A meristem is the tissue in all plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place.
The term “meristem” was first used by Karl Wilhelm von Nägeli (1817-1891) in his book “Beiträge zur Wissenschaftlichen Botanik” in 1858.[citation needed] It is derived from the Greek word “merizein” (μερίζειν), meaning to divide, in recognition of its inherent function.
Differentiated plant cells generally cannot divide or produce cells of a different type. Therefore, cell division in the meristem is required to provide new cells for expansion and differentiation of tissues and initiation of new organs, providing the basic structure of the plant body.
Meristematic cells are analogous in function to stem cells in animals, are incompletely or not at all differentiated, and are capable of continued cellular division (youthful). Furthermore, the cells are small and protoplasm fills the cell completely. The vacuoles are extremely small. The cytoplasm does not contain differentiated plastids (chloroplasts or chromoplasts), although they are present in rudimentary form (proplastids). Meristematic cells are packed closely together without intercellular cavities. The cell wall is a very thin primary cell wall.
Maintenance of the cells requires a balance between two antagonistic processes: organ initiation and stem cell population renewal.
Source: http://en.wikipedia.org/wiki/Meristem
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