Growth in plants happens in two stages
- first new cells are produced,
- secondly these cells expand via uptake of water by the vacuole.
The division of cells doesn't occur throughout the plant - but only in meristematic regions. Expansion, however, can occur anywhere. Thus, in any plant, there are regions containing young cells, maturing cells and matured cells that have lost the capacity to divide.
What is Meristematic Tissue?
Meristematic (Gk. meristos = divisible) tissue is defined as a mass of young, immature and undifferentiated calls, which remain young forever and divide throughout the life of the plant.
The tissues at apex of stem, root and leaf primordia and vascular cambium are examples of meristematic tissue. Carl Wilhem Nageli(1858) introduced the term meristem. When a cell divides, the cell that started the division and remains in the meristem is called initial and the other one is called derivative.
Characteristics of Meristematic Tissues
Normally half of the cells retain their capacity of division and forms meristem, while rest derivative cells gradually lose their power of division through differentiation and are called permanent tissues. Usually the meristematic tissues show following characters:
- localized in growing regions at shoot and root apex, that have continuous divisions.
- responsible for growth and development of plants.
- relatively smaller, thinner with primary cell wall and isodiametric in shape.
- rich in cytoplasm with prominent nuclei. The karyo-cytokinetic ratio is greater.
- vacuoles are either absent or very small except cambium.
- usually compactly arranged without intercellular spaces.
- metabolically, these are the most active cells; hence, mitochondria, endoplasmic reticulum, plastids and other organelles are not fully developed.
Classification of meristems
There are two types of meristems based on the nature of divisions : Eumeristems and Psuedomeristems. Eumeristems is formed from pre formed meristem; always keeps dividing and never loses its power of division. The pseudomeristem (also called fusiform meristem or non-eumeristem) is formed from differentiation of permanent tissues like cambium and cork cambium.
The meristem is classified in various ways. the major classifications are made based on position, origin, function, and plane of division.
1. Meristems based on position
- Apical meristem: These meristems occur at the growing point of the plant like apices of stem and roots. They are responsible for the increase in length. They are sub-terminal (situated just behind the tip) in root apex and terminal in shoot apex. They can be formed by groups of cells (promeristem) as in phanerograms or single celled as in cryptograms.
- Interecalary meristems: These meristems lie between the permanent tissues as remnants of the apical meristems. They occur at the base of node or ay the base of internodes or at the base of leaf or sheathing leaf of monocots. In dicots, leaf sheath is replaced by stipule. The intercalary meristems are also responsible for increase in length.
- Lateral meristems: Theses meristems lie laterally on stem and roots and caused secondary growth. Vascular cambium and cork cambium(phellogen) are examples of lateral meristem. The thin walled vascular cambium is highly vacuolated unlike other meristems and become meristematic for long period even throughout life.The phellogen or cork cambium is formed by dedifferentiation of permanent tissues and functions as secondary lateral meristem.
2. Meristems based on functions
- Protoderm is outer layer that develops epidermis.
- Procambium develops into primary vascular tissues.
- Ground meristem develops into fundamental or ground tissues( hypoderrmis, cortex, endodermis, pericycle, medullary rays) and pith. These meristems are large, thin walled and isodiametric.
3. Meristems based on division
- Mass meristem: It divides in almost planes and gives irregular shape like endosperm.
- Plate meristem: It divides through one plane only, i.e. anticlinal, amd form plate like structure forming uniseriate or multiseriate structure.
- Rip or file meristem: It divides at right angle to longitudinal axis to form longitudinal files of cells.
4. Meristems based on origin
- Promeristem (primordial meristem or procambium meristem) : The first formed cells embryonic stage and forms the promeristem.
- Primary meristems: These originate from embryonic stage or promeristems. These cells are always in active state of division and give rise to primary permanent tissues. Apical meristem, intercalary meristems and fascicular (vascular cambium) of the primary vascular bundles are primary meristems.
- Secondary meristems: Meristems develop from primary permanent tissues at a later stage (secondary growth) and give rise to secondary permanent tissue. Cork cambium or phellogen (develops from pericycle during periderm formation), inter fascicular cambium of dicot stem (develop from medullary rays), wound cambium (from surrounding cells of injuries), accessory cambium (cambium of moncots like Dracaena, Yucca) and cambium of roots are secondary meristems.