Class IX, BIOLOGY, "Multicellular Organism (Brassica Plant)"

Multicellular Organism

The organisms consisting of many cells are called multicellular organism. e.g. Brassica Plant, frog, man etc. diagram?

Brassica Plant (Mustard Plant)

1. This plant is sown in winter and at the end of season, it produces seeds and then dies.
2. This is an annual plant.
3. An oil is extracted from seeds of this plant which is known as mustard oil.
5. The scientific name of this plant is Brassica Campestris.
(Diagram)

Non-Reproductive Parts or Vegetative Parts

These parts do not directly take part in sexual reproduction e.g. root, stem, branches and leaves.

Reproductive Parts

These parts directly take part in sexual reproduction e.g. flower, fruit and seed.

Root of Brassica

The root is that part plant which is present inside the soil. It is produced from radical of seed. The first formed root is called Primary root. During its growth, it gives off secondary and tertiary root. Each root has a root cap at its tip or apex. Behind the root cap, root hairs are present which absorbs water and salts from the soil. Roots also anchor the plant firmly in the soil.
(Diagram)

Internal Structure of Root of Brassica

When transverse section of root of Brassica is observed under the microscope, the following parts are very prominent.

Epidermis
It is outermost and protective layer. It is single celled. Some cells grow outward to form root hair.

Cortex
It is present inner to epidermis. It is made up of several layers of thin walled living cells (parenchyma cells). There are present intercellular spaces. These cells store food.

Endodermis
it is the innermost layer of cortex. There are no intercellular spaces. There are thickenings of special materials around the cell which check diffusion of water from xylem to cortex.

Pericycle
The layer present inner to endodermis is called pericycle. There are also no intercellular spaces. All the branches of roots arise from pericycle.

Vascular Bundles
Xylem is present in middle of root which extends to pericycle in the form of four rays and controls one way transport of water and salts. In between the xylem rays, phloem bundles are present which transport food in two directions.

Stem of Brassica

The stem is that part of the plant which grows above ground. It arises from plumule of seed. It is herbaceous and branched. It bears leaves and flowers. The part of stem or its branch from where a leaf arises is called node. The part between two nodes is called internode. the important function of stem is the conduction of prepared food from leaves to other parts and that of water absorbed from roots to leaves. It supports leaves, flowers and fruits like a pillar. It keeps the leaves in such a position that they can get light to prepare food.
(Diagram)

Internal Structure of Stem of Brassica

When transverse section of stem of Brassica is observed under the microscope, following parts are visible.

Epidermis
It is the outermost protective layer of stem. Outer to epidermis, there is layer of cutin which reduces loss of water from stem. The cells are compactly arranged and there are no intercellular spaces.

Cortex
It is inner to epidermis. It is made up of many layers of parenchyma and collenchyma tissues. The main function of cortex is storage of water and food.

Endodermis
It is innermost layer of cortex. It is not prominent in stem. It allows suitable quantity of waer to enter cortex from xylem.

Pericycle
It is in the form of bundles in between the endodermis and vascular bundles. It is composed of sclerenchyma cells. It forms bundle cap.

Vascular Bundles
In stem, vascular bundles are arranged in the form of ring. Vascular bundle consists of phloem and xylem. Phloem is towards outside and xylem is towards inside. Few layers of cambium are present between the xylem and phloem. Cambium causes increase in diameter of the stem with passage of time.

Medullary Rays
There are present few layers of thin walled living cells between every two layers. These are called medullary rays. The medullary rays connect the cortex with pith for the transport of food.

Pith
The central part of stem consists of living, rounded, thin walled parenchyma cells. This part is called pith. Here food is stored.

Leaf of Brassica

1. Leaf is produced on node of stem or its branch.
2. Each leaf consists of two parts. The stalk of leaf is called petiole and upper broad part is called lamina.
3. Young leaves are without petiole and their margins are entire or smooth.
4. Lower leaves are large in size. These are also without petiole but there margins are wavy.
5. In the middle of leaf there is a thick midrib.
6. From midrib, arise veins of different thickness and form a network in leaf. This arrangement of veins is called reticulate venation.
7. Veins are composed of xylem and phloem.
8. The angle formed between stem and leaf is called axil. In this axil, buds are present which gwo and become branches.
9. The dorsal and ventral surfaces of leaves are different from each other. Such leaves are called bifacial leaves.
10. The main function of leaf is the preparation of food by process of photosynthesis.

Internal Structure of Leaf

When transverse section of leaf is observed under the microscope, following structure are visible.
(Diagram)

Epidermis
This layer of cells covers both upper and lower surfaces of leaf. Upper layer is called upper epidermis and lower layer is called lower epidermis. There are more number of stomata in lower epidermis than upper epidermis. This results in less transpiration and CO2 enters according to need. Each stomata consists of two guard cells, which are bean shaped, or kidney shaped. There is a pore between guard cells through which exchange of gases takes place and water vapours come out of leaves.

Mesophyll
The tissue present between upper and lower epidermis is called mesophyll. It consists of two parts.

Palisade Mesophyll
Uper part consists of elongated cells which hare lying vertical. These are double layered closely packed cells and are called Palisade Mesophyll.

Spongy Mesophyll
The lower part is sponge like and has more intercellular spaces. This is called spongy mesophyll.
Both types of cells have chloroplasts containing chlorophyll. So, photosynthesis takes place here. The function of mesophyll is to manufacture food for the plant.

Vascular Tissue
It consists of midrib and veins. The midrib is bundle. Upper part of midrib is xylem and lower part is phloem. Pericycle and endodermis surround this bundle. Besides this Lamina has other bundle which are called veins.

Flower of Brassica

The flower is reproductive part. With the maturation of age, the plant bears yellowish flowers. Many flowers are arranged on a branch in a special way. This branch is called peduncle. This arrangement of flowers on the peduncle is called Inflorescence.

Parts of Flowers

Flower consists of a stalk and floral leaves. The stalk is called Pedicel. The upper part of pedicel is comparatively swollen and flattened. This is called thalamus. On the thalamus, floral leaves are arranged in four whorls.
(Diagram)
The details of floral leaves is as follows:

Calyx
It is the outermost whorl of floral leaves. It consists of four sepals. On maturation, its colour changes to yellow. In young flowers sepals cover the inner parts of the flower. The main function of calyx is to protect inner parts of the flower.

Corolla
It is the second whorl of floral leaves. It consists of four free petals. Its yellow colour is conspicuous and can attract insects, honey bees and butterflies which help in pollination.

Androecium
It is the third whorl present inner to petals. It is the male reproductive part. It consists of six free stamens. These are arranged in two whorls, the outer whorl has two small stamens and inner whorl has four long stamens. Each stamen consists of two parts. Lower stalk is called filament. Upper swollen part is called anther, Inside anther, a large number of pollen grains are produced. When anther matures, a longitudinal slit appears in its walls from which pollen grains escape. AT the base of filament, four nectaries are present. These nectaries secrete nectar. To get nectar, insects visit the flowers. In this way, pollen grains get attached to the bodies of insects and are transferred from one flower to the other. This process is called pollination.

Gynoecium
It is the inner most part of flower. It is female reproductive part. It consists of two carpels, which are fused. Each carpel has three parts. The basal swollen part is called ovary. The stalk like part above the ovary is called Style. The top of style is somewhat swollen and it is called Stigma. Overy contains many ovules. Ovules are ripened to form seeds while ovary is ripened to form fruit. The fruit of Brassica is called Siliqua