In biology, a tissue is a group of cells having similar structure and origin and performing the same functions. In plants, the most diverse and complexly arranged tissues developed in the process of evolution in the overgrown (flowering) plants. Plant organs are usually formed by several tissues. Six types of plant tissues can be distinguished: educational, basic, conductive, mechanical, covering, secretory. Each tissue includes subtypes. There are intercellular spaces between tissues as well as within them.

Educational tissue
Due to the division of the cells of the educational tissue, the plant increases in length and thickness. At the same time, some of the cells of the educational tissue differentiate into cells of other tissues.

The cells of the educational tissue are small enough, tightly adjacent to each other, have a large nucleus and a thin shell.

The educational tissue in plants is located in the growth cones of the root (root tip) and stem (stem apex), it can be found at the base of internodes, and the educational tissue also makes up the cambium (which ensures the growth of the stem in thickness).

Parenchyma, or basic tissue
The parenchyma includes several varieties of tissue. A distinction is made between assimilative (photosynthetic), hoarding, water-bearing, and air-bearing basic tissue.

Photosynthetic tissue consists of cells containing chlorophyll, i.e. green cells. These cells have thin walls and contain a large number of chloroplasts. Their main function is photosynthesis. Assimilatory tissue makes up the pulp of leaves, is part of the bark of young tree stems and grass stems.

Assimilation tissue cells
The cells of the hoarding tissue accumulate stores of nutrients. This tissue makes up the endosperm of seeds and is part of tubers, bulbs, etc. The stem core, inner cells of stem bark and root, and succulent pericarp also usually consist of the hoarding parenchyma.

Water-bearing parenchyma is characteristic only of a number of plants, usually of arid habitats. The cells of this tissue accumulate water. The water-bearing tissue can be both in the leaves (aloe) and in the stem (cacti).

Air-bearing tissue is characteristic of aquatic and marsh plants. Its peculiarity is the presence of a large number of intercellules containing air. This facilitates the gas exchange of the plant when it is difficult.

Conductive tissue
A common function of the various conductive tissues is to conduct substances from one plant organ to another. In the trunks of woody plants, conductive tissue cells are located in the wood and the bast. Vessels (tracheae) and tracheids, which carry water solution from roots, are located in wood, and sieve tubes, which carry organic substances from photosynthesizing leaves, are located in the bast.

Vessels and tracheids are dead cells. Water solution rises faster through the vessels than through the tracheids.

The sieve tubes are living but nuclear-free cells.

Cover tissue
Cover tissue includes the scum (epidermis), cork, and crust. The skin covers the leaves and green stems; these are living cells. Cork consists of dead cells impregnated with a fat-like substance that is impermeable to water and air.

The main functions of any cover tissue are to protect the inner cells of the plant from mechanical damage, drying out, penetration of microorganisms, temperature changes.

Cork is a secondary covering tissue, as it occurs in place of the skin on the stems and roots of perennial plants.

Cork consists of cork and dead layers of the underlying tissue.

Mechanical tissue
The cells of the mechanical tissue are characterized by strongly thickened, woody sheaths. The function of mechanical tissue is to give the body and organs of plants strength and elasticity.

In the stems of overgrown plants, the mechanical tissue may be arranged in one integral layer or in separate strands, separated from each other.

In leaves, fibers of mechanical tissue are usually located next to fibers of conductive tissue. Together they form the leaf veins.

Secretory, or excretory, tissue
The cells of the secretory tissue secrete different substances, and therefore the functions of this tissue are different. Secretory cells in plants line resin and ether oil passages and form peculiar glands and glandular hairs. Nectaries of flowers belong to the secretory tissue.