Meristem
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. Differentiated plant cells generally cannot divide. The term meristem was derived from the Greek word merizein, meaning to divide. Meristematic cells are analogous in function to stem cells in animals.
Characteristics of Meristematic Cells
• Incompletely or not at all differentiated,
• Capable of continued cellular division (youthful).
• Small and protoplasm fills the cell completely.
• The vacuoles are extremely small.
• The cytoplasm does not contain differentiated plastids, although they are present in rudimentary form (proplastids).
• Meristematic cells are without intercellular cavities.
• The cell wall is a very thin primary cell wall.
Classification of Meristems According to Origin
A. Primary Meristems
• Protoderm - lies around the outside of the stem and develops into the 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 cortex and pith. It produces the cork cambium, another secondary meristem.
These meristems are responsible for primary growth, or an increase in length.
B. 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. .
• Cork Cambium - gives rise to the periderm which replaces the epidermis.
Classification of Meristems According to Position
• Apical Meristems - (Shoot apex) - The source of all above-ground organs, (Root apex) - It is covered by the root cap, which protects the apical meristem from the rocks, dirt and pathogens and (Floral meristem) - When plants begin flowering, the shoot apical meristem is transformed into an inflorescence meristem which goes on to produce the floral meristem which produces the familiar sepals, petals, stamens, and carpels of the flower.
• Lateral Meristems - they are involved in lateral growth e.g. Vascular cambium and Cork cambium.
• Intercalary Meristem - Occur only in monocot (particularly grass) in stems at the base of nodes and leaf blades. Intercalary meristems at the nodes allow for rapid stem elongation, while those at the base of leaf blades allow damaged leaves to rapidly regrow.
Theories of Development and Differentiation of Meristems
Apical Cell Theory
Solitary apical cells occur in many bryophytes and pteridophytes formed all tissues and organs of the plant. The apical cell theory was proposed as the basis for an understanding of the method of growth and morphology in many groups. But the theory was not applicable to seed plants.
Histogen Theory
Under this theory the more or less distinct major regions of the stem and root apex were called histogens (tissue builders). This theory, in contrast to the apical cell theory, placed the origin of axis apices in a group of initials. The histogens were:
The dermatogen: a uniseriate, external layer; formed the epidermis.
The periblem: the region between plerome and dermatogen. Formed the cortex.
The plerome: a central core; formed the pith and primary vascular tissues.
Tunica and corpus Theory
The growing apex of the stem is differentiated into:
Corpus: in the middle of apical meristem, divide to form the central vascular cylinder and cortex.
Tunica: outer enveloping layer around the corpus, divide to form the epidermis and part of the cortex in some cases.
Promeristem theory
The promeristem region is differentiated into:
Protoderm: formed the epidermis in stem and piliferous layer in the root.
Procambium: formed primary phloem and primar xylem.
Ground meristem: formed the cortex and the pith.
Mature Tissues (Permanent Tissues)
In which growth has stopped completely or for a time. The cells may be living or dead, thin or thick walled.
Tissue types based on kind of constituent cells:
A. Simple
Homogenous, consisting of one kind of cells. e.g. parenchyma, collenchyma, sclerenchyma.
B. Complex
Heterogenous, consisting of more than one kind
of cells working together as a unit. e.g. xylem and phloem.
The Tissue Systems
All the tissues of a plant which perform the same general function form a tissue system.
There are four main tissue systems:
1. The dermal or epidermal tissue system.
2. The fundamental or ground tissue system.
3. The vascular tissue system.
4. The secretory and excretory tissue system.
The Dermal or Epidermal Tissue System
The dermal system forms the outer protective covering of the plant and is represented in the primary plant body by the epidermis. During secondary growth the epidermis may be replaced by another dermal system; the periderm forming the new protective tissue.
1. Epidermis (epi= upon, derma= skin).
Composition: Continuous single layer of cells except for certain small pores, called stomata and lenticels. In many plants it may be bi- or multiseriate.
In the case of roots the outermost layer is known as epiblema, piliferous layer or rhizodermis.
Origin: From protoderm or dermatogen of apical meristem.
Functions:-
• Protects the internal tissue from damage.
• Prevents evaporation of water.
• Serves in gas exchange.
• Plays a role in photosynthesis and secretion.
• Acts as store for water in Xyrophytic plants.
• Some develops into secretory tissues, stomata and absorbing hairs.
Shapes and Types of epidermal cells:-
2. Multiple epidermis Develpos from protoderm.
3. Hypodermis Develops from ground meristem.
4. Piliferous layer
In Monocot roots, called (epiblema or rhizodermis) from which absorbing root hairs arise.
5. Exodermis
Replaces the ruptured piliferous layer during secondary growth, with thick walled cells.
6. Periderm
During secondary growth in roots and stems, the epidermis replaced
by the periderm, with dead,
suberized cork cells forming new protective tissue.
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