Tissue (biology)

In biology, a tissue is a group of similar cells, along with the extracellular matrix they produce, that all come from the same part of the embryo and work together to perform a specific job. Tissues sit at a level of biological organization between individual cells and a whole organ; an organ is made by combining several different tissues that function as a unit. The word "tissue" comes from the French "tissu," meaning "fabric," which itself comes from the verb "tisser," "to weave." The scientific 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, the study of tissues falls under both plant anatomy and plant physiology. The classic tools for examining tissues include the paraffin block (where tissue is embedded and then sliced into thin sections), histological stains, and the light microscope. More recent advances—such as electron microscopy, immunofluorescence, and the use of frozen tissue sections—have allowed scientists to see much finer detail. These tools let researchers look at tissues in both healthy and diseased states, which helps doctors make accurate diagnoses and predict how a disease will progress.

**Plant tissue**

In plant anatomy, tissues are broadly grouped into three systems: the epidermis, ground tissue, and vascular tissue. The epidermis is made of cells that form the outer surface of leaves and the young plant body. Vascular tissue’s main parts are xylem and phloem, which move fluids and nutrients around inside the plant. Ground tissue is less specialized than the other two; it makes nutrients through photosynthesis and stores reserve nutrients. Plant tissues can also be divided into two other types: meristematic tissues and permanent tissues.

**Meristematic tissue**

Meristematic tissue is made of cells that divide actively, causing the plant to grow longer and thicker. Primary growth happens only in certain spots, like the tips of stems or roots, and that is where meristematic tissue is found. The cells in this tissue are roughly round, polyhedral, or rectangular, with thin cell walls. New cells from the meristem start out as meristem cells themselves, but as they grow and mature, their traits slowly change and they become differentiated. They are classified as primary meristem or 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, whose cells mostly divide in one plane, making the organ thicker and wider; it usually lies beneath the tree bark as cork cambium or in the vascular bundles of dicots as vascular cambium, and its activity produces secondary growth. Intercalary meristem is found between permanent tissues, often at the base of nodes, internodes, or leaf bases, and it helps increase the length of the plant and the size of internodes, leading to branch formation and growth. The cells of meristematic tissue are similar in structure, with a thin, elastic primary cell wall made of cellulose. They are packed tightly together with no spaces between them. Each cell has dense cytoplasm and a prominent nucleus, and the dense protoplasm contains very few vacuoles. Normally these cells are oval, polygonal, or rectangular. Because their main job is to multiply and increase the plant’s girth and length, they have a large nucleus and small or no vacuoles, and no intercellular spaces.

**Permanent tissues**

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 within 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. Meristematic cells differentiate into different types of permanent tissues, which fall into two categories: simple permanent tissues and complex permanent tissues.

**Simple permanent tissue**

Simple permanent tissue is a group of cells that are alike in origin, structure, and function. There are three types: parenchyma, collenchyma, and sclerenchyma.

**Parenchyma**

Parenchyma (from Greek *para* meaning "beside" and *enchyma* meaning "infusion" or "tissue") makes up the bulk of a plant’s substance. It consists of relatively unspecialized living cells with thin cell walls that are usually loosely packed, leaving intercellular spaces. These cells are generally isodiametric in shape. They contain a small number of vacuoles, or sometimes none at all; if vacuoles are present, they are much smaller than those in typical animal cells. This tissue provides support to the plant and stores food. Chlorenchyma is a special type of parenchyma that contains chlorophyll and carries out photosynthesis. In aquatic plants, aerenchyma—tissue with large air cavities—helps the plant float by making it buoyant. Some parenchyma cells, called idioblasts, contain metabolic waste. Spindle-shaped fibers, known as prosenchyma, are also found in this tissue for support, and succulent parenchyma stores water in xerophytes.

**Collenchyma**

Collenchyma (from Greek *colla* meaning "gum" and *enchyma* meaning "infusion") is a living tissue of the primary plant body, like parenchyma. Its cells are thin-walled but have thickening made of cellulose and water.