TYPES OF TISSUES
The plant body comprises several types of tissues. Tissues are complex clusters of cells of common origin’. Tissue comprises of group of cells which may possess a common structure or may perform a common function.
Simple The tissues which consist of similar types of cells are referred to as simple tissues. They appear to be homogeneous. These include parenchyma, collenchyma and sclerenchyma.
Complex tissues- The tissues which consist of more than one type of cells are referred to as complex tissues. These include xylem and phloem.
Based on the stage of development of the plant body, the tissues are also classified into two types: 1) Meristematic tissue; and 2) Permanent tissue
1)Meristematic tissue or meristems: The cell(s) of this tissue are generally young and immature, and possess the potential to show continuous division. These tissues show the ability to divide and self perpetuate throughout the plant life. A meristem may consist of one or more groups of cells and these cells are defined as meristematic cells (meristem-like). Thus we can say that a cell or group of cells that are meristematic constitute a meristem.
2) Permanent tissue: These cells are derived from a meristem. The cells of this tissue cannot divide as they lose the capacity to divide. The cells of this tissue possess living or dead cells which may be thick or thin-walled but have well-developed organelles. These cells are also known as mature tissue.
SIMPLE TISSUES Simple tissues mainly consist of parenchyma, collenchyma and sclerenchyma.
SHAPE AND SIZE
Parenchyma— It consists of the least specialised and least differentiated cells. They are primarily isodiametric and possess a primary cell wall but may exhibit a variety of shapes and forms.
—–They are thin-walled living cells and may be capable of regaining the capacity to divide.
——They are compactly arranged or separated by abundant intercellular spaces.
ORIGIN
Parenchyma can have diverse origins. They originate either from an apical meristem or from lateral meristems. They are regarded as the basic ground tissue from which all specialised cells might have developed. The cells occur as scattered diffused elements or homogenous masses.
——They are generally polyhedral but sometimes morphological variations stellate and armed forms also occur. A typical parenchyma cell is 14-sided.
From the evolutionary point of view, the parenchymatic cell is regarded as the ancestor or precursor of the other cell types of the plant because it is not much differentiated and shows similar behaviour as meristematic cells. For example, it can dedifferentiate by decreasing the thickness of the cell wall and becoming a totipotent cell that can proliferate.
PHYSIOLOGICAL FUNCTION
——These cells are physiologically very active and participate in various metabolic activities such as synthesis, storage, transport of metabolites, wound healing, repair and regeneration.
—–It is the most common type of tissue and is present in many parts of the plants such as pith and cortex regions of the stems, roots, fleshy tissues of fruits and seeds, photosynthetic leaves and rays of secondary xylem and phloem as well as the axial secondary xylem and secondary phloem. When parenchyma are fully turgid they provide considerable stiffness to stems.
LOCATION
—–Parenchyma present in different regions arise in different ways. Parenchyma present within the primary plant body differentiates from the ground meristem, procambium and protoderm, while that present in the secondary body, originates from the activity of both vascular and cork cambium.
FUNCTION
—– Parenchyma that contains chloroplast participates actively in photosynthesis and is known as chlorenchyma. The thin cell wall of these cells allows light and carbon dioxide to pass through to reach the chloroplast. Some parenchyma cells can secrete nectar, mucilage resins and oils.
—— These cells contain large amounts of dictyosomes and endoplasmic reticulum. They assist in the transfer or transport of large quantities of sugar and minerals.
—–parenchyma is an excellent source to produce callus (in vitro mass of undifferentiated cells that proliferate and differentiate to give an adult plant).
—– Parenchyma is a continuous tissue in the cortex and medulla of stems and roots, as well as in leaves, fruit pulp, and seeds endosperm.
——The ability of plant tissues to be repaired after an injury depends partially on parenchymatic cells.
4 DIFFERENT TYPES OF PARENCHYMA
- PHOTOSYNTHETIC PARENCHYMA/ CHLORENCHYMA
- This type of parenchyma, also known as chlorenchyma, is specialized in photosynthesis thanks to the many chloroplasts present in the cells.
- Photosynthetic parenchyma is commonly found under the epidermis, where light is more intense, and it is abundant in leaves, but also the cortex of green shoots.
- The photosynthetic parenchyma of the leaves is known as mesophyll, which is usually divided into two types: palisade and spongy mesophyll.
- Palisade mesophyll is close to the upper epidermis of the leaves, where it gets more light, whereas the spongy mesophyll is in the lower and darker side of the leaves.
- Parenchymatic cells of the palisade mesophyll are more tightly packaged and contain more chloroplasts so that the photosynthetic activity is higher. In the spongy mesophyll, there are more empty intercellular spaces that facilitate the movement of gases and water.
- STORAGE PARENCHYMA
- The cells in this tissue synthesize and store several substances.
- These substances can be solid, like starch grains and crystallized proteins, they are mostly found in solution, such as lipids, proteins, and others.
- They are stored in vacuoles, which are the compartments specialized in storing molecules. In the cytoplasm, some molecules are also stored like carbohydrates and nitrogenated substances.
- Some parenchymatic cells store only one type of substance, but a mix of different substances can also be found in the same cell. The most frequently stored molecule is starch. Stored proteins are a good source of nitrogen, which is very important for the plant, and the destiny of these proteins is usually degradation
- AQUIFEROUSN PARENCHYMA
- Though parenchymatic cells store some amount of water, those of the aquiferous parenchyma are specialized in this function.
- They are large cells, with a thin cell wall and a very large vacuole where water is stored. In the cytoplasm or the vacuole, there is a mucilaginous substance that increases the capacity for absorption and retention of water.
- Aquifereous parenchyma is present in plants that live in dry environments, known as xerophyte plants. Plant underground organs that store nutrients are not specialized in the storing of water, although those cells that contain starch granules or other substances are capable of storing large amounts of water.
- AERIFEROUS PARENCHYMA/AERENCHYMA
Aeriferous parenchyma (aerenchyma). There are large interconnected empty intercellular spaces, where gases can diffuse and aerate the root.
- Aeriferous parenchyma or aerenchyma contains large intercellular empty spaces, larger than in other plant tissues.
- This tissue is well-developed in plants living in wet or aquatic environments (these plants are known as hydrophytes), although it can be also found in non-aquatic plants under stress. Both, the stem and root can develop aerenchyma.
- In the roots, two ways of aerenchyma formation have been observed: schizogeny and lysogeny.
- Schizogeny is a process that occurs by cell differentiation during the development of the organ. Lysogeny is a consequence of the stress and the intercellular cavities are produced by cell death. Lysogenic aerenchyma is found in wheat, rice, corn and barley. Some authors suggest that a third type known as
- Expansigeny,(expansion of leaf cells) where the intercellular cavities are by cell retraction, but cells do not lose the physical contact.
If you would like to read about anthropology consider clicking the link anthropology https://hoboknowledge.com/anthropology-archeology-prehistory-protohistory-civilization/
