Tissues

• Group of cells or associated intercellular substances which generally share similar origin in the embryo specialized for particular function / functions
• In multicellular organisms, cells arranged themselves to form tissues
• Fourth level in the organizational hierarchy of the living organisms

 

Plant tissues

Classified into two based on the nature of dividing ability of the cells

1.Meristamatic tissue

e.g. – apical meristem, cambium

Features-

• Cells are isodiametric and rectangular shaped
• Have only thin primary cell wall
• Small vacuole
• Non specialized cells
• Have no intercellular spaces
• Prominent nucleus
• Form from specialized and non-specialized cells

2. Permanent tissue

• Mature cells
• Do not involve to further cell division
• Highly specialized cells
• Permanent tissues found in plants are of two types

1. Simple permanent tissue– have cells all of which are of same type

• Parenchyma
• collenchyma
• sclerenchyma

     2. Complex permanent tissue– have cells which are of different types

• vascular tissues

Cell specialisation

• Cells become specialised in structure and function is known as cell specialisation.
• During cell specialisation chemical and physical changes occur in the shape and Organisation of the cell

 

 

 

Parenchyma

• living cells
• large central nucleus, cytoplasm are found
• have thin primary cell wall
• cell wall is mainly made up of cellulose, pectin and hemicellulose
• have clear intercellular spaces
• generally spherical shape
• some are elongated e.g. – palisade parenchyma cell

Location

• plant cortex
• pith
• ground tissue
• medullary rays
• xylem parenchyma
• phloem parenchyma

Functions

• mainly food storage
• supporting tissue in herbaceous plant

Modifications of parenchyma

• epidermis –
  • protection
• endodermis –
  • selective permeability of water and mineral
  • Suberin deposits (casparian strips) in the radial and tangential cell walls
• mesophyll cell –
  • photosynthesis
• pericycle –
  • secondary growth
  • lateral root formation
  • permeability to water
• parenchyma are not a specialised group of cells
• they have the ability to act as meristamatic tissue
• contain chloroplast

Collenchyma cells

• living at maturity
• only primary cell wall
• unevenly thickened at corners
• cell wall is made up of cellulose and hemicellulose
• rectangular shaped cell in longitudinal section with tapering ends
• polygonal shape in cross section
• lack of intercellular spaces or absent

Function–

• mechanical support in herbaceous plant

• only found in shoot parts of the plant, cannot be seen in any of the root parts
• Eventhough the collenchyma cells are of primary origin, they can also be seen in secondary plant parts
• formed by primary tissues
• some of them consist chloroplast

Location

• the peripheral region of cortex of tender stem
• above and below the mid rib of the leaves

• cannot be produced by secondary tissues

 

Sclerenchyma

• dead cells
• have no intercellular spaces
• have secondary cell wall made up of Lignin
• primary cell wall is made up of cellulose
• Pits are found in cell wall
• There are of two types of scelerenchyma cells
  • fibres
  • Sclereids

General function

• mechanical support

Adaptation

• cell wall is made up of cellulose, hemicellulose and Lignin

 

• Fibres
• elongated cells
• polygonal shape in cross section
• cell wall component cellulose
• function -mechanical support
• found in
  • pericycle of the stem
  • xylem
  • phloem
• Sclereids
• roughly cubical shape
• Lignin is deposited as layers
• Have several branched pits from the centre towards the external
• function – mechanical support and protection
• found in – seed coat
• in aquatic plants Sclereids provide the support

 

Vascular tissues

• found in the vascular bundle
• function
  • transport of the substances

There are two types

• xylem
• phloem

 

Phloem

• components;
  • sieve tube element
  • companion cell
  • phloem parenchyma
  • phloem fibres
• Sieve tube element are elongated tubular like structure
• living cell
• large central vacuole
• thin peripheral cytoplasm
• lack nucleus
• cell wall do not have Lignin deposit
• only primary cell wall made up of cellulose
• several sieve tube elements are joined end to end to form a continuous sieve tube
• terminal end of sieve tube element is sieve tube plate
• each sieve tube elements joint with companion cell by plasmodesmata
• this region of plasmodesmata is known as sieve area
• function;
  • Transport of food. This is a passive process (pressure flow). Does not depend on the time of the day. Bidirectional conduction. ATP energy is required for the loading and unloading of the food in phloem. but energy is not required for the conduction through sieve tube .substances transported are sucrose, plant growth substance, vitamins, minerals and chemicals applied to the plant
• Companion cell
• living cell
• prominent nucleus
• dense cytoplasm
• Packed with numerous mitochondria, well developed endoplasmic reticulum, Golgi complex, ribosome and small vacuole.
• Function;
  • Phloem loading and phloem unloading. ATP energy required.

At the terminals of small veins companion cells are larger than the sieve tube elements. They are specialised and have ingrowth. These cells are known as transfer cells. In Dicot stem generally phloem located exterior to xylem. Function of phloem- transport

 

Xylem

• it consists of following components
  • xylem parenchyma
  • xylem fibres
  • tracheids
  • xylem vessels
• Xylem vessels
• found only in Anthophyta
• extend from the root tip the tip of the leaves
• several xylem vessel elements are joined together to form xylem vessel
• elongated tubular structure
• secondary cell wall mainly made up of Lignin
• primary cell wall is made up of cellulose
• secondary cell wall provide support strength and adhesive force to the xylem
• lumen of each xylem vessels are inter connected with other
• function;
  • mechanical support to the plant body
  • water and mineral conduction

 

Tracheids

• found in Pterophyta, lycophyta, cycadophyta, coniferophyta and anthophyta
• Mainly function in Pterophyta, lycophyta and cycadophyta
• Single cell
• spindle shaped
• lumen of every cells are connected with the other which is adjacent to it
• secondary cell wall is made up of Lignin
• primary cell wall cellulose
• function;
• mechanical support
• water and mineral conduction
• transport of plant growth substances (abscisic acid and cytokinin)