Tissues class 9 science notes

Tissues class 9 science notes

tissues are the groups of similar cells, having the same origin and performing a specific function. The name tissue was given by Bichat. Tissue first evolved in coelenterates.

• Organs are group of different types of tissues performing some specific functions.  Such as Stomach, Heart, etc.
• Organ system: The entire organ that comes together to perform some specific functions.

Plant Tissues

A group of cells having same origin and co-operative with one another to performing a similar function is called as a tissue. The study of tissue is called histology. Depending upon constitution of cells, the tissues are of two types:

• Simple tissues – it is made up of similar cells.
• Complex tissues – it is made up of two or more than two types of cells.

Based on the capacity to divide the plant tissues have been classified into two fundamental types:

Meristematic tissue

It is a simple tissue that composed of a group of similar and immature cells which can divide and form new cells.

Characteristics

• Ability to grow and divide
• Absence of intercellular spaces
• Dense cytoplasm
• Mitochondria have simple structure
• Rate of respiration is very high
• Walls are thin, and made of cellulose

The meristematic cells from which all other types of cell are formed depending upon their origin, meristem is of two types: Primary & Secondary.

1. Primary meristems: these are those meristematic tissues which are derived directly from the meristem of the embryo. Depending on their position they are of three types:
2. Apical meristem: these are present on the tips of the stem, roots and their branches. They produce growth in length. 
3. Intercalary meristem: These meristematic tissues present at the internodes of the stems and at the leaf-bases. These help in elongation of organs.
4. Lateral meristem: The meristem occurs on the sides and takes part in increasing the girth of the plant. This present below the bark and in the vascular bundles in dicot stem of gymnosperm. It is responsible for increasing the diameter of the stem e.g. Cambium.
5. Secondary meristems: Meristem is formed secondarily from the permanent tissues. Secondary meristem is usually lateral. They are cylindrical meristem these meristem give rise to secondary tissue that constituent secondary growth. Common examples are Vascular cambium of root, Inter-fascicular vascular cambium of stem, Cork cambium, Wound cambium and Accessory cambia of monocots (yucca).

PERMANENT TISSUE

They are those tissues which have lost capacity to divide and have attained a permanent shape, size and function. The permanent cells may be dead or living. Depend upon the structure; the permanent tissues are classified into the following two types:

Simple permanent tissues

Permanent tissues having all cells Similar in structure and function are called simple tissues. They include protective and supportive tissues.

•  Protective tissues: these include epidermis and cork cambium.
•  Supporting tissues : these are categorised into three types

Parenchyma

Parenchyma forms the major component within organs. The cells of the parenchyma are generally isodiametric.

• They may be spherical, oval, round, polygonal or elongated in shape.
• Their walls are thin and made up of cellulose. They may either be closely packed or have small intercellular spaces. The parenchyma performs various functions like photosynthesis, storage, secretion.
• They form the ground tissue in the non – woody or soft areas of stems, leaves, roots, flowers, fruits etc.

                                  DIFFERENT TYPES OF PARENCHYMA

Functions

1. It is basically for the storage of food e.g. starch
2. It also provides turgidity to plants.
3. It provides rigidity to tissues.
4. Photosynthesis in the form of chlorenchyma.
5. Providing buoyancy and storage of metabolic gases in the form of aerenchyma.

Collenchyma

Collenchyma is made up of living cells which are usually elongated with thick corners. There is absence of intercellular spaces in this tissue. Few chloroplasts are present in the cell of Collenchyma. The Collenchyma occurs in layers below the epidermis in dicotyledonous plants. It consists of cells which are much thickened at the corners due to a deposition of cellulose, hemicellulose and pectin. These cells assimilate food when they contain chloroplasts. Inter cellular spaces are absent. They provide mechanical support to the growing parts of the plant such as young stem and petiole of a leaf.

DIFFERENT TYPES OF COLLENCHYMA

Functions

1. It provides mechanical strength to young dicot stem, leaves etc.
2. It also provides flexibility to the organ and allows their bending.
3. It prevent tearing of leafs.

Sclerenchyma

Sclerenchyma consists of long, narrow cells with thick and lignified cell walls having a few or numerous pits. They are usually dead and without protoplasts. On the basis of variation in form, structure, origin and development, Sclerenchyma may be either fibres or sclereids. The fibres are thick-walled, elongated and pointed cells, generally occurring in groups, in various parts of the plant. The sclereids are spherical, oval or Cylindrical, highly thickened dead cells with very.

T.S. OF FIBRES

Functions

1. It provides rigidity to leaves, and prevents collapsing during temporary wilting.
2. A number of fibre commercially exploited, e.g. jute.
3. Sclerenchyma is the chief mechanical tissues of the mature plant organ.

Complex PERMANENT TISSUES

The complex tissues are made of more than one type of cells and these works together as a unit. Xylem and phloem constitute the complex tissues in plants.

Xylem functions as a conducting tissue for water and minerals from roots to the stem and leaves. It also provides mechanical strength to the plant parts.

 It is composed of four different kinds of elements, namely, tracheids, vessels, xylem fibres and xylem parenchyma. Gymnosperms lack vessels in their xylem.

• Tracheids are elongated or tube like cells with thick and lignified walls and tapering ends. These are dead and are without protoplasm. In flowering plants, tracheids and vessels are the main water transporting elements.
• Vessel is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity. The vessel cells are also devoid of protoplasm. The presence of vessels is a characteristic feature of angiosperms.
• Xylem fibres have highly thickened walls and have central lumens. These may either be septate or aseptate. Xylem parenchyma cells are living and thin-walled, and their cell walls are made up of cellulose. They store food materials in the form of starch or fat, and other substances like tannins. The conduction of water takes place by parenchymatous cells

Phloem transports food materials, usually from leaves to other parts of the plant.

Phloem in angiosperms is composed of sieve tube elements, companion cells, and phloem parenchyma and phloem fibres.

Gymnosperms have albuminous cells and sieve cells. They lack sieve tubes and companion cells.

• Sieve tubes elements are also long, tube-like structures arranged longitudinally and are associated with the companion cells. The functions of sieve tubes are controlled by the nucleus of companion cells.
• The companion cells are specialised parenchymatous cells, which are closely associated with sieve tube elements.
• Phloem parenchymais made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus. The phloem parenchyma stores food material and other substances like resins, latex and mucilage. Phloem parenchyma is absent in most of the monocotyledons.
• Phloem fibres are made up of sclerenchymatous cells. These are generally absent in the primary phloem but are found in the secondary phloem. These are much elongated, un-branched and have pointed, needle like apices. The cell wall of phloem fibres is quite thick. At maturity, these fibres lose their protoplasm and become dead. Phloem fibres of jute, flax and hemp are used commercially.