page_title: Meristem
section_title: Apical meristems
hierarchical_section_title: Meristem / Apical meristems
context_page_description: The meristem is a type of tissue found in plants. It consists of undifferentiated cells capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants.
  Differentiated plant cells generally cannot divide or produce cells of a different type. Meristematic cells are undifferentiated or incompletely differentiated, and are totipotent and capable of continued cell division. Division of meristematic cells provides new cells for expansion and differentiation of tissues and the initiation of new organs, providing the basic structure of the plant body. The cells are small, with no or small vacuoles and protoplasm fills the cell completely. The plastids, are undifferentiated, but are present in rudimentary form. Meristematic cells are packed closely together without intercellular spaces. The cell wall is a very thin primary cell wall.
  The term meristem was first used in 1858 by Carl Wilhelm von Nägeli in his book Beiträge zur Wissenschaftlichen Botanik. It is derived from the Greek word merizein, meaning to divide, in recognition of its inherent function.
  There are three types of meristematic tissues: apical, intercalary, and lateral.
context_section_description: Apical meristems are the completely undifferentiated (indeterminate) meristems in a plant. These differentiate into three kinds of primary meristems. The primary meristems in turn produce the two secondary meristem types. These secondary meristems are also known as lateral meristems because they are involved in lateral growth.
  There are two types of apical meristem tissue: shoot apical meristem (SAM), which gives rise to organs like the leaves and flowers, and root apical meristem (RAM), which provides the meristematic cells for future root growth. SAM and RAM cells divide rapidly and are considered indeterminate, in that they do not possess any defined end status. In that sense, the meristematic cells are frequently compared to the stem cells in animals, which have an analogous behavior and function.
  The apical meristems are layered where the number of layers varies according to plant type. In general the outermost layer is called the tunica while the innermost layers are the corpus. In monocots, the tunica determine the physical characteristics of the leaf edge and margin. In dicots, layer two of the corpus determine the characteristics of the edge of the leaf. The corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems. Apical meristems are found in two locations: the root and the stem. Some Arctic plants have an apical meristem in the lower/middle parts of the plant. It is thought that this kind of meristem evolved because it is advantageous in Arctic conditions.
caption: Organisation of an apical meristem (growing tip)
   1 - Central zone
   2 - Peripheral zone
   3 - Medullary (i.e. central) meristem
   4 - Medullary tissue
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