Oct 11,2010 Presented by Symbiosis HISTOLOGY Oct 11,2010 Presented by Symbiosis

What is Histology?

Histology is the study of tissues. A tissue is a group of physically linked cells associated with intercellular substances that are specialized for carrying out specific functions. The systems made of tissues all having specific functions, hence improving the efficiency of the body function. This is called division of labour.

Many cells are linked to make up tissue, however the cells making up this tissue can be of the same type or of different type. The study of tissue structure relies heavily on the light microscopy and its associated techniques, which can be permanent or temporary. The light microscope is used mainly to study the structure of tissue however a electron microscope is used in cases for clarity.

Plants Histology Plant cells are divided into two groups. This division is cause by the cells making up the tissue. The two groups are: One type of Cells and More than one type of cells

One type of cell In this group as the name implies these tissues in plant are simple and made up of the same type of cell. There are three main types tissue in this group, namely: PARENCHYMA COLLENCHYMA SCLERENCHYMA

PARENCHYMA The parenchyma tissue are roughly spherical, and some instances seemingly elongated. These tissues are important in some of the plants vital activities.

Functions of the parenchyma Packing tissue – This function of the parenchyma tissue is important in the leaf, stem and root of the young dicotyledonous plants. Support – The means of support is clearly explain using the herbaceous where the main use of this tissue is for support. This is possible because of the excellent osmotic properties of the tissues.

Metabolic Activities – Although the structure is un-uniformed the parenchyma tissues take part in vital metabolic activities in the plant body. Food Storage –For the continue activity or existence storage is always a essence. In plants the parenchyma tissues are at the sites of storage in food storage organs. For example in potato tuber, where the parenchyma cells stores starch.

Gaseous exchange – between the cells of the parenchyma are air spaces that allow for the diffusion of gasses for respiration and photosynthesis. This occurs readily in the spongy mesophyll layer of the leaf. Transport – The parenchyma tissues aids in the transport of materials such as, water and mineral salts, through the cells or cell walls of the plant. This process is also apart of the apoplast pathway.

Parenchyma modification Apart from the great functions of the tissue Parenchyma, In certain areas of the plant the cells may be modified for further specialization. These areas includes: EPIDERMIS ENDODERMIS PERICYCLE MESOPHYLL

EPIDERMIS This is the outer covering of the plants primary structure. It is one cell thick, and its wall made of cellulose, pectin and covering of cutin. The epidermal cells secretes cutin which forms the cuticle, which is both on the outer and inside of the cell wall. The arrangement of the epidermal cells in monocotyledons (more rectangular and regular shape) differs from the that of dicotyledons (irregular shape, wavy margins).

Other specialized epidermal cell like guard cells appear at intervals in pairs side by side with an opening called stomata which sizes are determined by the turgidity of the guard cells. These cells aids in the process of transpiration and photosynthesis. The guard cell is the only epidermal cell that contains chloroplast. In some cases epidermal cell seeks reinforcement of its function by the growth of hair like extensions call Epidermal Hair.

Functions of the epidermal hair. They increase the surface area in root increasing the absorption of water and mineral salts. They prevent slippage in roots of climbing plant. They helps cuticle in reducing water lost They helps to store water in plant that are adapted for dry conditions.

They act as defense or body guards for Nectarous plants. In some plants they act as killing sites for insect. In some cases these insects are digested. These types of epidermal cell are called Glandular.

ENDODERMIS The endodermis is a layer of cells surrounding the vascular tissue of plant. This can be regarded as the innermost layer of plants. In plants roots it is more easy to see, where as it is one cell thick, than in stems. This is so because the cells in the root develop a Casparian strip.

PERICYCLE Pericycle is the layer of parenchyma one to several cell thick which possess in the root. This is located between the vascular tissue and the endodermis. Pericycle contributes to the secondary growth of the plant if it retains its capacity for cell division. In its roots it maintain meristematic activity producing lateral roots. Pericycle are roughly shape

MESOPHYLL Mesophyll is the packing tissue found between the two epidermal layers. This consist of Modified parenchyma to carry out photosynthesis. There are two distinct layers of mesophyll in dicotyledonous plants, namely, Palisade mesophyll (which is the upper layer, consisting of column shape cells), and the Spongy mesophyll ( which is the lower layer consisting of fewer chloroplast and is irregular shape).

COLLENCHYMA Collenchyma is a mechanical tissue, providing support for those organs it is in. Just like parenchyma the structure of collenchyma is the same except for the fact that there is extra deposition of cellulose at the corners of the cell. Collenchyma is a essential strengthening agent in young plant. It is also an important strengthening tissue in supplementing the effects of turgid parenchyma. They are the first strengthening agent to grow in primary plants body because it has the ability to grow and stretch without impeding any other cell.

SCLERENCHYMA The sole function of sclerenchyma is to assist in providing support and mechanical strength for the plant. The cells of the sclerenchyma is dead hence growth and elongation is prohibited. Hence the maturity of the cells in sclerenchyma is not complete until the elongation of the living cells around is completed.

There are two types of sclerenchyma cell, namely; Fibres which are usually elongated and Sclereids which are normally roughly spherical. The primary cell wall of both cells are heavily thicken by the deposition of lignin around them. As this process continued the living contents of the matured cell die.

FIBRES Sclerenchyma fibres are strong owing to their lignified walls. The ends of the cells are inter-locked to each other adding to the strength of the cell. These fibres can be found in pericycle stems and the layer in the cortex below the epidermis.

Function of the Fibres They form solid rod of tissue capping the vascular bundles of dicotyledons. Creating a hollow cylinder in the cortex below the epidermis holding the vascular tissues.

SCLEREIDS Sclereids are generally scattered in the organ in groups and or single. They are mainly distributed in the cortex, pith, phloem and fruit seeds, but they are in other parts of the plant body too. Sclereids are mainly used for support and or mechanical protection in its contained organ.

As mentioned in previous slides, they are found in fruit flesh As mentioned in previous slides, they are found in fruit flesh. Depending on their positions and number, they add to the rigidity of the fruits flesh. In fruit seeds, their main function is the toughening of the testa.

More than one types of cells These tissues in the containing organ are made up of more than one type of cells. These multi-cells tissues can be found and clearly seen in the vascular bundles. The vascular bundles functions in translocation.

Xylem Vessels Xylem contributes to physiological and structural function in plants. The xylem vessels are made up four types of cells namely: Tracheids Vessel elements Parenchyma Fibres

TRACHEIDS These are single cells that are elongated and lignified. They offer mechanical strength and gives supports to plants. They are dead with empty lumen when mature. They are excellent water conductive cells, the only one in the xylem, of ancestral vascular plants.

VESSELS They are very long tubular strutures form by fusion of several cells end to end in a row. Vessel elements are shorter or wider than tracheids. These are better source of water transport in flowering plants. They out number the amount of tracheids.

Protoxylem and Metaxylem Just behind the meristems in plants root are areas where growth and elongation takes place. Here the first vessels grow and starts to develop. This is called the protoxylem When these cells are mature and growth as ended this is called metaxylem. Metaxylem vessels are dead

Metaxylem They have three basic pattern of scalariform, reticulate and pitted.

XYLEM PARENCHYMA These are both in primary and secondary xylem. In the secondary parenchyma two system exist namely, rays initial and fusiform initial this is because of the meristematic cells.

Xylem Fibres These fibres are originated from tracheids. They are shorter and narrower than tracheids.They have thicker walls. Xylem fibres resemble the sclerenchyma fibres.

Phloem There are five cell types in phloem,namely sieve tube elements, companion cells, parenchyma,fibres and sclereids. Phloem tubes are are composed of living cells with cytoplasm and have no mechanical function.

Sieve tubes and companion cells These are long tube like structures that translocate solutions of organic solutes. They are formed by the end to end fusion of cells called sieve tube elements. Sieve tube elements walls are made of cellulose and pectic substances. Sieve tube element have no nuclei and are dependent on companion cells.

Phloem parenchyma, fibres and sclereids The Phloem parenchyma are found in the dicotyledon.They have the same function as the Xylem parenchyma. Fibres occur in primary phloem but more frequently in the secondary phloem of dicotyledons. Fibres form vertical running band of cells in the secondary phloem. Sclereids occur in mature phloem.