In plants, the meristem is the area of tissue from which new growths are formed. At the meristem, the plant cells are continually proliferating and are not yet differentiated. Depending on where on the plant the meristem is, and on which signals it receives, the meristem tissue can give rise to new leaves, flowers, or roots. The meristem is integral in plant growth—without it, plants would have no source for the production of new cells.
Apical Meristem Function
The apical meristem is found at the ends of roots (root apical meristem) or the tops of shoots (shoot apical meristem) of a plant, and is responsible for the plant’s growth in length or height. This type of growth is known as primary growth. The presence of an apical bud (or terminal bud) exerts apical dominance over the axillary buds, ultimately promoting vertical growth and hindering lateral growth. When the apical bud is removed, signals blocking growth from the axillary buds disappear to activate lateral growth.
Shoot Apical Meristem
The shoot apical meristem, found above ground, is composed of undifferentiated cells that have one of three destinies. They can develop to become one of three primary meristems: the protoderm, ground meristem, or procambium. The protoderm will go on to form the epidermal tissues of the plant; the ground meristem will form the cortext and pith of the plant; and the procambium will become xylem and phloem, the vascular tissues of the plant.
The shoot of a plant also includes its leaves, which grow from the sides of the apical meristem. The beginning growth of the leaf at the node results in a bump, or an axillary bud, at the node. If the terminal bud is in close proximity to the axillary bud, the axillary buds will remain dormant. However, if the terminal bud is removed or if the distance between the terminal bud and axillary bud increases—as happens when the plant grows—then the inhibiting factors exerting apical dominance diminish or disappear, allowing for the growth of leaves at the lateral buds of the apical meristem. When flowering plants are ready to bloom, the shoot apical meristem of the plant becomes an inflorescence meristem where petals, sepals, stamens, and other flower parts emerge.
Root Apical Meristem
The root apical meristem, found below ground, is responsible for the growth and development of a plant’s roots. The root meristem produces cells in a bilateral direction, meaning that it yields two types of tissues at the same time. One tissue comprises the main roots of the plant that supply proliferative, undifferentiated cells for continued growth, and the other forms a root cap that protects the apical meristem and the source of new cells. Because the roots are growing and the root cap is continuously being ground down into the soil, cells of the root cap are constantly being shed and replaced by new cells, as provided by the main root. This is typical of a tap root. Lateral root meristems account for the lateral growth of roots from the main root, into vastly branched root systems. Lateral root growth help to increase the plant’s efficiency in water and nutrient absorption, nutrient storage, and stability for aerial growth.
Basal Meristem Function
Also known as an intercalary meristem, the basal meristem is found between mature, differentiated tissues. Although located relatively near an apical meristem and also composed of mostly undifferentiated cells, the intercalary meristem is distinctly different. It works independently of the apical meristem to promote the vertical growth of the plant. Growth here, however, happens not at the tip of the plant growth, but at the base. This allows leaves to continue growing despite being cut, in the way blades of grass continue to grow after being mowed.
Lateral Meristem Function
While the apical meristem is responsible for vertical growth, the lateral meristem is responsible for lateral growth, or growth in diameter. This type of growth is known as secondary growth because it is growth around an already established stem.
In all woody plants and some herbaceous plants, there are two types of lateral meristems: the vascular cambium and the cork cambium. Similar to the procambium of the apical meristem, the vascular cambium produces secondary xylem and phloem; however, the procambium is also accountable for the development of wood that increases the girth of a plant. The cork cambium give rise to the periderm, which is similar to the protoderm. While the protoderm produces the primary epidermis growth of a plant, the periderm replaces that epidermis to produce bark. The bark acts like a shield for the plant, barring it from physical damage and preventing water loss via a waxy substance called suberin.