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The elephant is the largest and one of
the most powerful and intelligent animals. Even though blue whales are the
largest animals for they grow up to 30 metres, elephants top the list as far as
land animals are concerned. A fully-grown elephant reaches a height of 4 metres
and a weight of 6 metric tons.
The most remarkable feature of the
elephant is its tusks and trunk. The tusks, in fact, are two of its upper teeth
that have grown on the outside and become enlarged. A tame or trained elephant
uses its tusks to support heavy loads while wild elephants use them as weapons
of attack. Tusks are made of ivory which is highly! valuable.
And it is for tusks that elephants are
hunted down and killed. Even though articles made of ivory are banned in most
countries they are available in the black market, and this leads to elephant
poaching.
The trunk is another interesting feature
of the elephant. It serves as an arm and a hand for picking up twigs and leaves
and pushing them into its mouth. The trunk is actually the elephant’s
lengthened nose and upper lip. It is thick and well-protected on the outside
but has a tender inside.
There are two kinds of elephants - the
Asian elephant and the African elephant. The Asian elephant is relatively
smaller than its African counterpart.
While the Asian elephant is 2.75 metres tall and
weighs 3.5 metric tons, the African is 3.5 metres tall and weighs about 5
metric tons, and is darker than the Asian species.
Moreover, the Asian elephant can be easily
tamed or trained and it sleeps lying down while the African elephant is wild
and sleeps upright.
The onion is one of the most important
commercial vegetable crops grown in India: The demand for onion is worldwide.
It is used both in raw and mature bulb stage as vegetable and spices.
The bulb of onion consists of swollen
bases of green foliage leaves and fleshy scales. Maharashtra, Tamil Nadu, A.P.,
Bihar and Punjab are the important onion producing states.
Soil and its Preparation:
Onion can be grown on various soils. But
sandy loam, silly loam and deep friable soils are best suited for onion crop.
The land is prepared by giving 5-6 ploughings.
Seed rate & Time of Sowing:
In onion 10 to 12 kg seeds require/ha. For
kharif it is sown in June & for rabi sown in October-November.
Manures & Fertilizers:
25 to 30 cart loads of well rotten FYM
or compost applied at the time soil preparation per hectare.
Harvesting, Curing, Yield and
Storage:
Follow harvesting of onion bulbs at
right stage of maturity. It is important in deciding storage life of onion as
bulbs may be stored for about six months.
The onion bulbs, reach maturity when the
plants cease to produce new leaves and roots. In onion, neck fall is the
indication of maturity. Time of harvesting depends on several factors tike
planting season, cultivar, market price and condition of the crop.
In general, when about 50 percent neck
fall is seen crop is harvested. Onion for storage should be fully developed.
Thick-neck bulbs which result due to premature harvesting do not store well.
Late harvesting leads to increased
respiration, subsequent susceptibility to diseases and excessive sprouting
during prolonged storage and left in field sunburn is also noticed. Bulbs are
harvested by hand pulling if soil is light; they are also harvested by hand
implements.
Children you have studied the general
structure of Eukaryotic cells. Though all the eukaryotic cells have the same
basic structure. There are some fundamental differences between animal andplant cells. These are shown in the Table.
Cell wall is present outside the plasma membrane of
bacteria, algae, fungi and all higher plants. But, its structure and chemical
composition is different in different groups. Carbohydrates are the major
components of the cell wall. The type of carbohydrate present in the cell wall
of each of these groups in different.
In bacteria, the cell wall is made up of carbohydrates
other than cellulose. In other words, cellulose is absent in the cell walls of
bacteria. In higher plants, cell wall is made up of cellulose, hemicelluloses,
pectin (as calcium or magnesium salts) lignin.
Cell wall gives mechanical strength to the cell.
Without the cell walls, plants cannot support their long and tall body.
Moreover, when the plants are watered, cells absorb water and swell. It the
cell wall is not present, the swollen cell will burst and die. As the cell wall
is tough and hard, it prevents excess swelling of the cell. As a result, the
cell becomes slightly stretched and turgid.
When there is no water, the cell shrinks and no
pressure is exerted on the cell wall. The plant looses the stiffness and
droops. This is called wilting.
Adjacent cells in a plant are connected through long
tube like structures called plasmodesmata. These tubes penetrate through the
cell walls.
Vacuoles are fluid filled bag like structures
surrounded by a membrane. These are present in the cytoplasm of plant cells. In
a newly formed plant cell, the vacuoles are small. As the cell becomes old,
these vacuoles fuse to form a single large vacuole that fills the entire volume
of the cell.
In such
cells, cytoplasm is in the form of a thin rim around vacuole and the nucleus is
pushed to one side. The membrane surrounding the vacuoles is called Tonoplast
and it allows only certain substances to pass through. The fluid in the
vacuoles is called vacuolar sap and contains a variety of substances such as
carbohydrates, amino acids, proteins, pigments and waste material.
Plastids are the organelles which are responsible
for the colour of the plant cell. Various colours you observe in plants,
especially in flowers and leaves are due to the presence of plastids in the
cells of these parts. Depending on their structure, colour and function,
plastids are of three types.
Leucoplast (Leuco=white) :- these are white in
colour and are usually seen in plant parts which are not exposed to sun light.
They are present in the cells of seeds, tubers, rocs…… and rhizomes. They are
of different shapes – oval, rod like, spherical or filamentous. They store food
materials.
Chloroplasts :- These are green in colour. They
occur in plant parts exposed to sun light. They contain chlorophyll, in
addition to the DNA present in the nucleus and ribosomes in the cytoplasm a
small amount of DNA and ribosomes are also present in the chloroplasts.
Major function of chloroplasts is to trap solar
energy and convert this to chemical bond energy. As you have studied,
chlorophyll plays a major role in photosynthesis.
These contain fat soluble pigments which may be red,
orange or yellow. They are present in fruits and flowers and are responsible
for their colours.
All the three plastids have the capacity to change
from one to another. For example, young tomatoes are white in colour – as they
mature they turn to green and then to red in colour. You might have seen green
chillies becoming red and several flowers and fruits changing their colour.
Cell wall and plastids are present only in plant
cells and absent in animal cells. Centrioles are present in animal cells and
are absent in plant cells.
In animal cells, two centrioles are present usually
on the top of the nucleus. Together, they are called centrosome. Centrioles are
short, stout cylindrical structures and participate in the cell division you will learn more about them in next lesson.
Points to remember:
Cell wall plastids and vacuoles are present in plant
cells and are absent in animals cells. Animal cells have centrioles and plant
cells do not have centrioles.
Cell wall is also present in bacteria but is
structurally and chemically different from that of plant cell wall.
Cell wall gives mechanical strength to the cell and
also helps the cells when they are turgid. Plasodesmata connect adjacent cells.
Vacuoles occupy most of the volume of the cell and
are filled with various substances. The membrane surrounding the vacuole is
called tonoplast.
Leucoplasts store food materials while chloroplasts
help in the preparation of food materials by participating photosysnthesis.
chromoplsts give colour to flowers and fruits.
Choloroplasts have chlorophyll which required for photosynthesis
. they also have DNA and robisomes inside them. Centrioles participate in cell division.
