Structures of Mustard (Brassica Campestris) Stem

Give an illustrated account of the external and internal structures of mustard (Brassica Campestris) stem

EXTERNAL STRUCTURE OF STEM:

 A stem develops from the plumule of a seed away from the soil upwards. The stem is hemoaceous and branched. It bears leaves and flowers. The point on the stem or a branch from where leaf arises is called a node. The part of the stem between two adjacent nodes is called internode. The stem provides support to the leaves in such a way that they may receive maximum sunlight tor preparing me food during photosynthesis. The stems bear flowers which are the reproductive organs of plants and take part in reproduction.

Internal Structure of Mustard Stem: 

A transverse section of a mustard stem under a microscope shows the following parts.

(1) Epidermis: It is the outermost single layer of stems. There are no intercellular spaces in the epidermal cells. The epidermis of stem has a thick and waxy cuticle which is a special chemical substance called cutin. Stomata and lenticel pores are present. 

Function: 

• It prevents water loss by waterproofing stem. 
• It protects the inner tissues. 
• Exchange of gases and water vapour take place during transpiration through the epidermis.

(2) Cortex: Inside the epidermis lies the cortex which is packed with parenchyma. Sometimes chloroplasts are present in the outer region forming the collenchyma tissues. 

Function: 

• Transport water. salts and gases across the stem. 
• Photosynthesis takes place in green stems. 
• Sometimes they store food as in rhizomes.

(3) Vascular Bundles: (several in a ring or scattered). Vascular bundles are present in a ring. Each vascular bundle consists of an outside phloem of sieve tubes and an inner xylem of vessels. A few layers of cells called cambium is present between the xylem and phloem.

Function: 

• Translocation of organic food materials up and down from the leaves to all parts of the plant. 
• Transport of water and salts upwards from the roots to leaves. 
• Formation of new phloem and xylem tissues. 

(4) Endodermis: The innermost layer of the cortex is called the endodermis. It is not prominent in stem. 

Function: 

• This layer controls the transport of water from xylem to cortex. 

(5) Pith: The central part of the stem is called pith. It is made up of living, rounded, thin walled parenchyma cells. 

Function: 

• It stores food. 
• It gives limited support.

Give an illustrated account of both the external and internal structures of mustard (Brassica campestris) root.

EXTERNAL STRUCTURE OF BRASSICA ROOT:

The root arises from the radicle of the seed and grows downwards in the soil. The first root arising from the radicle is called the primary root. The primary root gives rise to secondary and tertiary roots. The primary root is thicker than any other root. The tips of all roots bear a root cap. It protects the root. Behind the root cap are present root hairs. The roots absorb water and mineral salts only through the root hairs. The roots fix the plants to the soil.

Internal Structure of Brassica Root: 

(1) Epidermis: It is the outer layer of the root. (epi=above, derma=skin). It is single layered. Root hairs are the outgrowths of epidermal cells, they are unicellular. There is no cuticle on the epidermis. The epidermis is also called the piliferous layer. 

Functions: 

• Absorption of water, salts and soil air through root hairs. 
• Protection of inner tissues. 

(2) Cortex: Lying inside the epidermis, is cortex. It is composed of many layers of thin walled living cells, parenchyma (packing tissue) cells.

There are intercellular spaces between parenchyma cells which are filled with water. 

Functions: 

• Transport of water, salts and air across root, either through or between cells. 
• Food is stored in the cortex. e.g. carrot.

(3) Vascular Bundles or Stele: The stele consists of central xylem, xylem is star shaped. 

In between the arms of the xylem is phloem. A cambium layer is present in the stele along with the xylem and phloem.

Functions: 

• Xylems transport water and salts upwards through transpiration. 
• Phloem transport organic food material up and down the plant through translocation. 
• Cambium layer is responsible for the formation of new phloem and xylem tissues.

(4) Endodermis: It is the innermost layer of the cortex with no intercellular spaces is There are thickening of special materials around cells. The endodermis surrounds the stele. 

Function: 

• It checks/stops the diffusion of water from xylem to cortex.

(5) Pericycle: The layer next to endodermis is called the pericycle There are no intercellular spaces in pericycle.

Function:

• All the branches of root originate from pericycle.

STRUCTURAL ORGANIZATION OF LIFE

IMPORTANT QUESTIONS   

Q.1: How was cell discovered? 

ANS: DISCOVERY OF CELL:

In 1610. Galileo, an Italian astronomer and physicist developed a microscope to observe small organisms.

• In 1665, Robert Hook made a compound microscope by combining lenses. It was a better microscope. Robert Hook examined a slice of cork under it. The cork was made from the bark of Oak. He found small honey comb like chambers and called them cells. 
• In 1842, Dutrochet boiled some plant material in nitric acid and examined it under a microscope. It consisted of cells.
• In 1831. Robert Brown discovered a spherical body, the nucleus, in the cells of orchids.
• In 1838, a German. botanist. Schleiden suggested that all plants are made of cells. 
• In 1839, Schwann, a German zoologist. suggested that all animals are made of cells.

Q.2: What are the three principles of the cell theory? 

ANS: CELL THEORY: 

The cell theory was formulated separately by a German botanist, Schleiden, in 1838 and a German zoologist, Schwann, in 1839. The salient features or three principles of this theory are as under:

• All organisms are composed of one or more cells. 
• All cells arise from pre-existing cells. 
• The cell is the structural and functional unit of all organisms. 

Measurements and the systems of units

System of units: 

     A set of fundamental and derived units is called a system of units. 

Names of system of units: 

• M.K.S System 
• C.G.S System 
• British Engineering System 
• System International (S.I ) 

(i) M.K.S System (Meter, Kilogram, Second System): 

In this system fundamental units of length, mass and time are meter, kilogram and second respectively. 
Units of other physical quantities can be expressed in terms of meter , kilogram and second. 

(ii) C.G.S System (Centimeter, Gram, Second System): 

In this system the fundamental units of length, mass and time are centimeter, gram and second respectively. 

(iii) British Engineering System:

In this system length, force and time are taken as fundamental quantities. Hence the fundamental units are taken as foot for length, pound for force and second for time. 

• This system is also known as F.P.S system. 
• Mass is taken as a derived quantity, it's unit is Slug.

(iv) System International (S.I ) units:

This system of units was introduced in 1960 and is now in use all over the world. This system of units is based on the following seven independent fundamental units.

Definition of Physics

Physics is a branch of science that deals with the study of properties of matter and energy and the interaction between them. Physics is basically an experimental science, it depends upon objective observations and accurate measurements of various natural phenomenon. 

Main Branches of Science: 
Science is divided into two main branches:

• Physical sciences 
• Biological sciences 

Main Branches of Physical Sciences: 
Physical sciences are divided into several branches such as: 

• Physics 
• Chemistry 
• Astronomy 
• Geology etc. 

Main Branches of Biological Sciences : 
Biological sciences are divided into following branches: 

• Botany 
• Zoology 
• Physiology etc.

Main Branches of Physics: 
Physics is divided into several branches such as: 

• Mechanics 
• Electricity 
• Thermodynamics 
• Atomic physics 
• Nuclear physics 
• Astro physics 
• Solid state physics 
• Plasma physics 
• Medical physics 
• Particle physics 
• Spectroscopy 
• Low temperature physics and many more