Plant Tissue | Botany | Lecture notes of Botany

Botany Lecture Notes of Plant Tissue

Botany lecture notes preparmy covers the entire botany syllabus for botany courses in graduation and post graduation degree.

What are Tissues ?

Tissues are groups of cells performing a similar function. Study of tissue is called Histology.

Plant Tissues

Each plant organ is composed of specific number of different tissue and each are categorized according to their function, structure or origin. The plant tissue mainly divide into two types

1. MERISTEMATIC TISSUES
2. Permanent tissues.

Meristematic Tissues

Unlike animals, plants have permanent regions of growth called meristems, or meristematic tissues, where cells actively divide. The new cell is produced is small, six sided. As the cell grow the vacuole occupy the 90% of the cell volumes. The meristematic tissues are classfied as

1. Apical Meristem

Apical meristem are meristematic tissues which are produced near the tip of root and shoot, they are responsible for the increase in length of roots and shoots in vertical direction. This type of growth is known as primary growth.  Three primary meristems, as well as embryo leaves and buds, develop from apical meristems. These primary meristems are called protoderm, ground meristem, and procambium. The tissues they produce are called primary tissues.

2. Lateral Meristem

Lateral Meristem composed of cells which are responsible for the lateral growth of plant, or increase the diameter of plant stem. This type of growth is also called secondary growth. The vascular cambium and cork cambium  in the diagram are Lateral Meristem.

Vascular Cambium

Vascular Cambium function primarily in support and conduction. The cambium, which extends throughout the length of roots and stems in perennial and many annual plants, is in the form of a thin cylinder of mostly brickshaped cells. The cambial cylinder often branches, except at the tips, and the tissues it produces are responsible for most of the increase in a plant’s girth as it grows.

Cork Cambium

It lies outside the Vascular cambium. Cork cambium is in the form of a thin cylinder that runs the length of roots and stems of woody plants.

Both Cork cambium and vascular cambium are called secondary tissue because they are produced after the primary tissue are matured

3. Intercalary Meristem

Intercalary Meristem is located in between permanent tissues. Grasses and related plants have intercalary Meristem tissues (they lack vascular and cork cambium) which are present in the vicinity of nodes. They are responsible for growth in length of the plant and increasing the size of the internode, They result in branch formation and growth.

PERMANENT TISSUE

When the Meristematic tissues divide in the end they will produce the permanent tissue.

Some permanent Tissue formed by the Meristems are

1. Parenchyma
2. Collenchyma
3. Sclerenchyma

Parenchyma Tissue

Parenchyma meaning (para – ‘beside’; chyma – ‘in filling, loose, unpacked’) is the bulk of a substance. Parenchyma tissues are composed of parenchyma cell, which are abundant in the higher plants. When the parenchyma cells are produced they are spherical in shape, but when all parenchyma cells are push against one another, their thin wall is flattened as a result of which parenchyma cells of different share formed.

Some parenchyma cells have space in between them (in water lilies, and other aquatic plants), the intercellular space is that much extensive that it make a network across the entire plant. These types of tissues are called aerenchyma.

Some parenchyma cells contains numerous chloroplast and helps in photosynthesis. These types of parenchyma tissue are called chlorenchyma. When chloroplast is absent in the prenchyma cells these are mainly function as food or water storage .

Some parenchyma cells develop the extensive inner cell wall, which will increase the surface area of cell, called transfer cell (found in carnivorous plants and nectaries flowers).

Collenchyma tissue

Collenchyma cells (Fig. 4.3), like parenchyma cells, have living cytoplasm and may remain alive a long time. Their walls generally are thicker and more uneven in thickness than those of parenchyma cells. The unevenness is due to extra primary wall in the corners. Collenchyma cells often occur just beneath the epidermis; typically, they are longer than they are wide, and their walls are pliable as well as strong. They provide flexible support for both growing organs and mature organs, such as leaves and floral parts. The “strings” of celery that get stuck in our teeth, for example, are composed of collenchyma cells.

Sclerenchyma tissue

Sclerenchyma tissue consists of cells that have thick, tough, secondary walls, normally impregnated with lignin. Most sclerenchyma cells are dead at maturity and function in support.

Two forms of sclerenchyma occur: sclereids and fibers.

Sclereids may be randomly distributed in other tissues. For example, the slightly gritty texture of pears is due to the presence of groups of sclereids, or stone cells, as they are sometimes called. The hardness of nut shells and the pits of peaches and other stone fruits is due to sclereids. Sclereids tend to be about as long as they are wide and sometimes occur in specific zones (e.g., the margins of camellia leaves) rather than being scattered within other tissues.

Fibers may be found in association with a number of different tissues in roots, stems, leaves, and fruits. They are usually much longer than they are wide and have a proportionately tiny cavity, or lumen, in the center of the cell.

Complex Tissue

These are tissue which are formed by two or more types of cells which work together as a unit to perform certain task. Complex tissue helps in performing the major functions in the plants, ie. transportation of food, water and other material up and down. That is why they are also called conducting and vascular tissue.

1. Xylem or wood
2. Phloem or bast

Xylem

Xylem consist of combinations of parenchyma cells, fibers, vessels, tracheids, and ray cells. Vessels are long tubes composed of individual cells called vessel elements that are open at each end. Tracheids, which, like vessel elements, are dead at maturity and have relatively thick secondary cell walls, are tapered at each end, the ends overlapping with those of other tracheids. Tracheids have no openings similar to those of vessels, but there are usually pairs of pits present wherever two tracheids are in contact with one another. The pit pairs allow water to pass from cell to cell. The lateral conduction takes place in the rays. Ray cells, which also function in food storage, are actually long-lived parenchyma cells that are produced in horizontal rows by special ray initials of the vascular cambium.  In woody plants, the rays radiate out from the center of stems and roots like the spokes of a wheel.

Phloem

Phloem helps in distribute the food produced by photosynthesis to entire plants. The Phloem is composed of mostly two types of cells without secondary walls. Phloem mainly consist of Sieve tube, sieve cell, companion cell, phloem fiber and phloem parenchyma. Phloem is derived from the parent cells of the cambium, which also produce xylem cells; it often also includes fibers, parenchyma, and ray cells. Sieve tube members, like vessel elements, are laid end to end, forming sieve tubes. Unlike vessel elements, however, the end walls have no large openings; instead, the walls are full of small pores through which the cytoplasm extends from cell to cell. These porous regions of sieve tube members are called sieve plates. 

Sieve tube members have no nuclei at maturity, even though their cytoplasm is very active in the conduction of food materials in solution throughout the plant. Living sieve tube members contain a polymer called callose that stays in solution as long as the cell contents are under pressure. If an insect such as an aphid injures a cell, however, the pressure drops, and the callose precipitates. The callose and a phloem protein are then carried to the nearest sieve plate where they form a callus plug that prevents leaking of the sieve tube contents.

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