Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific functions. Different tissues work together to perform a unique function and form an organ. Organs working together form organ systems. Vascular plants have two distinct organ systems: a shoot system and a root system. The shoot system consists of two portions: the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems, and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, absorbing the light needed for photosynthesis. The root system, which supports the plants and absorbs water and minerals, is usually underground.
Plant tissue systems comprise meristematic and permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated and continue to divide and contribute to the plant's growth. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.
Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots, at the bases of leaf blades, and at nodes (the areas where leaves attach to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing.
Secondary tissues are either simple (composed of similar cell types) or complex (consisting of different cell types). Dermal tissue, for example, is a superficial tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different plant parts. It includes three cell types: vessel elements, tracheids (both of which conduct water), and xylem parenchyma. Phloem tissue transports organic compounds from photosynthesis to other plant parts and consists of four different cell types: sieve cells (which conduct photosynthates), companion cells, phloem parenchyma, and phloem fibers. Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other. In stems, the xylem and the phloem form a vascular bundle. In roots, this is termed the vascular stele or vascular cylinder.
Adapted from Openstax biology 2e, section 25.1
