the entire organization of vertebrates.
The emergence of paired fins - limbs - is the next major aromorphosis in the evolution of vertebrates.
In the Silurian period, the first air-breathing animals - arthropods - came to land along with psilophytes. The intensive development of lower vertebrates continued in water bodies. It is assumed that vertebrates arose in shallow freshwater reservoirs and only then moved to the seas. The Devonian period was marked by the development of land by other arthropods - spiders; at the end of the period, the first terrestrial vertebrates appear - amphibians (stegocephals). In the Carboniferous, reptiles (cotylosaurs), flying insects, and lung molluscs arose. In the last, Permian period of the Paleozoic era, a rapid development and increase in systematic groups of reptiles is observed; animal-toothed reptiles appear - the ancestors of mammals.
Question 4. What features of the structure of vertebrates served as prerequisites for their release on land?
In the Silurian period, the first air-breathing animals - arthropods - came to land along with psilophytes. The intensive development of lower vertebrates continued in water bodies. It is assumed that vertebrates arose in shallow freshwater reservoirs and only then moved to the seas. In the Devonian, vertebrates are represented by three groups: lungfish, ray-finned fish, and lobe-finned fish. It was the lobe-finned fish that gave rise to terrestrial vertebrates. The lobe-finned fish were typically aquatic animals, but they could breathe atmospheric air with the help of primitive lungs, which were protrusions of the intestinal wall. Only lobe-finned fish could adapt to life on land. Their fins were blades consisting of individual bones with muscles attached to them (Fig. 2). With the help of fins, lobe-finned fish - large animals from 1.5 to several meters long - could crawl along the bottom. Thus, they had two main prerequisites for the transition to the terrestrial habitat: muscular limbs and lungs. At the end of the Devonian, lobe-finned fish gave rise to the first amphibians - stegocephals.
Rice. 2. Skeleton of the paired fin of a lobe-finned fish and a stegocephalus:
A - shoulder girdle and fin of a lobe-finned fish;
B - internal skeleton of the fin;
B - skeleton of the forelimb of a stegocephalus:
1 - element corresponding to the humerus;
2 - element corresponding to the radius;
8 - element corresponding to the ulna;
4, 5, 6 - bones of the wrist; 7 - phalanges of fingers.
the germinal stage of a seed plant, which is formed in the process of sexual reproduction and serves for settling. Inside the seed is an embryo, consisting of an embryonic root, a stalk and one or two leaves, or cotyledons. flowering plants According to the number of cotyledons, they are divided into dicots and monocots. In some species, such as orchids, individual parts of the embryo are not differentiated and begin to form from certain cells immediately after germination.
A typical seed contains a supply of nutrients for the embryo, which will have to grow for some time without the light needed for photosynthesis. This reserve can occupy most of the seed, and sometimes is located inside the embryo itself - in its cotyledons (for example, in peas or beans); then they are large, fleshy and determine general form seed. When the seed germinates, they can be taken out of the ground on an elongating stalk and become the first photosynthetic leaves of a young plant. In monocots (for example, wheat and corn), the food supply is the so-called. endosperm is always separated from the embryo. The ground endosperm of grain crops is a well-known flour.
In angiosperms, the seed develops from the ovule - a tiny thickening on the inner wall of the ovary, i.e. the bottom of the pistil located in the center of the flower. The ovary may contain from one to several thousand ovules.
Each of them contains an egg. If, as a result of pollination, it is fertilized by sperm that penetrates the ovary from the pollen grain, the ovule develops into a seed. It grows, and its shell becomes dense and turns into a two-layer seed coat. Its inner layer is colorless, slimy and is able to swell strongly, absorbing water. This will come in handy later, when the growing embryo will have to break through the seed coat. The outer layer can be oily, soft, filmy, hard, papery, and even woody. On the seed peel, the so-called. hilum - the area by which the seed was connected to the peduncle, which attached it to the parent organism.
The seed is the basis for the existence of modern flora and fauna. Without a seed, there would be no coniferous taiga, deciduous forests, flowering meadows, steppes, grain fields on the planet, there would be no birds and ants, bees and butterflies, humans and other mammals. All this appeared only after the plants in the course of evolution had seeds, inside which life can, without declaring itself in any way, be preserved for weeks, months and even for many years. The miniature plant germ in the seed is capable of traveling long distances; he is not tied to the earth by roots, like his parents; does not need water or oxygen; he is waiting in the wings to get into appropriate place and waiting for favorable conditions, begin development, which is called the germination of the seed.
The evolution of seeds.
For hundreds of millions of years, life on Earth did without seeds, as it does without them now on two-thirds of the planet's surface covered with water. Life originated in the sea, and the first plants to conquer land were still seedless, but only the appearance of seeds allowed photosynthetic organisms to fully master this new habitat for them.
The first land plants.
Among large organisms, the first attempt to gain a foothold on land was most likely made by marine macrophytes - algae that found themselves on stones heated by the sun at low tide. They reproduced by spores - single-celled structures dispersed by the parent organism and capable of developing into a new plant. Algae spores are surrounded by thin shells, so they do not tolerate drying. Under water, such protection is quite enough. Spores there are spread by currents, and since the temperature of the water fluctuates relatively little, they do not need to wait long for favorable conditions for germination.
The first land plants also reproduced by spores, but the obligatory change of generations was already fixed in their life cycle. The sexual process included in it ensured the combination of the hereditary characteristics of the parents, as a result of which the offspring combined the advantages of each of them, becoming larger, more enduring, and more perfect in structure. At a certain stage, such a progressive evolution led to the appearance of liverworts, mosses, club mosses, ferns and horsetails, which had already completely left the reservoirs on land. However, spore reproduction did not yet allow them to spread beyond marshy places with moist and warm air.
Spore plants of the Carboniferous period.
At this stage of the development of the Earth (about 250 million years ago), giant forms with partially lignified trunks appeared among the ferns and lycopsids. Horsetails were not inferior in size to them, the hollow stems of which were covered with green bark soaked in silica. Wherever plants appeared, they were followed by animals, mastering new types of habitats. In the humid twilight of the coal jungle, there were many large insects (up to 30 cm in length), giant centipedes, spiders and scorpions, amphibians that looked like huge crocodiles, and salamanders. There were dragonflies with a wingspan of 74 cm and cockroaches 10 cm long.
Tree ferns, club mosses and horsetails possessed all the qualities necessary for living on land, except for one thing - they did not form seeds. Their roots effectively absorbed water and mineral salts, the vascular system of the trunks reliably carried the substances necessary for life to all organs, and the leaves actively synthesized organic substances. Even the spores have improved and acquired a strong cellulose shell. Not afraid of drying out, they are carried by the wind over considerable distances and could not germinate immediately, but after a certain period of dormancy (the so-called dormant spores). However, even the most perfect spore is a unicellular formation; unlike seeds, it dries quickly and does not contain a supply of nutrients, and therefore is not able to wait long for favorable conditions for development. Yet the formation of resting spores was an important milestone on the way to seed plants.
For many millions of years, the climate on our planet remained warm and humid, but evolution in the fertile wilds of the coal bogs did not stop. Tree-like spore-bearing plants first developed primitive forms of true seeds. Seed ferns appeared, lycopsids (famous representatives of the genus Lepidodendron- in Greek, this name means "scaly tree") and cordaites with solid woody trunks.
Although there are few fossils of these organisms that lived hundreds of millions of years ago, tree-like seed ferns are known to have existed even before the Carboniferous. In the spring of 1869, the Skohary Creek in the Catskills, New York, flooded heavily. The flood swept away bridges, knocked down trees, and heavily washed away the shore near the village of Gilboa. This incident would have long been forgotten if the falling water had not revealed to the observers an impressive collection of strange stumps. Their bases greatly expanded, like those of swamp trees, the diameter reached 1.2 m, and their age was 300 million years. Details of the structure of the bark were well preserved, fragments of branches and leaves were scattered nearby. Naturally, all this, including the silt from which the stumps rose, was petrified. Geologists have dated the fossils to the Upper Devonian, pre-Carboniferous, and determined that they correspond to tree ferns. For the next fifty years, only paleobotanists remembered the find, and then the village of Gilboa presented another surprise. Together with the fossilized trunks of ancient ferns, this time their branches with real seeds were discovered. Now these extinct trees belong to the genus Eospermatopteris, which translates as "dawn seed fern." (“dawn”, because we are talking about the earliest seed plants on Earth).
The legendary Carboniferous period ended when geological processes complicated the planet's relief, crushing its surface into folds and dismembering it with mountain ranges. Lowland swamps were buried under a thick layer of sedimentary rocks washed off the slopes. The continents changed their shape, pushing the sea and deviating ocean currents from their previous course, ice caps began to grow in places, and red sand covered vast expanses of land. Giant ferns, club mosses and horsetails died out: their spores were not adapted to the harsher climate, and the attempt to switch to propagation by seeds turned out to be too weak and uncertain.
The first true seed plants.
The coal forests perished and were covered with new layers of sand and clay, but some trees survived by forming winged seeds with a strong shell. Such seeds could spread faster, longer, and therefore over longer distances. All this increased their chances of finding favorable conditions for germination or waiting for them to come.
Seeds were destined to revolutionize life on Earth at the beginning of the Mesozoic era. By this time, two types of trees, cycads and ginkgos, had escaped the sad fate of other carboniferous vegetation. These groups began to co-populate the Mesozoic continents. Encountering no competition, they spread from Greenland to Antarctica, making the vegetation cover of our planet almost homogeneous. Their winged seeds traveled through mountain valleys, flew over lifeless rocks, sprouted in sandy patches between rocks and among alluvial gravel. Probably, small mosses and ferns, which survived the climate change on the planet at the bottom of ravines, in the shade of cliffs and along the shores of lakes, helped them explore new places. They fertilized the soil with their organic remains, preparing its fertile layer for the settlement of larger species.
Mountain ranges and vast plains remained bare. Two types of "pioneer" trees with winged seeds, having spread around the planet, were tied to wet places, since their eggs were fertilized by flagellated, actively swimming spermatozoa, like those of mosses and ferns.
Many spore plants produce spores different size- large megaspores, giving rise to female gametes, and small microspores, during the division of which motile spermatozoa arise. To fertilize an egg, they need to swim up to it through the water - while a drop of rain and dew is quite enough.
In cycads and ginkgoes, megaspores are not dispersed by the parent plant, but remain on it, turning into seeds, however, spermatozoa are motile, so dampness is needed for fertilization. The external structure of these plants, especially their leaves, also brings them closer to the fern-like ancestors. Preservation ancient way fertilization by floating spermatozoa in the water meant that, despite the relatively hardy seeds, prolonged drought remained an insurmountable problem for these plants, and the conquest of land was suspended.
The future of terrestrial vegetation was provided by trees of a different type, growing among cycads and ginkgoes, but having lost flagellated spermatozoa. These were the araucaria (genus Araucaria), coniferous descendants of Carboniferous cordaites. In the era of cycads, araucaria began to form huge amounts of microscopic pollen grains, corresponding to microspores, but dry and dense. They were carried by the wind to megaspores, more precisely, to the ovules formed from them with eggs, and germinated with pollen tubes that delivered immobile sperm to the female gametes.
Thus, pollen appeared in the world. The need for water for fertilization disappeared, and plants rose to a new evolutionary stage. The formation of pollen led to an enormous increase in the number of seeds developing on each individual tree, and consequently to the rapid spread of these plants. The ancient araucaria also had a method of settlement, preserved in modern conifers, with the help of hard winged seeds, easily carried by the wind. So, the first conifers appeared, and over time, well-known species of the pine family.
Pine produces two types of cones. Men's length approx. 2.5 cm and 6 mm in diameter are grouped at the ends of the uppermost branches, often in bunches of ten or more, so that large tree there may be several thousand of them. They scatter pollen, showering everything around with a yellow powder. The female cones are larger and grow on the tree below the male ones. Each of their scales resembles a scoop in shape - wide on the outside and tapering towards the base, with which it is attached to the woody axis of the cone. On the upper side of the scale, closer to this axis, two megaspores are openly located, waiting for pollination and fertilization. Pollen grains carried by the wind fly inside the female cones, roll down the scales to the ovules and come into contact with them, which is necessary for fertilization.
Cycads and ginkgoes could not compete with more progressive conifers, which, effectively dispersing pollen and winged seeds, not only pushed them out, but also mastered new, previously inaccessible corners of the land. Taxodiaceae became the first coniferous dominants (now they include, in particular, sequoias and swamp cypresses). Spreading all over the world, these beautiful trees for the last time covered all parts of the world with homogeneous vegetation: their remains are found in Europe, North America, Siberia, China, Greenland, Alaska and Japan.
Flowering plants and their seeds.
Conifers, cycads and ginkgos belong to the so-called. gymnosperms. This means that their ovules are located openly on the seed scales. Flowering plants constitute the department of angiosperms: their ovules and the seeds developing from them are hidden from the external environment in an expanded base of the pistil, called the ovary.
As a result pollen grain cannot reach the ovule directly. For the fusion of gametes and the development of the seed, a completely new plant structure is needed - a flower. Its male part is represented by stamens, the female part by pistils. They may be in the same flower or in different flowers, even on different plants, which in the latter case are called dioecious. Dioecious species include, for example, ash trees, hollies, poplars, willows, date palms.
For fertilization to occur, the pollen grain must land on the top of the pistil—the sticky, sometimes pinnate stigma—and adhere to it. Stigma highlights chemical substances, under the influence of which the pollen grain germinates: the living protoplasm, emerging from under its hard shell, forms a long pollen tube that penetrates into the stigma, spreads further deep into the pistil along its elongated part (column) and eventually reaches the ovary with ovules. Under the influence of chemical attractants, the nucleus of the male gamete moves along the pollen tube to the ovule, penetrates into it through a tiny hole (micropyle) and merges with the nucleus of the egg. This is how fertilization occurs.
After that, the seed begins to develop - in a humid environment, richly supplied with nutrients, protected by the walls of the ovary from external influences. Parallel evolutionary transformations are also known in the animal world: external fertilization, which is typical, say, for fish, is replaced by internal fertilization on land, and the embryo of mammals is formed not in eggs laid in the external environment, as, for example, in typical reptiles, but inside the uterus. The isolation of the developing seed from extraneous influences allowed the flowering plants to boldly “experiment” with its shape and structure, and this, in turn, led to the avalanche-like appearance of new forms of land plants, the diversity of which began to increase at a rate unprecedented in previous eras.
The contrast with gymnosperms is obvious. Their “naked” seeds lying on the surface of the scales, regardless of the type of plant, are approximately the same: teardrop-shaped, covered with a hard skin, to which a flat wing is sometimes attached, formed by the cells surrounding the seed. It is not surprising that for many millions of years the form of gymnosperms remained very conservative: pines, spruces, firs, cedars, yews, cypresses are very similar to each other. True, in junipers, yew and ginkgo seeds can be confused with berries, but this does not change the overall picture - the extreme uniformity of the general plan of the structure of gymnosperms, the size, type and color of their seeds in comparison with the huge wealth of flowering forms.
Despite the paucity of information about the first stages of the evolution of angiosperms, it is believed that they appeared by the end of the Mesozoic era, which ended about 65 million years ago, and at the beginning of the Cenozoic era they already conquered the world. The oldest known to science flowering genus - Claytonia. Its fossils have been found in Greenland and Sardinia, i.e., it is likely that even 155 million years ago it was as widespread as the cycads. Leaves at Claytonia palmately complex, as in the current horse chestnuts and lupins, and the fruits are berry-like with a diameter of 0.5 cm at the end of a thin peduncle. Perhaps these plants were brown or green in color. The bright colors of angiosperm flowers and fruits came later, paralleling the evolution of the insects and other animals they were meant to attract. berry Claytonia four-seeded; on it you can discern something resembling the remnant of a stigma.
In addition to the extremely rare fossil remains, unusual modern plants, grouped in the order Gnetales, provide some insight into the first flowering plants. One of their representatives is a conifer (genus Ephedra), found particularly in the deserts of the southwestern United States; outwardly, it looks like several leafless rods extending from a thick stem. Another genus is velvichia ( Welwitschia) grows in the desert off the southwestern coast of Africa, and the third is gnetum ( Gnetum) is a low shrub of the Indian and Malay tropics. These three genera can be considered "living fossils", demonstrating possible pathways for the transformation of gymnosperms into angiosperms. The cones of the conifer outwardly resemble flowers: their scales are divided into two parts, resembling petals. Velvichia has only two wide ribbon-like leaves up to 3 m long, completely different from conifer needles. Gnetum seeds are provided with an additional shell, making them look like angiosperm drupes. It is known that angiosperms differ from gymnosperms in the structure of wood. Among the oppressors, it combines the features of both groups.
Seed dispersal.
The viability and diversity of the plant world depend on the ability of species to spread. The parent plant is rooted in one place all its life, therefore, its offspring must find another. This task of developing new space was entrusted to the seeds.
First, the pollen must land on the pistil of a flower of the same species, i.e. pollination must take place. Second, the pollen tube must reach the ovule, where the nuclei of the male and female gametes will fuse. Finally, the mature seed has to leave the parent plant. The probability that a seed will germinate and a seedling successfully take root in a new place is an insignificant fraction of a percent, so plants are forced to rely on the law big numbers and disperse as many seeds as possible. The last parameter is generally inversely proportional to their chances of survival. Let's compare, for example, a coconut palm and orchids. The coconut palm has the largest seeds in the plant world. They are able to swim indefinitely in the oceans until the waves throw them onto the soft coastal sand, where the competition of seedlings with other plants will be much weaker than in the forest more often. As a result, the chances of settling down for each of them are quite high, and one mature palm tree, without risk to the species, usually produces only a few dozen seeds per year. Orchids, on the other hand, have the smallest seeds in the world; in tropical forests, they are carried by weak air currents among high crowns and germinate in moist cracks in the bark on tree branches. The situation is complicated by the fact that on these branches they need to find a special type of fungus, without which germination is impossible: small orchid seeds do not contain nutrient reserves and in the first stages of seedling development they receive them from the fungus. It is not surprising that in one fruit of a miniature orchid there are several thousand of these seeds.
Angiosperms are not limited to the production of diverse seeds as a result of fertilization: the ovaries, and sometimes other parts of the flowers, develop into unique seed-containing structures - fruits. The ovary can become a green bean that protects the seeds until they ripen, turn into a strong coconut capable of making long sea voyages, into a juicy apple that an animal will eat in a secluded place, using the pulp, but not the seeds. Berries and drupes are a favorite delicacy of birds: the seeds of these fruits are not digested in their intestines and fall into the soil along with excrement, sometimes many kilometers from the parent plant. The fruits are winged and fluffy, and the form of appendages that increase volatility is much more diverse than that of pine seeds. The wing of the ash fruit resembles an oar, in the elm it looks like a brim of a hat, in the maple the paired fruits - diptera - resemble soaring birds, in the ailanthus the wings of the fruit are twisted at an angle to each other, forming, as it were, a propeller.
These adaptations allow flowering plants to be very effective in dispersing seeds. external factors. However, some species do not count on outside help. So, the fruits of touchy are a kind of catapult. Geraniums use a similar mechanism. Inside their long fruit there is a rod, to which are attached four for the time being straight and joined together flaps - they hold firmly from above, weakly from below. When ripe, the lower ends of the valves come off the base, twist sharply towards the top of the rod and scatter the seeds. In the ceanothus shrub well-known in America, the ovary turns into a berry, similar in structure to a time bomb. The pressure of the juice inside is so high that after ripening it is quite warm sunbeam so that its seeds scatter in all directions like living shrapnel. Boxes of ordinary violets, having dried up, burst and scatter seeds around them. Hamamelis fruits act like a howitzer: in order for the seeds to fall farther, they shoot them at a high angle to the horizon. In the knotweed virgin, in the place where the seeds are attached to the plant, a spring-like structure is formed that discards mature seeds. In oxalis, the shells of the fruit first swell, and then crack and shrink so sharply that the seeds fly out through the cracks. Arceutobium is tiny, due to the hydraulic pressure inside the berries, it pushes the seeds out of them like miniature torpedoes.
Viability of seeds.
The embryos of many seeds are provided with nutrients and do not suffer from drying out under an airtight shell, and therefore they can wait for favorable conditions for many months and even years: in sweet clover and alfalfa - 20 years, in other legumes - more than 75, in wheat, barley and oats - to ten. Weed seeds are distinguished by good viability: in curly sorrel, mullein, black mustard and pepper knotweed, they germinate after lying in the ground for half a century. It is believed that 1.5 tons of weed seeds are buried per 1 hectare of ordinary agricultural soil, which are just waiting for an opportunity to get closer to the surface and sprout. Cassia and lotus seeds remain viable for centuries. The record for viability is still held by the seeds of the walnut lotus, discovered several years ago in the bottom silt of one of the dried-up lakes in Manchuria. It was established by radiocarbon method that their age is 1040 ± 120 years.
