Plant tissues

• Ground tissue system performs support, storage and photosynthetic functions and it is made up of three main tissues which are PARENCHYMA, COLLENCHYMA AND SCLERENCHYMA
• Parenchyma - thin primary cell wall, middle lamella composed of calcium and magnesium pectates which cement adjacent cells. The cortex is filled with PARENCHYMA CELLS in the dicot stem. Large central vacuole, shape is variable, can be spherical, elongated or lobed.
• Palisade mesophyll and spongy mesophyll cells contain parenchyma cells.
• Parenchyma cells contain intercellular air spaces such as in spongy mesophyll
• Middle lamella is present in between the adjacent Parenchyma cells. Basically the middle lamella is where 2 cells meet.
• Main functions of Parenchyma cells - Packing tissue - Most common type of tissue making the bulk of organs such as in cortex and pith. They act as packing tissues between more specialised tissues such as xylem or phloem (xylem parenchyma and phloem parenchyma)
• Main function of Parenchyma - when the parenchyma cells are turgid, they are tightly packed and provide support for the organ they are found in. Especially in harbaceous plants as they provide the main means of support. Support and packing tissues work together, and by them being packing tissues they provide support to the organ they are found in.
• Main functions of Parenchyma -Gaseous exchange - There are many intercellular air spaces especially in the spongy mesophyll. They provide a network of air spaces so gaseous exchange can occur between the living cells and the environment through the lenticels and stomata.
• Chlorenchyma are modified Parenchyma + chloroplasts
• Main functions of Parenchyma - Metabolism such as photosynthesis - Modified parenchyma with chloroplasts can perform photosynthesis.
• Maion functions of Parenchyma - Parenchyma cells can transport water and mineral salts. The transport occurs via the apoplastic pathway which is the pathway of water and mineral salts through adjacent cell walls.
• Main functions of Parenchyma cells - Storage
• Water travels from root hairs to xylem via three routes- APOPLASTIC, SYMPLASTIC AND TRANSMEMBRANE ROUTES
• Parenchyma occurrence - PITH, CORTEX, MESOPHYLL (PALISADE AND SPONGY) , BULK OF VARIOUS ORGANS (stems and roots), FLORAL PARTS, PULP OF FRUIT, AMONG XYLEM AND PHLOEM TISSUE
• Collenchyma - definite shape with uneven THICK CELL WALLS. they are thick as they contain extra deposit of cellulose and pectins in the corner.
• Collenchyma contain small intercellular air spaces or even absent as there is the thick cell wall present.
• collenchyma - Flexible but strong and provides mechanical support to the organ in which it occurs, like dicot stems, petioles, midrib of leaves
• Collenchyma's function is basically support. It is also very important support role in young organs and herbaceous plants
• collenchyma is found under the epidermis of stem as a ring , normally 3-4 cell layers thick
• Collenchyma cells are found as bundles instead of rings on the outer cortex of certain plant cells.
• Sclerenchyma (2 forms) - FIBRES AND SCLEREIDS
• Sclerenchyma fibres - elongated and needle like with overlapping tapering edges
• Sclerenchyma sclereids - roughly spherical but other forms are known
• Sclerenchyma's principle characteristic (in both fibres and sclereids) - Have a thick lignified secondary cell wall. Lignification prevents further growth so that is why sclerenchyma is found around tissues that stopped elongating
• Lignin

  • has high tensile strength - does not break easily upon stretching
  • has high compressional strength - does not buckle easily when compressed
• Both fibres and sclereids - contain simple pits

  • simple pits are areas were lignin IS NOT DEPOSITED in the primary cell wall because there are plasmodesmata present.
• Simple pits have NO FUNCTION in mature sclerenchyma because mature sclerenchyma ARE DEAD
• No intercellular air spaces are present in between sclerenchyma cells
• Sclerenchyma cells undergo APOPTOSIS (programmed cell death) at MATURITY and so they have NO PROTOPLASM and their PITS have NO FUNCTION!
• Main function of both fibres and sclereids -

  • Support plant structures and provide mechanical strength
• fibres protect vascular bundles as bundle caps in DICOTS and bundle sheaths in MONOCOTS.
• sclereids fuction is to give firmness or rigidity to the tissue in which it is found .
• Fibres are found in - indivisual fibres or in groups within xylem and phloem.
• fibres are found in - leaves in both monocots and dicots
• Fibres are found below the epidermis of monocot stems.
• As a bundle cap in dicots protecting the vascular bundles
• Fibres are found as Bundle sheath in monocots protecting the vascular bundles.
• Stone cells - Clump of sclereids found in some fruits like pear or apples
• The main component cell types of xylem are TRACHEIDS, XYLEM VESSEL ELEMENTS, FIBRES AND PARENCHYMA .
• Tracheids, xylem vessel elements and fibres are all dead. But parenchyma cells are alive.,