meristem
Meristem cells have the ability to divide and are located in the parts where plants grow. The apical growth and thickening growth of roots and stems are directly related to the activity of meristem.
According to the nature and source of meristem, it can be divided into three categories: primary meristem, primary meristem and secondary meristem. Primary meristem is located at the top of the growth cone of roots and stems. They are left directly from the embryo. The primary meristem has strong cell division ability, small cell volume, large nucleus and dense cytoplasm, showing equal-diameter polyhedron. Primary meristem is composed of cells of primary meristem. Their characteristics are that on the one hand, cells have begun to differentiate, on the other hand, they still have the ability to divide, but the division activity is not as vigorous as that of primary meristem. Secondary meristem is a tissue with the ability to divide again after physiological and structural changes of mature parenchyma cells, which is related to the thickening of roots and stems and the re-formation of protective tissues.
Meristem can be divided into apical meristem, lateral meristem meristem and metameristem.
Apical meristem is the meristem located at the top of root and stem, that is, the growth cone at the top of root and stem. Lateral meristems are cambium and cork cambium located on the side. The activity of these meristems is related to the thickening and growth of roots and stems. Without the thickening growth of monocotyledons, there would be no lateral meristem. Sandwich meristem is a meristem located between mature tissues, which is not common in seed plants. There are inserted meristems at the base of internodes of Gramineae plants and at the base of leaves of onion and leek, and their activities are related to inserted growth.
fundamental tissue
It occupies the largest weight in plants and exists in the roots, stems, leaves, flowers, fruits and seeds of vegetative organs. The main feature of this tissue is that it consists of living parenchyma cells, so it is also called parenchyma. Basic tissues generally include absorption tissues, assimilation tissues, storage tissues, ventilation tissues and transmission cells, which are responsible for absorption, assimilation, storage, ventilation, transmission and other functions, and were also called nutritional tissues in the past.
protective tissue
The epidermis on the surface of organs exposed to air (such as stems, leaves, flowers, fruits and seeds) is a kind of protective tissue, which generally consists of a layer of cells. The cells in this layer are closely arranged without intercellular space, and there are keratinocytes on the cell wall in contact with air. These fat keratinocytes can form a layer of cuticle on the surface of the outer wall when added into the gaps of fiber molecules, which makes it difficult for water to escape from the cell wall, thus preventing the loss of water in plants. In addition, it also has the function of preventing microbial invasion. Some plants have wax on the outer surface of the epidermis (such as sugarcane stems, grapes and apples), which also plays a protective role.
The epidermis of roots is not a protective tissue, but a water-absorbing tissue with water-absorbing ability. A part of the outer wall of epidermis at the apex bulges to form root hairs. The cell wall of root hair is composed of cellulose, with only a thin stratum corneum, so the water in the soil can enter the cells from the cell wall. Root hairs have a certain life span, usually from a few days to several weeks. After the root hair withers, the parenchyma cells inside are embolized and play a protective role.
Conductive tissue
Transport organization is the organization that transports water and various substances in plants. Their main feature is that cells are long tubes, and cells are related to each other in different ways.
According to different transportation materials, transportation organizations can be divided into two categories. One is a conduit for transporting water and minerals dissolved in water; The other is a sieve tube for transporting organic matter.
echanical tissue
This kind of tissue plays an auxiliary role in plants. Most cells are slender, and the main feature is that they all have thickened cell walls. There are two common mechanical structures: one is thick-walled structure; One is thick keratinous tissue.
Thick-horned cells are living cells, and their structural characteristics are that the cell wall is thickened at the corners of the cells (some of them are plate-thickened), so they are called thick-horned cells. These cell walls are mainly composed of cellulose, so the hardness of the walls is not strong, but they are elastic. Thick horny tissues are generally distributed in young stems and petioles, and their existence does not affect the growth of these organs.