Tissue (biology)

In biology, a tissue is a group of similar cells, along with their extracellular matrix, that share a common embryonic origin and work together to perform a specific job. Tissues sit at a level of biological organization between individual cells and a complete organ; an organ is formed when several different tissues are grouped together to carry out a broader function. The word "tissue" comes from the French "tissu," meaning "fabric," which itself comes from the verb "tisser," meaning "to weave." The study of tissues is called histology, and when it focuses on disease, it is known as histopathology. Xavier Bichat is regarded as the father of histology. In plants, tissue study falls under both plant anatomy and physiology. Traditional tools for examining tissues include the paraffin block (where tissue is embedded and then sliced), histological stains, and the optical microscope. Advances like electron microscopy, immunofluorescence, and frozen tissue sections have allowed for greater detail to be seen. These methods let scientists and doctors look at tissues in both healthy and diseased states, which helps in making accurate medical diagnoses and prognoses.

Plant tissues are broadly grouped into three systems: the epidermis, ground tissue, and vascular tissue. The epidermis forms the outer surface of leaves and young plant bodies. Vascular tissue, made up of xylem and phloem, moves fluids and nutrients around the plant. Ground tissue is less specialized than the other two; it produces nutrients through photosynthesis and stores reserve nutrients. Plant tissues can also be split into two other types: meristematic tissues and permanent tissues.

Meristematic tissue is made of cells that divide actively, causing the plant to grow longer and thicker. Primary growth happens only in specific spots, like the tips of stems or roots, where meristematic tissue is found. These cells are roughly spherical, polyhedral, or rectangular, with thin cell walls. New cells from the meristem start out as meristem themselves, but as they grow and mature, their features change and they become differentiated. Meristematic tissue is classified into primary meristem and secondary meristem. Primary meristem includes apical meristem, which is at the growing tips of stems and roots and increases their length (primary growth). Secondary meristem includes lateral meristem, which divides mainly in one plane to increase diameter and girth, found beneath bark as cork cambium and in dicot vascular bundles as vascular cambium (producing secondary growth). Intercalary meristem sits between permanent tissues, usually at the base of nodes, internodes, and leaf bases, and is responsible for growth in length and internode size, leading to branch formation. Meristematic cells are similar in structure, with thin, elastic primary cell walls made of cellulose. They are packed tightly without intercellular spaces. Each cell has dense cytoplasm and a prominent nucleus, with very few vacuoles. Their shape is oval, polygonal, or rectangular. Because their main job is to multiply and increase the plant's girth and length, they have large nuclei and small or no vacuoles, and no intercellular spaces.

Permanent tissues are groups of living or dead cells that come from meristematic tissue but have lost the ability to divide and are fixed in place in the plant body. When meristematic cells take on a specific role, they stop dividing. This process of taking on a permanent shape, size, and function is called cellular differentiation. There are two types of permanent tissues: simple permanent tissues and complex permanent tissues.

Simple permanent tissue is made of cells that are similar in origin, structure, and function. There are three types: parenchyma, collenchyma, and sclerenchyma. Parenchyma (from Greek, meaning "beside" and "infusion") makes up the bulk of a plant. It consists of relatively unspecialized living cells with thin walls that are usually loosely packed, leaving intercellular spaces. These cells are generally isodiametric in shape and contain few vacuoles—sometimes none, and even if present, the vacuole is much smaller than in animal cells. This tissue provides support and stores food. Chlorenchyma is a special parenchyma that contains chlorophyll and performs photosynthesis. In aquatic plants, aerenchyma (large air cavities) helps them float by making them buoyant. Parenchyma cells called idioblasts hold metabolic waste. Spindle-shaped fibers called prosenchyma also appear in these cells for support, and succulent parenchyma stores water in xerophytes. Collenchyma (from Greek, meaning "gum" and "infusion") is a living tissue of the primary body, like parenchyma. Its cells are thin-walled but have thickening made of cellulose and water.