Vascular tissue is composed of xylem and the phloem. Xylem, which is the material you think of as wood, conducts water and dissolved mineral salts forming a continuous system that runs throughout the plant body. Water molecules enter, or leave a cell by osmosis. This is the movement of water molecules across a semi- permeable membrane from an area of high concentration of water molecules to a law concentration of water molecules.
Plants take up water mainly through the root hairs. Each root hair is a single, long cell with a structure similar to the palisade. Within the system, water and minerals dissolved in water passes from the roots up through the shoot in a continuous stream. When water reaches the leaves, much of it passes into the air as water, through the stomata, a process called transpiration.
Phloem conducts food and other dissolved materials in both directions along the length of the plant. Most of the cells that form mature xylem are dead. They do not have cytoplasm.
As water transpires, more is drawn from the xylem. This movement of water exerts ‘suction ‘on the water filling the xylem vessels of the stem. As the water is ‘sucked’ upward through the xylem of the stem, more water is supplied to the bottom of the xylem by the roots causing a continuous movement of column of water from the roots to the leaves. This is called the transpiration stream.
Protective tissues
The protective tissue that forms the outer layer on leaves, green stems and roots is called the epidermis. The cells of the epidermis secrete a wax substance called cutin. Cutin forms a layer over the outer surface of the epidermis called cuticle. This layer cuts down on water and protects against infection by microorganisms.
Cork protects woody stems and roots. Cork is produced by the cells of the cork cambium and lives for only a short time. Actually fully grown cork is dead cells which protect the underlying living tissue.
