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Meristem
Plant Body Organization Meristems There are several basic types of plant meristems. These include the: Apical meristem Floral meristem Lateral meristem Leaf meristems (blastozones) Intercalary meristem Back to main anatomy menu Next Back to plant body menu Main menu Plant Body Organization Apical meristems The main growing points in plants are the shoot and root apical meristems. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Shoot Apical Meristems The shoot apical meristem (SAM) can be separated into two basic regions – the promeristem and the meristematic zones. The promeristem is at the very tip of the apical meristem and consists of the apical initials and adjacent cells. The meristematic zones lie behind the promeristem and are separated into the protoderm (leads to epidermal tissue), procambium (differentiates into vascular tissue) and ground meristem (origin of cortex and pith cells). Back to main anatomy menu Back Next Photomicrograph of a coleus shoot tip. Back to plant body menu Main menu Plant Body Organization Shoot Apical Meristems Meristematic cells are typically small and densely cytoplasmic. Apical dome Leaf primordium As the shoot tip grows it lays down tissue in a consistent pattern. Developing leaf The meristematic zones occupy the apical dome. Adjacent to the apical dome, leaf primordia are initiated at regular intervals. Procambium produces vascular tissue connecting the leaf to the stem. Eventually an axillary bud forms in the leaf axil. Back to main anatomy menu Back Ground tissue Axillary bud Procambium Next Back to plant body menu Main menu Plant Body Organization Shoot Apical Meristems Most angiosperm shoot apical meristems can be described by the tunica-corpus theory of meristem organization, where outer cell layers or the tunica covers the inner body or corpus layers of the meristem. The tunica layers are characterized by having anticlinal (perpendicular to the surface) cell divisions, while the corpus layers can have periclinal (parallel to the surface) cell divisions. Tunica Tunica Corpus Corpus Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Shoot Apical Meristems Angiosperm apical meristems usually have a two or three-layered tunica. The tunica layers are designated as L-I, L-II, and L-III. The L-I gives rise to the protoderm (epidermal layers). The L-II gives rise to the sexual gametes, the procambium (vascular system) and ground meristem. The L-III when present forms parts of the ground meristem. Tunica L-I L-II L-III Corpus Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Shoot Apical Meristems Cryptomeria - Abies type Gymnosperm meristems do not have distinct tunica layers and are better described by apical meristematic zones. Central mother cells Apical initials Surface layer cells The major zones include: Surface layer meristem Central mother cells Peripheral zone (flank meristem) Peripheral zone Central rib meristem. Back to main anatomy menu Back Next Peripheral zone Rib meristem Back to plant body menu Main menu Plant Body Organization Root Apical Meristem The root meristem lies under a protective root cap. Vascular cylinder Cortex Meristem Root cap Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Root Apical Meristem The root tip can be divided into three regions. The cell division zone contains the apical meristem. Most of the root growth occurs in the elongation zone as cells expand. The maturation zone produces the root hairs and lateral roots. Elongation zone Maturation zone Cell division zone Root cap Root apical meristem Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Root Apical Meristem During development: The protoderm becomes the epidermis. Protoderm Procambium The procambium becomes the vascular system. Ground meristem Apical meristem The ground meristem becomes cortex. Back to main anatomy menu Root cap Back Next Back to plant body menu Main menu Plant Body Organization Root Apical Meristem The center of the root apical meristem contains a region with relatively few cell divisions called the quiescent zone. Although cell division is slow in this region, the quiescent zone plays an important role in establishing tissue organization in the root meristem. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Floral Meristem Under proper conditions, a vegetative shoot apical meristem can transition into a floral meristem. Upon flower evocation, the meristem becomes broader and less dome-shaped. The floral appendages initiate in whorls in order starting with the outer sepals, petals, stamens and inner pistil. Leaf primordia Sepals Pistil Petals Stamens Vegetative shoot apical meristem Back to main anatomy menu Transition meristem Back Floral meristem with flower parts initiating Next Back to plant body menu Main menu Plant Body Organization Floral Meristem Floral meristem formation in Clarkia amoena. The floral meristem has a broader, flatter appearance and floral appendages are being initiated to the periphery of the meristem. Vegetative meristem Back to main anatomy menu Back Floral meristem Next Back to plant body menu Main menu Plant Body Organization Lateral Meristem Lateral meristems are responsible for increase in axial growth (diameter) of the stem or root. The two basic types are the vascular cambium and the cork cambium (phellogen). Vascular cambium Back to main anatomy menu Back Cork cambium Next Back to plant body menu Main menu Plant Body Organization Lateral Meristem – Vascular Cambium The cambium is a lateral meristem that is responsible for radial growth in stem and root tissue. The cambium is responsible for producing primary and secondary vascular tissue. Xylem cells are produced to one side and phloem cells to the other side of the cambium. Xylem Back to main anatomy menu Back Next Cambium Phloem Back to plant body menu Phloem fibers Main menu Plant Body Organization Lateral Meristem - Vascular Cambium The meristematic cells in the cambium are called fusiform initials and periclinal divisions in the fusiform initials create cells to both sides within the cambial zone. Cambial cells Cambium in a stem cross-section. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Lateral Meristem - Vascular Cambium A fusiform initial is longer than it is wide and is wedgeshaped at either end. Periclinal divisions lead to the production of either xylem or phloem cells. Periclinal plane of division. One set of divisions leads to a xylem cell. An additional division produces a phloem cell. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Lateral Meristem Vascular Cambium Storied cambium Non-storied cambium Fusiform initials may be either formed as a storied or non-storied cambium. In a storied cambium, the cells are shorter and line up in a way that only the tips overlap. Cells in a non-storied cambium are more elongated and a greater portion of each cell overlaps. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Lateral Meristem Vascular Cambium Another type of initial cell found in the cambium is the ray initial. Ray initials are smaller and less elongated than fusiform initials. They give rise to the ray cells. Periclinal plane of division. Back to main anatomy menu Back Fusiform Ray initials initials Diagram of a transverse stem section showing ray cells within a nonstoried cambium. Next Ray cells Back to plant body menu Main menu Plant Body Organization Lateral Meristem - Vascular Cambium Ray initials originally form from fusiform initials. They can form in a number of ways. They may form from a cut-out portion of the fusiform initial or the entire fusiform initial may form transverse sections that become ray initials. Tier of ray initials Fusiform initial Ray initial Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Lateral Meristem - Vascular Cambium Rays form files of living cells within the secondary vascular system. They function as vegetative storage tissue. Rays Xylem Back to main anatomy menu Back Next Cambium Phloem Back to plant body menu Main menu Plant Body Organization Lateral Meristem - Vascular Cambium In young stems, xylem and phloem appear in separated vascular bundles. Cambium within the vascular bundle is called fascicular cambium, while the cambium between bundles is called interfasicular cambium. Vascular bundle Back to main anatomy menu Back Next Interfasicular cambium Back to plant body menu Main menu Plant Body Organization Lateral Meristem - Vascular Cambium With age, the vascular system and the cambium forms a complete ring within the stem and the secondary vascular system is formed. Xylem Back to main anatomy menu Back Next Cambium Back to plant body menu Main menu Plant Body Organization Lateral Meristem - Cork Cambium In addition to the vascular cambium, there is a cork cambium responsible for making bark (periderm). The periderm must continue to expand as the stem expands and the cork cambium is responsible for that axial growth. Periderm Back to main anatomy menu Back Next Cork cambium Back to plant body menu Vascular cambium Main menu Plant Body Organization Lateral Meristem - Cork Cambium The cork cambium is also called the phellogen. It gives rise to cells that form the periderm, which is the term for the protective bark tissue that replaces the epidermis in a woody stem. Back to main anatomy menu Back Next Phellogen Periderm Back to plant body menu Main menu Plant Body Organization Lateral Meristem - Cork Cambium The periderm is made of three layers. The outer phellem (or cork) layer. The middle phellogen (cork cambium). And the inner phelloderm, which is living parenchyma cells formed to the inner side of the phellogen. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Leaf Meristems Unlike shoots and roots, leaves are produced in an enormous diversity of shapes and sizes. Differential development of marginal meristems along the edge of the leaf are responsible the various shapes including leaf edges and compound leaves. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Leaf Meristems - Marginal Meristems In the marginal meristem concept, a developing leaf primordium there are two regions of growth associated with the apical and marginal meristems. Apical meristem The leaf apical meristem is responsible for leaf length. Marginal meristem Marginal meristems occur along the edge of the developing leaf giving the size and shape of the leaf blade (lamina). Back to main anatomy menu Midvein Petiole Back Next Back to plant body menu Main menu Plant Body Organization Leaf Meristems - Blastozones Evidence for initial cells in meristematic layers similar to those found in the apical meristem have not been found to support specific marginal meristems. It is currently thought that the leaf primordium produces topological meristematic regions along the leaf margin termed blastozones. Segmentation and differential development within the blastozone leads to the observed variations in leaf shape. Leaf primordium Meristematic blastozone regions Apical dome Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Leaf Meristems - Blastozones Leaf formation proceeds in three basic stages. Initially, a leaf primordium initiates on either side of the apical dome. This is followed by primary morphogenesis where meristematic blastozones form along the leaf margin. Finally, during the secondary morphogenesis stage, the cells expand and differentiate into specific tissue types. The image to the right shows the formation of blastozone regions leading to development of a compound leaf with opposite leaflets. Meristematic blastozone regions Leaf primordium Leaf primordium Each leaflet margin would also have blastozone regions leading to the serrated leaf edge. Back to main anatomy menu Future leaflet Apical dome Back Next Back to plant body menu Main menu Plant Body Organization Marginal Meristems – Plate Meristem The lateral growth of a leaf occurs with a relatively uniform thickness. This type of growth where parallel tissue layers divide anticlinally is termed a plate meristem. Parallel cell layers in the mesophyll of a developing coleus leaf. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Intercalary Meristem Intercalary meristems are produced in shoots at the nodes especially in monocots. It is typical for monocots to produce numerous compact nodes behind the apical meristem that elongate later in development using intercalary meristems. Apical meristem The corn embryo to the right has initiated at least 6 nodes in the apical region prior to germination. Back to main anatomy menu Back Next Back to plant body menu Main menu Plant Body Organization Intercalary Meristem Intercalary is defined as growth and cell division that occurs away from where the cells were initially formed in the original apical meristem. In the tall monocots like palms and bamboo, the extension growth from nodal intercalary meristems can be quite dramatic. Emerging bamboo shoots Back to main anatomy menu Back Next Bamboo stems at full height Back to plant body menu Main menu Plant Body Organization Intercalary Meristem The apical meristem produces numerous nodes early in development near the base of the plant. Stem extension growth proceeds in a telescopic fashion with cells in each node dividing and elongating from the intercalary meristem. Bamboo shoot section Intact bamboo shoot with outer coverings removed Apical meristem Nodes each with a leaf primordia Back to main anatomy menu Back Intercalary meristems at each node Next Back to plant body menu Main menu Plant Body Organization Intercalary Meristem Some monocot leaves also produce an intercalary meristem in the form of a narrow band in the collar region near the base of the leaf. The intercalary meristem accounts for much of the leaf blade extension growth and this meristem can resume growth if a portion of the immature leaf is removed. Continued leaf growth in mowed turf grass is from the intercalary meristem. Back to main anatomy menu Back Leaf blade Collar region (intercalary meristem) Leaf sheath Fescue (Festuca) Back to plant body menu Main menu
