- 1 Types of Plant Tissue Systems & Their Function
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Types of Plant Tissue Systems & Their Function
Plant tissues are of two types, meristematic and permanent. Permanent plant tissues are of two subtypes, simple and complex.
Characteristics and Types of Meristematic Tissue
Nature: Cells of meristems divide continuously and help in increasing the length and girth of the plant.
These cells show the following characteristics:
- The cells of meristematic tissue are similar in structure and have thin cellulose cell walls.
- The meristematic cells may be spherical, oval, polygonal, or rectangular in shape.
- The meristematic cells are compactly arranged and do not contain any intercellular space between them.
- Each meristematic cell contains dense or abundant cytoplasm and a single large nucleus.
- The meristematic cells contain few vacuoles or no vacuoles at all.
Meristematic tissues are growth tissues and are found in the growing regions of the plant. According to their position in the plant, meristems are apical, lateral, and intercalary.
1. Apical meristems: These are situated at the growing tip of stems and roots, i.e., at the shoot apex and root apex. Apical meristems are also found at the apices of the leaves.
2. Lateral meristems: These are found beneath the bark (called cork cambium) and in vascular bundles of dicot roots and stems (called vascular cambium). They occur in thin layers. Cambium is the region that is responsible for growth in thickness.
3. Intercalary meristems: They are located at the base of leaves or internode, e.g., stems of grasses and other monocots. Such tissues also occur below the nodes (e.g., mint).
Functions of Meristematic Tissue
- 1. Meristematic tissue acts as a parent tissue from which other tissues develop.
- 2. These tissues take part in growth by the formation of new cells.
- 3. With the help of meristems, plants continue to produce new leaves, branches of stem and root, flowers, fruits, and root hairs.
- 4. The place of injury in plants is healed up by the formation of new cells by meristems.
- 5. The plant shoots lodged or bent by wind are made to grow upright by the activity of intercalary meristem.
Different types of meristems have the following functions:
- Apical meristem: It brings about the elongation of the root and stem. It results in an increase in the height of the plant, which is called primary growth.
- Lateral meristem: It causes the organ (stem or root) to increase in diameter and girth. This is called secondary growth.
- For example, cork cambium or phellogen produces a protective cork on the outside and secondary cortex tissue inside.
- Intercalary meristem: It produces an increase in the length of an organ such as leaves and internodes.
Types of Permanent Tissues
What happens to the cells formed by meristematic tissue? Cells derived from the division of meristematic tissue take up specific roles and lose the ability to divide. They, thus, form, a type of permanent tissue. The developmental process by which cells derived from meristematic tissue, take up a permanent shape, size, and function is called differentiation. In this way, cells of meristematic tissue differentiate to form cells of permanent tissues.
Different types of permanent tissues are formed due to differences in their specialization. Permanent tissues may be simple or complex. Their cells may be living or dead, thin-walled or thick-walled. Thickening may be regular or irregular.
Differences between Meristematic and Permanent Tissue
|Meristematic Tissue||Permanent Tissue|
|1. Its component cells are small, spherical, or polygonal and un-differentiated.||1. Its component cells are large, differentiated with different shapes.|
|2. Cytoplasm is dense. Vacuoles are nearly absent.||2. Large central vacuole occurs in living permanent cells.|
|3. Intercellular spaces are absent.||3. Intercellular spaces are often present.|
|4. Cell wall of its cells is thin and elastic.||4. Cell wall of its cells is thin or thick.|
|5. Nucleus of each cell of this tissue is large and prominent.||5. Nucleus of each cell of this tissue is less conspicuous.|
|6. Its cells grow and divide regularly.||6. Its cells do not normally divide.|
|7. It is a simple tissue.||7. It can be simple, complex or specialized.|
|8. Its cells are metabolically active.||8. Metabolic rate of cells of this tissue is slow.|
|9. Cell organelles of its cells are simple.||9. Cell organelles of its cells are well developed.|
|10. Cells of this tissue do not contain crystals and other inclusions.||10. Cells of this tissue possess crystals and other inclusions.|
|11. Its cells are living.||11. Its cells may be living or dead.|
|12. It provides growth to the plant.||12. It provides protection, support, conduction photosynthesis, storage, etc.|
The study of the gross internal structure of an organ, as observed in a section, is called anatomy (Greek ana = up; temnein = to cut). In other words, anatomy is the study of the way in which tissues and organs are arranged in organisms.
Simple Permanent Tissues
These tissues are composed of cells that are structurally and functionally similar. Thus, these tissues are all made of one type of cell.
Parenchyma – Description & Functions
Nature: Parenchyma (Gr., para = beside; enchyma = in-filling) forms the bulk of the plant body. Parenchyma cells are living and possess the power of division. The cells are rounded or isodiametric, i.e., equally expanded on all sides. The parenchymatous cells are oval, round, polygonal or elongated in shape. The cell wall is thin and encloses a dense cytoplasm which contains a small nucleus and surrounds a large central vacuole. In other words, parenchyma cells have living protoplasm. Inter-cellular spaces are abundant (i.e., parenchyma has loosely-packed cells).
Occurrence: The parenchyma is widely distributed in plant bodies such as stems, roots, leaves, flowers, and fruits. Thus, the parenchyma tissue is found in the soft parts of the plant such as the cortex of roots, ground tissues in stems, and mesophyll of leaves. It is also distributed in the pith, medullary rays, and packing tissue in the xylem and phloem.
Functions of Parenchyma:
- Parenchyma serves as a packing tissue, to fill the spaces between other tissues and maintain the shape and firmness of the plant due to its turgid cells.
- Due to turgidity (osmotic) property, parenchyma acts as a primary support to the stem of herbaceous plants.
- The main function of parenchyma is to store and assimilate food. Parenchyma serves as food storage tissue, e.g., starch (present in amyloplasts) in the parenchyma of the cortex of potato tuber.
- Transport of materials occurs through cells or cell walls of parenchyma cells.
- Parenchyma cells are metabolically active; their intercellular air spaces allow gaseous exchange.
- Parenchymatous tissue stores waste products of plants such as tannin, gum, crystals, resins or inorganic waste, etc.
- If chloroplast is present, the parenchyma tissue is called chlorenchyma and it performs photosynthesis, e.g., the mesophyll of leaves.
- In hydrophytes (aquatic plants) such as water hyacinth, Hydrilla (Fig. 3.9), large air cavities are present in parenchyma to give buoyancy to the plants. Such a type of parenchyma is called aerenchyma.
- In xerophytes (arid plants), e.g., succulents (plants having fleshy parts) parenchyma acts as a water storage tissue.
What is Collenchyma in Plants?
Nature: Collenchyma (Gr., kolla – glue) tissue also consists of living cells. It shows many features of parenchyma but is characterized by the deposition of extra cellulose at the corners of the cells (Fig. 3.10). In collenchyma, intercellular spaces are generally absent. Collenchyma cells are elongated in shape. They often contain a few chloroplasts.
Occurrence: The cells of the collenchyma are located below the epidermis (i.e., hypodermis) of the dicotyledon stem and petiole (leaf stalk) (i.e., in the outer region of cortex). These cells also occur in the midribs of dicot leaves. Collenchyma is absent in monocot stems, roots, and leaves.
Functions of Collenchyma:
Collenchyma is a mechanical tissue in young dicotyledonous stems and provides mechanical support and elasticity. Thus, collenchyma provides tensile strength with flexibility to those organs in which it is found. It allows easy bending in various parts of a plant (leaf, stem) without actually breaking it. When cells of collenchyma contain some chloroplasts, they manufacture sugar and starch.
Differences between Parenchyma and Collenchyma
|1. The tissue consists of thin-walled living cells.||1. The tissue consists of cells having localized thickening in their cell walls.|
|2. It is distributed in almost all the parts of the plant body.||2. It occurs mostly in the aerial parts of the plants and is restricted to the other layers.|
|3. The living cells of the parenchyma assimilate and store food. They also store waste products.||3. Collenchyma is the chief mechanical tissue in parts of a young plant particularly in the young dicotyledonous stems.|
Collenchyma. A-T.S. cells; B-T.S. cells showing deposition of cellulose at comers; C-L.S. cells.
Sclerenchyma – Description, Types, & Functions
Sclerenchyma cells (Gr. sclerous – hard) are dead cells and they are devoid of protoplasm. The cell walls of sclerenchyma are greatly thickened with lignin. Such cell wall, walls are called lignified. Due to the excessive thickening of a sclerenchyma cell wall, its cell cavity or lumen becomes nearly absent. The cells of sclerenchyma are closely packed without intercellular spaces. Thus, these cells are fitted together like tiles on a mosaic floor. A conspicuous middle lamella exists between two sclerenchymatous cells. Middle lamella is a thin layer of cementing substance containing pectin, lignin, and protein; it occurs between the cell walls of two adjacent plant cells.
Lignin is a complex polymer that acts as a cement and hardens cell walls. It provides flexibility and great tensile and compressional strength. A high tensile strength means that it does not break easily on stretching, and a high compressional strength means that it does not buckle easily. Lignin makes the cell wall impermeable, so important substances are unable to pass through it. As a result, cells that are heavily lignified do not have living content (= protoplasm).
Cells of sclerenchyma are of two types: fibres and sclereids. Fibres consist of very long, narrow, thick, and lignified cells. The length of sclerenchymatous cells varies from 1 mm to 550 mm in different plants. Fibres are usually pointed at both ends and are clustered into strands and look polygonal in the transverse section. In contrast to fibres, sclereids (also called grit cells or stone cells) are irregular-shaped. (e.g., sclereids may be spherical, oval, cylindrical, T-shaped, dumbell-shaped, or stellate). They are dead and develop in various parts of the plants such as the cortex, pith, phloem, hard seeds, etc. Often oblique thin areas are found in the thick walls of both fibres and sclereids. These are called pits.
Occurrence: The sclerenchyma occurs in abundance either in patches or definite layers. They are found in stems (around the vascular bundle), roots, veins of leaves, and hard coverings of seeds and nuts. Sclereids form the gritty part of most of ripe fruits and contribute hardness to the seed coat and nutshells.
The husk of the coconut is made of sclerenchymatous tissue. It is present in the mesocarp of the fruit of coconut (Nariyal) and yields coir a well-known fibre used for mats, cordage (ropes and cords), brushes, etc.
Differences between Sclerenchyma Fibres and Sclereids
|1. They are elongated, spindle-shaped, thick-walled dead cells.||1. They are broad thick-walled dead cells.|
|2. They are arranged in bundles, nets, and cylinders.||2. They occur singularly or in small groups.|
|3. They do not form the covering of any plant organ.||3. They form the hard covering of nuts and seeds.|
|4. They provide mechanical strength.||4. They provide stiffness.|
Functions of Sclerenchyma:
The sclerenchyma is mainly mechanical and protective in function. It gives strength, rigidity, flexibility, and elasticity to the plant body and, thus, enables it to withstand various strains.
Differences between Collenchyma and Sclerenchyma
|1. It consists of living cells.||1. It consists of dead cells.|
|2. Its cells contain cytoplasm.||2. Its cells are empty.|
|3. Its cell walls are cellulosic.||3. Its cell walls are lignified.|
|4. The thickening of the cell wall is not uniform.||4. Cell wall thickening is uniform.|
|5. Lumen of the cell is wide.||5. Lumen of the cell is narrow.|
|6. It provides mechanical support and elasticity to the plant body.||6. It is chiefly mechanical tissue.|
Stem – Functions, Structure, and Types
Under the microscope, observe the various types of cells and their arrangement. On the basis of your observation, try to answer the following questions:
1. Are all cells similar in structure? How many types of cells can be seen?
2. Can you think of a reason why there are so many types of cells?
You will find that sections of the sunflower stem (Helianthus annus) have different types of cells in it.
Together they have 7 types of cells: epidermal cells, collenchyma, parenchyma, sclerenchyma, xylem, phloem, and cambium (= lateral meristematic cells). All of these cells perform different functions; such as conduction of water (xylem), transport of food (phloem), protection (epidermis), support (collenchyma, sclerenchyma), food storage (parenchyma) and growth (cambium). Thus, from this activity you have learned that different groups of tissue cells present in a plant organ help the organisms to perform other functions efficiently.
You can try to cut sections of stems and roots of different plants (monocots and dicots) and study them.
Protective Tissues of Plants
Protective tissues include the epidermis and cork (or phellem).
The epidermis (Gr. epi = upon, derma = skin) is usually present in the outermost layer of the plant body such as leaves, flowers, stem, and roots. Epidermis is one cell thick and is covered with a cuticle. Cuticle is a waterproof layer of a waxy substance called cutin which is secreted by epidermal cells. Cuticle possesses variable thickness in plants, for instance, it is more thicker in xerophytic (or desert) plants. Cells of the epidermis are elongated and flattened and do not contain any intercellular space between them. Their inner contents are similar to parenchyma cells (they are living cells).
The main function of epidermis is to protect the plant from desiccation and infection. In fact, the cuticle of the epidermis helps to reduce water loss by evaporation from the plant surface and also helps in preventing the entry of pathogens (bacteria, fungi, etc.).
The aerial surfaces of many plants bear cutinized hair over their epidermis. They are called trichomes. They reduce the rate of transpiration. The seeds of cotton contain numerous long unicellular hairs which form the husk of cotton. In roots, the younger parts are covered with an unscrutinised layer of the epidermis called epiblema. Some of the epiblema cells give rise to tubular outgrowths called root hairs. These long unicellular root hairs increase the absorptive surface area of the root. They pass into the soil interspaces to absorb water and minerals.
As plants grow older, the outer protective tissue (i.e., epidermis) undergoes certain changes. A strip of the secondary meristem, called phellogen or cork cambium replaces the epidermis of the stem. Cork cambium is a simple tissue having only one type of cell. The cells of cork cambium are rectangular and their protoplasts are vacuolated and contain tannins and chloroplasts. Cork cambium gives off new cells on both sides, thus, forming cork (phellem) on the outer side and the secondary cortex or phelloderm on the inner side.
The layer of cells which is cut by cork cambium on the outer side ultimately becomes several layered thick cork (bark) of trees. Cells of cork are dead and compactly arranged without intercellular spaces. The walls of cork cells are heavily thickened with an organic substance (a fatty substance), called suberin deposits. Suberin makes these cells impermeable to water and gases. The cork cells do not contain protoplasm but are filled with resin or tannins. In the case of an onion bulb too, in the skin of an onion, the cell walls become thick and waterproof due to the addition of suberin.
Cork and bark are not the same structures. While cork includes the outer products of cork cambium, the bark includes the outer products of cambium such as secondary phloem and also cork cambium and cork.
Cork is protective in function. Cork cells prevent desiccation (loss of water from the plant body), infection, and mechanical injury. Cork is light and does not catch fire easily. Due to these properties, cork is used as insulators, shock – absorbers, linoleum (used as flooring), and sports goods (in the making of shuttle cocks, cricket balls, wooden paddles of table tennis, etc.) Commercial cork is obtained from the stem surface of the cork oak tree (Quercus suber) found in Southern Europe and North Africa.
Structure, Functions, Types & Mechanism of Stomata
The Epidermis of a leaf is not continuous in some places due to the presence of small pores, called stomata. Each stoma is bounded by a pair of specialized epidermal cells or two kidney-shaped cells called guard cells. The concave sides of these guard cells face each other and have a space forming a stomatal opening. Guard cells are the only epidermal cells that contain chloroplasts, the rest being colourless. The stoma allows the gaseous exchange to occur during photosynthesis and respiration. During transpiration too, water vapour also escapes through stomata.
During photosynthesis, carbon dioxide gas is taken in by the stomata from the atmosphere, and oxygen gas is released (i.e. O2 is a byproduct of photosynthesis). However, during the respiration of plants, oxygen is taken in and carbon dioxide is released via stomata. Photosynthesis takes place during the daytime (in light), but respiration occurs both in the day and night time. The process of transpiration helps the xylem tissue in the conduction of water and dissolved mineral salts by mass flow mechanism.
Complex Permanent Tissues
Complex tissues consist of more than one type of cells having a common origin. All these cells coordinate to perform a common function. Complex tissues transport water, mineral salts (nutrients), and food material to various parts of the plant body.
Complex tissues are of the following two types:
- Xylem or wood
- Phloem or bast
The xylem and phloem are both conducting tissues and are also known as vascular tissues; together both of them constitute vascular bundles.
Xylem – Definition, Types, and Function
Nature: Xylem (Gr. xylos = wood) is a vascular and mechanical tissue. In other words, it is a conducting tissue. The xylem is composed of cells (called elements) of four different types:
- Vessels or tracheae
- Xylem parenchyma
- Xylem sclerenchyma (or fibre).
Except for the xylem parenchyma, all other xylem elements are dead and bounded by thick lignified walls. Of these four types of cells of the xylem, the most important cells are vessels. Vessels are shorter and wider than tracheids. Vessels are very long tube-like structures formed by a row of cells placed end to end. The transverse walls between the vessel elements are partially or completely dissolved to form continuous channels or water pipes. Tracheids are elongated cells with tapering ends. They also conduct water. Since tracheids do not have open ends like the vessels water has to pass from cell to cell via the pits. Xylem parenchyma stores food and helps in the lateral conduction of water.
Differences between Tracheids and Vessels
|1. Single-celled.||1. Made up of a row of cells.|
|2. The end walls remain intact.||2. End (= transverse) walls get dissolved and become perforated.|
|3. 3. The walls of tracheids are very thick with a narrow lumen.||4. The walls of vessels are less thick and they have a wider lumen.|
How pits are formed in Vessels?
No lignin is laid down where plasmodesmata were present in the original cell walls. These non-lignified areas are known as pits and they allow water to pass sideways between one xylem vessel and the next. As vessels and tracheids of xylem have the lignified cell walls, so, this simply means that these cells are hollow and there are no cell contents to restrict the flow of water.
Functions of Xylem:
- The main function of the xylem is to carry water and mineral salts upward from the root to different parts of the shoots.
- Since walls of tracheids, vessels, and sclerenchyma of the xylem are lignified, they give mechanical strength to the plant body.
A Detailed Overview of Phloem
Nature: Like the xylem, the phloem (Gk. phloos = bark) contains tubes but performs no mechanical function. The phloem is composed of the following four elements or cells.
- Sieve tubes
- Companion cells
- Phloem parenchyma
- Phloem fibres
Except for phloem fibers, phloem cells are living cells.
Differences between Xylem and Phloem
|1. It conducts water and minerals.||1. It conducts organic solutes or food materials.|
|2. Conduction is mostly unidirectional, i.e., from roots to apical parts of the plant.||2. In it conduction may be bidirectional, i.e., from leaves to storage organs or growing parts or from storage organs to growing parts of plants.|
|3. Conducting channels or tracheary elements are tracheids and vessels.||3. Conducting channels are sieve tubes.|
|4. Components of the xylem include tracheids, vessels, xylem parenchyma, and xylem fibers.||4. Components of phloem include sieve tubes, companion cells, phloem parenchyma, and phloem fibers.|
|5. Three of the four elements of the xylem are dead (i.e., tracheids, vessels, and fibers). Only the xylem parenchyma is living.||5. Three of four elements are living (i.e., sieve tubes, companion cells, and phloem parenchyma) only phloem fibers are dead.|
|6. In addition to conduction, the xylem provides mechanical strength to the plant.||6. Phloem performs no mechanical function for the plants.|
Although sieve tube elements do not have nuclei, they still remain living. It is so because they are dependent on adjacent companion cells, which develop from the same original meristematic cell. The two cells, together form a functional unit. Companion cells have an extra number of mitochondria and ribosomes.
1. Sieve Tubes:
Sieve tubes are slender, tube-like structures composed of elongated thin-walled cells, placed end to end. Their end walls are perforated by numerous pores and are called sieve plates. Walls of sieve tubes are perforated. The nucleus of each sieve cell degenerates at maturity, however, cytoplasm persists in the mature cell. Thus, nuclei are absent in mature sieve tube elements. The cytoplasm of one sieve tube element is continuous with those of other sieve elements above and below due to cytoplasmic connections passing through the pores of the sieve plate.
2. Companion Cells:
Generally associated with the sieve tube is a small thin-walled cell containing dense and very active cytoplasm and a large elongated nucleus. It is called a companion cell and it is connected to the sieve tube with numerous plasmodesmata.
3. Phloem Parenchyma:
These are thin-walled, living cells of the parenchyma of the phloem. They have two functions, storage and slow lateral conduction of food.
4. Phloem Fibres or Bast Fibres:
These are thick-walled, elongated spindle-shaped dead cells that possess narrow lumen. They provide mechanical strength to the tissue. Bast fibers obtained from some plants such as jute, hemp, and flax have commercial or economic value.
Functions of Phloem:
Phloem transports (conducts) photosynthetically prepared food materials from the leaves to the storage organs and later from storage organs to the growing regions of the plant body.