Plants are one of the main forms of life, including familiar organisms such as trees, shrubs, vines, grasses, ferns, green algae, and lichens. Plants can be divided into seed plants, mosses, ferns, etc. It is estimated that there are about 350,000 species in existence. Most of the energy of green plants is obtained from sunlight through photosynthesis. Temperature, humidity, light, and fresh water are the basic needs of plant survival. Angiosperms have six major organs: roots, stems, leaves, flowers, fruits, and seeds. Green plants have the ability to photosynthesize – with the help of light energy and chlorophyll, under the catalysis of enzymes, they use water, inorganic salts and carbon dioxide for photosynthesis, release oxygen, and produce organic substances such as glucose for use by the plant body.
Definition
All living organisms are considered living things. Living things are divided into several kingdoms. Plants are those that can live on land and are autotrophic. Plants have distinct cell walls and nuclei. Their cell walls are made of a polymer of glucose called cellulose. Plants are capable of photosynthesis – that is, they can produce food from water, minerals and carbon dioxide using light energy and chlorophyll, which is not present in animals. After releasing oxygen, glucose is left – an energy-rich substance that is a component of plant cells. [1] Aristotle divided living things into plants (which are usually immobile) and animals (which often move to get food). In the Linnaean system, there are two kingdoms: the plant kingdom and the animal kingdom. Later, people gradually realized that the originally defined plant kingdom contained several unrelated groups, and moved fungi and several algae to a new kingdom. However, there are still many different views on plants, both professionally and in the eyes of the general public. Indeed, if you try to perfectly put “plants” into a single category, there will be problems, because for most people, the word “plant” is not very clear about the relationship between the concept of phylogeny on which modern taxonomy and systematic taxonomy are based. The main methods of plant propagation are layering, division, cuttings, grafting, seeds, spores, etc. Note: There is now a classification of Panphyta.
Plant classification
The origin of plant life is DNA and protoplasm composed of chemical substances. The history of plants dates back 2.5 billion years ago (Proterozoic Era). The earliest plants in the history of the earth belonged to fungi and algae. After that, algae flourished for a time. It was not until 438 million years ago (Silurian Period) that green algae broke free from the constraints of the aquatic environment and landed on the earth for the first time.
Evolved into naked ferns and ferns. Added green to the earth for the first time. 360 million years ago (Carboniferous), naked ferns became extinct and ferns declined. Instead, lycophytes, sphenops, true ferns and seed ferns emerged, forming swamp forests. The main plants that flourished in the Paleozoic Era almost all became extinct 248 million years ago (Triassic), while gymnosperms began to rise, evolved pollen tubes, and completely got rid of their dependence on water, forming dense forests. At the beginning of the Cretaceous period 140 million years ago, newer and more advanced angiosperms had already differentiated from some gymnosperms. After entering the Cenozoic Era, as the earth’s environment gradually changed from the globally uniform tropical and subtropical climate of the Mesozoic Era to a diversified climate with distinct four seasons in mid- and high-latitude regions, ferns further declined due to lack of adaptability, and gymnosperms also began to go downhill due to the limitations of adaptability. At this time, the advancement of angiosperms in many processes of heredity and development, as well as in structures such as stems and leaves, especially the great advancement they showed in the reproductive organ of flowers, played a role, enabling them to adapt to those harsh environmental conditions through their own genetic variation and develop faster, differentiate into more types, and now there are more than 80 orders and more than 200 families. It is the blooming and withering of angiosperms that makes the Cenozoic Earth with distinct four seasons so beautiful.
It is estimated that there are about 350,000 plant species in existence, which are classified as seed plants, mosses, ferns and algae. As of 2004, 287,655 species have been confirmed, including 258,650 flowering plants, 16,000 mosses, 11,000 ferns and 8,000 green algae.
Component organs
Plants have six major organs: roots, stems, leaves, flowers, fruits, and seeds. The stem is the central axis of the plant body. It stands upright or creeps in the water, with branches on the stem. The branches have meristematic cells at the top for apical growth. The stem is generally differentiated into two parts: short nodes and long internodes. The stem has the function of transporting nutrients and water and supporting the distribution and formation of leaves, flowers and fruits in a certain space. Some stems also have the functions of photosynthesis, storage of nutrients and reproduction. Leaves are one of the nutritional organs of vascular plants. The function is to photosynthesize organic matter, and transpiration provides the power for the roots to absorb water and mineral nutrients from the outside world. Flowers are abnormal short branches with reproductive functions. Fruits are mainly used as a medium for spreading seeds. Seeds have the functions of reproduction and dissemination. Seeds also have various structures suitable for dissemination or resistance to adverse conditions, creating good conditions for the continuation of plant races.
Roots
Roots are the nutritional organs of plants, usually located below the surface, responsible for absorbing water and dissolved ions in the soil, and have the functions of supporting and storing synthetic organic matter. (Except for aerial roots and anchor roots) Roots are composed of parenchyma, vascular tissue, protective tissue, mechanical tissue and meristem cells.
Roots can be divided into four zones. The top is a cap-like structure – the root cap. Above it are the meristem and elongation zone, and above that is the root hair zone with root hairs.
The root cap is located outside the root apical meristem. The high mucilage of the outer cell wall can reduce the friction between the root and the soil during its downward growth, thus playing a protective role. At the same time, the amyloplasts in the cells can also ensure the root’s gravitropism.
The meristem is the apical meristem located inside the root cap. The cells in the meristem can divide continuously. On the one hand, a small part of them is used to form root cap cells, while the majority of them grow and differentiate backward to form various root structures; on the other hand, they maintain their original volume. [2]
The cells in the elongation zone develop from the cells in the meristem. Their ability to divide has weakened, and the cell elongation axis is elongated. Elongation activity can cause damage to the primary phloem and primary xylem, resulting in lacunae.
The root hair cells are mature cells. Root hairs are generated by the hair cells (Trichoblast) in the epidermis, which can effectively increase the absorption area of the plant roots. The absorption area of the roots of trees can reach 400m².
Stem The stem is one of the plant’s vegetative organs. It is the visible trunk of most plants. Of course, for example, the modified stem of a cactus. The stem is connected to the root, and the water and minerals absorbed by the root are transported upward to the various vegetative organs through the xylem, and the products of photosynthesis are transported downward through the phloem. The stem originates from the embryo of the plant embryo. The stem of the hypocotyl is the part below the cotyledon to be precise. The earliest plant with a stem was the now extinct Pteris kusii, and the existing one is the pine leaf fern, which has no real roots or leaves. Therefore, in vascular plants (vessel plants), the earliest organ to appear is the stem, and the roots and leaves evolved from the stem. Modified stems The stems of some plants have specialized functions that are not only support and transportation functions, and their shapes are not just branches and leaves, which are called modified stems. Common ones include the tubers of cacti, the bulbs of onions, the corms of water chestnuts, the rhizomes of ginger, the runners of strawberries, the tendrils (stem tendrils) of grapes, and asparagus whose stems (branches) are specialized into leaves.
Leaves
Leaves are the vegetative organs of higher plants, which develop laterally from the leaf primordium of the plant’s stem. Leaves contain chlorophyll and are the main site for photosynthesis in plants. At the same time, evapotranspiration of plants is achieved through the stomata of the leaves. Leaves only appear on true stems, that is, only vascular plants have leaves. All higher plants, such as ferns, gymnosperms and angiosperms, have leaves. In contrast, mosses, algae, fungi and lichens do not have leaves. Only structures similar to leaves can be found in these flat bodies (Thallus), but they can only be used as analogs (Analoga). Complete leaves consist of three parts: blade, petiole and stipules. The blade refers to the flat main structure on the complete leaf. It absorbs sunlight as much as possible and regulates the water and temperature in the plant body through stomata. The petiole is the part that connects the blade to the stem node. Stipules are attached to both sides of the base of the petiole or in the axils of the leaves, are small, and fall off early. The shape of stipules varies in different plant species. For example, peas have large leaf-like stipules, while the stipules of acacia and jujube are needle-shaped, and the stipules of mountain cherry are pinnate. Their function is to protect young leaves. Metamorphosis Metamorphosis is a leaf whose shape and structure have changed due to a change in function. Such as the thorns of cacti, the large sepals of jade leaf gold flowers and the carpels of flowering plants. The flowers grow on the receptacle, with the petals (or perianth pieces) on the outside, and the reproductive organs of the plant, stamens and pistils, wrapped in the middle. The bright colors and attractive fragrance of the flowers are all to attract insects. With the help of insects, the pollination process is completed to achieve the purpose of passing on the family line. The flowers of most grasses and trees are dull in color and have no fragrance, which cannot attract insects to come for pollination. Such plants generally rely on wind power to complete the pollination process. Depending on the plant, most plants will bloom hundreds of flowers every year, while a few plants, such as tulips, will only bloom one flower a year. The length of the flowering period also varies greatly.
Flowers grow on the receptacle, with the petals (or tepals) on the outside and the reproductive organs of the plant, the stamens and pistils, in the middle. The bright colors and attractive fragrance of flowers are to attract insects. With the help of insects, the pollination process is completed to achieve the purpose of passing on the family line. The flowers of most grasses and trees are dull in color and have no fragrance, which cannot attract insects to pollinate. Such plants usually rely on wind power to complete the pollination process. Depending on the plant, most plants will bloom hundreds of flowers per year, while a few plants, such as tulips, will only bloom once a year. The length of the flowering period also varies greatly. [3]
The calyx is located in the outermost whorl of sepals, usually green, but some plants have petal-like shapes.
The corolla is located in the inner whorl of the calyx and is composed of petals. It is relatively thin and soft, and often has colors to attract insects to help with pollination.
The stamen group is the general name for the stamens in a flower. The anther is born at the top of the filament and is where the pollen is formed. The pollen contains male gametes.
The pistil group is the general name for the pistil in a flower, which can be composed of one or more pistils. The reproductive organ that makes up the pistil is called the carpel, which contains the ovary, and the ovary chamber contains the ovules (containing the female gametes). A pistil may be composed of multiple carpels. In this case, if each carpel separates to form a single pistil, it is called a centrifugal pistil. On the contrary, if the carpels are fused, it is called a compound pistil. The sticky top of the pistil is called the stigma, which is the receptor for pollen. The style connects the stigma and the ovary, and is the channel for the pollen tube to enter the ovary after the pollen grains germinate.
Fruits develop from the pistil of flowers. In most plants, the seeds are contained inside the fruit. Strawberries are an exception, as their “fruit” grows from the receptacle. The number of seeds in a fruit varies, with some containing only one seed and others containing many. When the fruit matures, some are rich in water, while others become dry. Watery fruits are usually brightly colored, which can attract animals to eat them and carry the seeds to distant places. When the seeds are excreted from the body, they will take root and sprout. Some legumes and other plants burst open when their fruits are ripe, spraying seeds to nearby areas where they will germinate. Some fruits are very light and can be carried to distant places by the wind, completing their mission of passing on their family line. The fruits of some plants have thorns on the surface, which can be attached to passing animals and carried to distant places by them. When they fall off the animal, the seeds take root and sprout on the spot. [4] Fruits formed by the single development of the ovary from the pistil after fertilization are called true fruits, such as peaches and soybeans. Fruits formed by the ovary alone are usually called true fruits, such as peaches and soybeans. The fruit formed by the ovary plus the other parts of the flower (calyx, perianth, rachis, etc.) is called a false fruit, such as apples, pears, etc. In fruits with calyx and calyx involved, such as strawberries, the fruits are mostly enlarged and fleshy receptacles.
Seed
The structure that the ovule of a plant grows into after fertilization generally consists of seed coat, embryo and endosperm. The embryo is the most important part of the seed and grows into a new individual after germination. The endosperm contains nutrients. The seed coat develops from the integuments and has the function of protecting the embryo and endosperm. The seed coat of gymnosperms consists of an outer layer, an inner layer (fleshy layer) and a middle layer (stone layer). Cycads and ginkgo have thick outer fleshy layers that are pigmented when mature; the outer layers of many conifers are underdeveloped. The inner layer generally tends to shrink and is a thin paper-like layer in mature seeds, lining the middle layer. The embryo develops from the fertilized egg. It consists of the plumule, hypocotyl, cotyledon and radicle. The embryo of gymnosperms is arranged along the central longitudinal axis of the seed. Different types of seeds have different numbers of cotyledons, ranging from 1 to 18. Two are common, such as cycads, ginkgo, yew, Torreya grandis, redwood, Gnaphalium chinense, Ephedra sinica, etc. The endosperm of gymnosperms is a haploid female gametophyte, which is generally well developed and stores starch or fat, and some contain fuzzy powder. The endosperm is generally light yellow, a few are white, and the endosperm in mature seeds of ginkgo is green. In the process of double fertilization, the endosperm of angiosperms fuses with the polar nucleus in the embryo sac and develops into polyploid. Most angiosperms have endosperm formation during seed development, but some mature seeds have no or very little endosperm because their endosperm is decomposed and absorbed by the embryo during development. Generally, mature seeds are divided into endosperm seeds and non-endosperm seeds. The embryo in non-endosperm seeds is very large, and various parts of the embryo, especially in the cotyledons, store a large amount of nutrients. Main function broadcast editor Most of the solid matter of plants is obtained from the atmosphere. Through a process called photosynthesis, plants use the energy in sunlight to convert carbon dioxide in the atmosphere into simple sugars. These sugars are used as building materials and constitute the main structural components of plants. Plants mainly rely on soil for support and water, as well as important basic nutrients such as nitrogen and phosphorus. Most plants also need oxygen in the atmosphere (for respiration) and oxygen around their roots to grow successfully. However, some specialized vascular plants, such as mangroves, can grow their roots in an oxygen-deficient environment.
Photosynthesis
Plants have the ability to photosynthesize – that is, they can use light energy and chlorophyll, which is not present in animals, to photosynthesize water, inorganic salts and carbon dioxide, release oxygen, and produce glucose – a substance rich in energy for the plant body to use.
The chlorophyll of plants contains magnesium.
Plant cells have obvious cell walls and nuclei, and their cell walls are composed of cellulose, a polymer of glucose.
The ancestors of all plants were single-celled non-photosynthetic organisms that engulfed photosynthetic bacteria, and the two formed a mutually beneficial relationship: photosynthetic bacteria lived in plant cells (the so-called endosymbiosis). Finally, the bacteria metamorphosed into chloroplasts, which are organelles that exist in all plants but cannot survive independently. Most plants belong to the angiosperms, which are flowering plants, including many trees. Plant respiration mainly takes place in the mitochondria of cells; photosynthesis takes place in the chloroplasts of cells. [7]
Green plant photosynthesis is the most common and largest reaction process on Earth. It plays a major role in the synthesis of organic matter, the accumulation of solar energy, the purification of air, the maintenance of atmospheric oxygen content and the stability of the carbon cycle. It is the basis of agricultural production and has great significance in theory and practice. It is calculated that the world’s green plants can produce about 400 million tons of protein, carbohydrates and fats every day. At the same time, they can release nearly 500 million tons of oxygen into the air, providing sufficient food and oxygen for humans and animals.
Leaves are the main organs for photosynthesis, and chloroplasts are important organelles for photosynthesis. The chloroplast pigments of higher plants include chlorophyll (a and b) and carotenoids (carotene and lutein), which are distributed on the photosynthetic membrane. The absorption spectrum and fluorescence phenomenon of chlorophyll indicate that it can absorb light energy and be excited by light. The biosynthesis of chlorophyll is formed under light conditions, which is not only subject to genetic constraints, but also affected by light, temperature, mineral nutrition, water and oxygen.
Photosynthesis includes two interrelated steps: the light reaction process and photosynthetic carbon assimilation. The light reaction process includes two stages: primary reaction and electron transfer and photosynthetic phosphorylation. The former absorbs, transfers and converts light energy into electrical energy, while the latter converts electrical energy into two active chemical energies: ATP and NADPH2 (collectively known as assimilative power). The conversion of active chemical energy into stable chemical energy is completed through the carbon assimilation process. There are three pathways for carbon assimilation: C3, C4 and CAM. Plants are divided into C3 plants, C4 plants and CAM plants according to the different carbon assimilation pathways. However, the C3 pathway is the main form of carbon assimilation shared by all plants, and the enzyme that fixes CO2 is RuBP carboxylase. The C4 pathway and the CAM pathway are just different ways of fixing CO2. In the end, CO2 must be released again in the plant body to participate in the synthesis of starch, etc. by the C3 pathway. The enzymes that fix CO2 in the C4 pathway and the CAM pathway are both PEP carboxylases, which have a greater affinity for CO2 than RuBP carboxylases. The C4 pathway acts as a CO2 pump; the CAM pathway is characterized by open stomata at night, absorbing and fixing CO2 to form malic acid, and closed stomata during the day, using the CO2 released by the decarboxylation of malic acid formed at night to form sugars through the C3 pathway. This is an adaptation formed during a long evolutionary process.
Photorespiration is the process in which green cells absorb O2 and release CO2, and its substrate is glycolic acid formed by the oxygenation of RuBP, an intermediate product of the C3 pathway. The entire glycolic acid pathway is carried out in chloroplasts, peroxisomes, and mitochondria in sequence. C3 plants have obvious photorespiration, while C4 plants do not have obvious photorespiration.
The photosynthetic rate of plants varies depending on the species, growth period, accumulation of photosynthetic products, etc., and is also affected by environmental conditions such as light, CO2, temperature, water, mineral elements, and O2. The effects of these environmental factors on photosynthesis are not isolated, but are interconnected and act together. Within a certain range, the more suitable the conditions, the faster the photosynthetic rate.
The utilization rate of plant light energy is still very low. The current crop yield is far from the theoretical value, so there is great potential for increasing yield. To improve the utilization rate of light energy, we should reduce the light energy loss caused by light leakage and improve the light energy conversion rate, mainly by appropriately increasing the photosynthetic area, extending the photosynthetic time, improving the photosynthetic efficiency, improving the economic yield coefficient and reducing the consumption of photosynthetic products. Improving photosynthetic performance is the fundamental way to increase crop yields.
Plant photosynthesis produces oxygen, and the ecological value of plants only has a temporary fixation effect on carbon atoms… because these fixed carbon atoms will eventually be respired, decomposed by microorganisms, or burned…
Respiration
Respiration is an important part of the metabolism of higher plants. It is closely related to the life activities of plants. Living cells continuously decompose substances through respiration, which plays an important role in providing energy for various life activities in plants and synthesizing raw materials for important organic matter. At the same time, it can also enhance the disease resistance of plants. Respiration is the hub of metabolism in plants.
Respiration is divided into two types: aerobic respiration and anaerobic respiration, depending on whether oxygen is required. Under normal circumstances, aerobic respiration is the main form of respiration in higher plants, but plants can perform anaerobic respiration in hypoxic conditions and special tissues to maintain metabolism.
Respiratory metabolism can be carried out through multiple pathways, and its diversity is a manifestation of adaptability to changing environments formed in the long-term evolution of plants. The EMP-TCA cycle is the main pathway for the oxidation and decomposition of organic matter in plants, and pathways such as PPP also occupy an important position in respiratory metabolism.
The respiratory substrate is completely oxidized, and finally CO2 is released and water is produced, while the energy in the substrate is converted into active activation energy in the form of ATP. Only CO2 and a small amount of ATP are formed in the EMP-TCA cycle. Most of the energy is still stored in NADH and FADH2. These substances store part of their energy in ATP through electron transfer and oxidative phosphorylation in the respiratory chain, which is the main form of energy storage and respiration release.
Plant respiratory metabolism is affected by many internal and external factors. Respiration affects the progress of plant life activities, and is therefore closely related to crop cultivation, breeding, and the storage of seeds, fruits and vegetables, roots, and tubers, as well as the preservation of cut flowers. Humans can use the relevant knowledge of respiration to adjust the respiration rate so that it can better serve production.
Plants refer to another biological system corresponding to animals. The difference between animals and plants was formed in the long-term evolution process. However, for tiny organisms, the difference between them is sometimes not obvious. As the evolutionary trend of plants, the establishment of independent material metabolism types such as individual occurrence formed by cell stacking, the formation of cell walls, and photosynthesis by chlorophyll to become an independent nutritional system is the main feature, while non-motility on this basis is a secondary feature. It is estimated that there are about 300,000 species of plants in existence, and fungi account for more than half of the plant kingdom. Because of the important characteristic of lacking chlorophyll, plants are divided into two major groups. Some people also believe that the entire biological world can be divided into three major groups: animals, fungi, and plants. In terms of the classification system, seed plants (flowering plants) were previously the focus of classification, and then it shifted to the so-called cryptogamous plants. At present, the plant kingdom is divided into 10 to 13 phyla, and seed plants are only one of them. But even today, the differences of opinion among scholars on the location and content of important phyla may be greater than that in the animal kingdom. Generally speaking, the classification system of Engler (H.G.A. Engler) was the most popular in the first half of the 20th century, and the classification system of Pascher (A. Pascher) gradually became dominant in the second half.