View source for Meristem

A '''meristem''' is a [[biological tissue|tissue]] in [[plant]]s consisting of undifferentiated cells ('''meristematic cells''') and found in zones of the plant where growth can take place - the [[root]]s and [[shoot]]s.

Differentiated plant cells generally cannot divide or produce cells of a different type. Therefore, [[mitosis|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 cell]]s in [[animal]]s, are incompletely or not at all [[cellular differentiation|differentiated]], and are capable of continued cellular division (youthful). Furthermore, the cells are small and [[protoplasm]] fills the cell completely. The [[vacuole]]s are extremely small. The [[cytoplasm]] does not contain differentiated [[plastid]]s ([[chloroplast]]s or [[chromoplast]]s), although they are present in rudimentary form ([[proplastid]]s). 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.

== Meristematic Zones ==
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. 

Meristems located at a bud on a branch or shoot are known as a [[node (botany)|node]]. Tissue between nodes is known as the '''internode'''.

===Apical Meristems===
The most general form of meristem is the [[apical meristem]] (also called terminal meristem). These are found in [[bud]]s at the tips of [[Plant stem|shoot]]s and at the [[root]] tip, they are responsible for shoot and root growth respectively. At the tip (apex) of the root the apical meristem is covered and protected by a ''root cap'' of differentiated cells. Buds can be naked (with the growing leaves visible), protected by non-overlapping scales (''valvate buds'') or by overlapping scales (''imbricated buds'').

Apical meristems are completely undifferentiated (indeterminate). They consist of 4 distinct cell groups: -

* [[Stem Cell]]s
* The immediate [[daughter cell]]s of the stem cells
* A [[subjacent]] organising centre
* [[Founder cell]]s for [[Organ (anatomy)|organ]] initiation in surronding regions

The four distinct zones mentioned above are maintained by a complex [[signalling pathway]]. The organisation centre expresses ''WUS'' proteins which maintains the stem cell identity of the overlying cells. The stem cells signal back with CLAVATA3 (CLE3) which is assumed to be a [[ligand]] for the CLE1 [[receptor kinase]]. When CLV1 interacts with CLV3 it initiates a signalling pathway that results in the repression of the expression of ''wus''. This controls the size of the organising centre.

===Primary Meristems===
Apical meristems may differentiate into three kinds of primary meristem:
* '''Protoderm''' - lies around the outside of the stem and develops into the [[Epidermis (botany)|epidermis]].
* '''Procambium''' - lies just inside of the protoderm and develops into primary [[xylem]] and primary [[phloem]].  It also produces the [[vascular cambium]], a secondary meristem. 
* '''Ground meristem''' develops into the [[pith]]. It produces the [[cork cambium]], another secondary meristem.

===Secondary Meristems===
There are two types of secondary meristems:
* '''Vascular cambium''' -  produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. This is what gives rise to [[wood]] in plants. Such plants are called [[arborescent]]. This does not occur in plants which do not go through secondary growth (known as [[herbaceous]] plants).
* 
'''Cork cambium''' - gives rise to the bark of a tree. 

These are also called the ''lateral meristems'' because they surround the established stem of a plant and cause it to grow laterally (i.e. larger in diameter).

== Indeterminate Growth of Meristems ==
Though each plant grows according to a certain set of rules, each new root and shoot meristem can go on growing for as long as it is alive; In many plants meristematic growth is potentially '''indeterminate''', making the overall shape of the plant not determinate in advance. This is the '''primary growth'''.

== Cloning ==
Under appropriate conditions, each shoot meristem can develop into a complete new plant or [[cloning|clone]].  Such new plants can be grown from shoot cuttings that contain an apical meristem.  Root apical meristems are not readily cloned, however.  This cloning is called '''asexual reproduction''' or '''vegetative reproduction''' and is widely practiced in [[horticulture]] to mass-produce plants of a desirable [[genotype]].  This process is also known as mericloning.

== See also ==
* [[Apical meristem]]
* [[Cork cambium]]
* [[Vascular cambium]]

{{Commons|Méristème}}
== References ==
*Neil A. Campbell and Jane B. Reece ''Biology'', 6th edition. Benjamin Cummings.
* Schoof ''et al.'' The stem cell population of ''Arabidopsis'' shoot meristems is maintained by a regulatory loop between CLAVATA and WUSCHEL genes. Cell 100: 635-644.
* Research on meristems: meristemania.org [http://meristemania.org]
[[Category:Plant anatomy]]
[[Category:Plant physiology]]
[[Category:Tissues]]