Plant Tissue Definition
Plant tissue is a collection of similar cells performing an organized function for the plant. Each plant tissue is specialized for a unique purpose, and can be combined with other tissues to create organs such as leaves, flowers, stems and roots. The following is a brief outline of plant tissues, and their functions within the plant.
Types of Tissue in Plants
Meristematic plant tissue is different than all other plant tissue, in that it is the main growth tissue of the plant. All cells originate from one meristem or another. The apical meristem is the plant tissue which drives above ground growth, and decides the direction of the plant. Root meristems dig into the soil in search of water and nutrients. Subapical meristems divide the plant and carry leaves in different directions. Intercalary meristems provide growth from the middle of the plant, to extend the leaves upward into the sunlight.
Meristematic plant tissue, at the central point, is undifferentiated and ready to divide into any other type of plant cell. Meristematic cells divide asymmetrically. This means that one plant remains undifferentiated, while the other cell takes on a more specialized form. This cell will then continue to divide and develop into a plant tissue, which can help form a new organ, such as a leaf. In this way meristematic plant tissue is equivalent to animal stem cells. These cells are totipotent or pluripotent, meaning they can divide into many different types of plant tissue.
Simple Plant Tissue
There are several basic forms of plant tissue, formed from mostly identical types of cells. The first is the epidermis. The epidermis in plants serves the same function as it does in animals. It is a plant tissue formed of thin and densely packed cells, meant to separate the inside of the organisms from the outside. The epidermis is often covered in a layer of waxy protection, to stop the plant from burning or drying out in the sun. The epidermis also contains guard cells, which operate small opening called stoma. These stoma control the passage of air and water through the leaves, allowing plants to move water and nutrients up from the soil.
Sometimes, another form of simple plant tissues covers the epidermis, cork. Cork is a plant tissue seen in woody plants, which dies and becomes an outer layer of bark. This tissue is also soaked with a special waxy substance which protects against insects, the sun, and the elements.
As you turn inside the plants, the next plant tissue is parenchyma. This tissue is comprised of thin-walled cells with very large central vacuoles. The turgor pressure of these vacuoles is elevated when they are full of water, which gives structure and support to the plant. Parenchyma plant tissue is found in all parts of the plant, and makes up large portions of the leaves, stems and roots. In the leaves, parenchyma plant tissue is highly involved in the process of photosynthesis. All parenchyma plant tissue is living, and carries out functions continually. Parenchyma tissue, when wounded, can revert back into meristematic plant tissue to regrow damaged areas.
Like cork, sclerenchyma plant tissue is a structural tissue which dies, but the cell wall and structure remain. Sclerenchyma plant tissue forms long, connected fibers called sclereids. These fibers can extend throughout a plant to provide support and strength to various organs. This plant tissue is commonly found in stems, bark, and in the hard shells of some fruits and nuts, such as pears. Collenchyma plant tissue is similar to sclerenchyma, in that it provides support. Often, collenchyma plant tissue is seen in young plants, with a limited number of cells. As such, only a portion of the cell wall in these cells will be thickened for support. This plant tissue is usually found wherever there is new growth and the other structural cells have not set in yet.
Complex Plant Tissue
The complex tissues in a plant deal with moving nutrients and water to the leaves, while removing the products of photosynthesis from the leaves. Photosynthesis produces the sugar glucose. Modified and bound to other 6-carbon sugars, the substance becomes sucrose or a variety of other disaccharides. In this form it can be moved with small amounts of water and can be transported efficiently throughout the plant. The complex tissues of the plant aid in this overall effort to supply the roots with food as they supply the leaves with water and nutrients.
The two main forms of plant tissue used in this process are xylem and phloem. Xylem is a plant tissue specially designed for transporting water and nutrients. This plant tissue can come in several forms, depending on the species. Sometimes, the xylem plant tissue is made up of a long chain of small tubes, called vessels, which interconnect and allow water to travel through unimpeded.
This main tube is supported by other cells, which help pull nutrients from the water and transport it to the cells within the leaves. Starting at the roots, the water is driven by pressure at the bottom and transpiration at the leaves, which sucks the water through the xylem like as straw. It is estimated that up to 95% of the water used by plants is transpired, rather than used in photosynthesis or in the metabolism. This is thought to be necessary to concentrate nutrients found in the soil, a
At certain places, the xylem extends small tubes into the other type of complex plant tissue, the phloem. Like the xylem, the phloem consist of a variety of different cell types which work together to produce a continual interconnected passageway connecting cells of the plant. The phloem, rather than bringing water up from the roots, needs to carry sugar down to the roots and stems. With a little water from the xylem, it can complete this process. It is further aided by companion cells, which surround the actual sieve-tube. The whole structure is then supported by phloem fibers, which give the tube shape and structure.
Other Ways to Classify Plant Tissue
There are other ways to classify the basic plant tissue types, if the above separation seems too complicated. Some choose to classify three types of plant tissue, ground tissue, vascular tissue, and dermal tissue. This is basically the same as above, although it separates the epidermis and related tissue into the dermal category. The remaining tissues which are not vascular, it refers to as ground tissue.
Another way to classify plant tissue is based on its function. Certain tissues are only used for the purposes of photosynthesis and growth. Theses tissues can be referred to as vegetative tissue. The more specialized organs of the plant, such as flowers, fruits, and seeds, are all reproductive tissue. This method of classifying plant tissues is often used by those interested in plant genetics and reproduction, as these forms of the plant are often vastly different, genetically speaking, than the vegetative portions of the plant. Plants have a life-cycle which exhibits the alternation of generations, in which the internal portions of the flower are actually small, multicellular organisms differing genetically from the parent plant. For this reason, some scientists choose to view these tissues as separate.