SEXTO GRADO: PLANT TISSUES

PLANT TISSUES

Plant tissues can also be divided differently into two types:

1.  Meristematic tissues

2.  Permanent tissues

Meristematic tissues

Meristematic tissue consists of actively dividing cells, and leads to increase in length and thickness of the plant. The primary growth of a plant occurs only in certain, specific regions, such as in the tips of stems or roots. Cells in these tissues are roughly spherical or polyhedral, to rectangular in shape, and have thin cell walls. Depending on the region of occurrence of meristimatic tissues, they are classified as:

a)   Apical Meristem - It is present at the growing tips of stems and roots and increases the length of the stem and root. This meristem is responsible for the linear growth of an organ.

b)   Lateral Meristem - This meristem consist of cells which mainly divide in one plane and cause the organ to increase in diameter and growth. Lateral Meristem usually occurs beneath the bark of the tree. This activity is called secondary growth.

c)   Intercalary Meristem - This meristem is located in between permanent tissues. It is usually present at the base of node, inter node and on leaf base. They are responsible for growth in length of the plant. This adds growth in the girth of stem.

The cells of meristematic tissues are similar in structure and have thin and elastic primary cell wall made up of cellulose.

Meristematic tissue cells have a large nucleus with small or no vacuoles, they have no inter cellular spaces.

 

Permanent tissues

The meristematic tissues that take up a specific role lose the ability to divide. This process of taking up a permanent shape, size and a function is called cellular differentiation. Cells of meristematic tissue differentiate to form different types of permanent tissue. There are 2 types of permanent tissues:

1. simple permanent tissues

2. complex permanent tissues

Simple permanent tissues

These tissues are called simple because they are composed of similar types of cells which have common origin and function. They are further classified into:

1.   Parenchyma

2.   Collenchyma

3.   Sclerenchyma

4.   Epidermis

Parenchyma

It consists of relatively unspecialized cells with thin cell walls. They are live cells. They are usually loosely packed, so that large spaces between cells (intercellular spaces) are found in this tissue. This tissue provides support to plants and also stores food. In some situations , it contains chlorophyll and performs photosynthesis, and then it is called chlorenchyma.

 

Collenchyma

Cells are thin-walled but possess thickening of cellulose and pectin. This tissue gives a tensile strength to the plant and the cells are compactly arranged and do not have inter-cellular spaces. It occurs chiefly in hypodermis of stems and leaves. It is absent in monocots and in roots.

Sclerenchyma

Sclerenchyma consists of thick-walled, dead cells. These cells have hard and extremely thick secondary walls, the cell walls are strong, rigid and impermeable to water. Sclerenchymatous cells are closely packed without inter-cellular spaces between them. Sclerenchymatous cells mainly occur in hypodermis, pericycle, secondary xylem and phloem.

The cells of sclerenchymatous cells can be classified as :

1.  Fibres- Fibres are long, elongated sclerenchymatous cells with pointed ends.

2.  Sclerides- Sclerenchymatous cells which are short and possess extremely thick, lamellated walls with long singular piths. They are called sclerides.

The main function of Sclerenchymatous tissues is to give support to the plant.

Epidermis

The entire surface of the plant consists of a single layer of cells called epidermis or surface tissue. The entire surface of the plant has this outer layer of epidermis. Hence it is also called surface tissue. Most of the epidermal cells are relatively flat. the outer and lateral walls of the cell are often thicker than the inner walls. The cells forms a continuous sheet without inter cellular spaces. It protects all parts of the plant.

        

Complex permanent tissue

A complex permanent tissue may be classified as a group of more than one type of tissue having a common origin and working together as a unit to perform a function. These tissues are concerned with transportation of water, mineral, nutrients and organic substances. The important complex tissues in vascular plants are xylem, phloem.

Xylem

Xylem is a chief, conducting tissue of vascular plants. It is responsible for conduction of water and mineral ions.

Xylem is a very important plant tissue as it is part of the ‘plumbing’ of a plant. Think of bundles of pipes running along the main axis of stems and roots. It carries water and dissolved substances throughout. These cells are joined end to end to form long tubes. Vessel members and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. The tracheids ends overlap with each other, with pairs of pits present. The pit pairs allow water to pass from cell to cell. While most conduction in the xylem is up and down, there is some side-to-side or lateral conduction.

Phloem

Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells that are without secondary walls. Sieve tubes are formed from sieve-tube members laid end to end. The end walls, unlike vessel members in xylem, do not have openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. In spite of the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. It is the companion cells that are nestled between sieve-tube members that function in some manner bringing about the conduction of food. Phloem transports food and materials in plants upwards and downwards as required.

QUESTIONS

Copy and answer the following questions in your notes.

1.     State and define the 2 types of plant tissues.

2.     In which direction do meristematic tissues allow for growth?

3.     What are the different types of meristematic tissues?

a.     State where they are located and what kind of growth they allow.

b.     Draw each kind of meristematic tissue.

4.     Where do permanent tissues come from?

5.     What are the 2 types of permanent tissue?

6.     State the 4 different types of simple permanent tissue.

7.     Make a drawing and a summary of each of the 4 different types of simple permanent tissue.

8.     What is complex permanent tissue’s function?

9.     What are the 2 types of complex permanent tissue?

10.                        Make a drawing and a summary of what xylem and phloem mean.