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karthikmns
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eval_testing_mns

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through the root hairs. The salts are in the form of electrically charged
ions. Sodium chloride (NaCl) is in the form of Na+ and Cl-, and Magnesium
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Sulphate (MgSO4) occurs as Mg2+ and SO42-. But, they are not absorbed
into the root hairs by the simple process of diffusion, but it involves the
use of energy by the cytoplasm which will be discussed in later classes.
Once ions are absorbed, the ions travel along with water in the xylem vessels
and pass to the growing points of the plants where they are used for growth
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purpose. They may also pass laterally from xylem to phloem. Thus, mineral
salts are one of the natural factors in plant growth phenomena.
Transport of materials in plants
Food such as sugar is synthesised in the green parts of plants, mainly
the leaves, but this food has to be transported to all the living cells,
especially to actively growing cells and the cells which stores food.
Free distribution by T.S. Government 2019-20 67
The veins of a leaf consist of xylem and phloem,
proboscis
and these tissues are continuous with the stem. The
following experiments provide evidence that food is
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transported in the phloem cells.
aphid
Phloem sieve tubes are extremely small and the
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analysis of their contents is not easy. Biologists
studied about food transportation in plants with the
help of aphids (greenfly). When you see aphids
phloem clustering round the young stems of plants as they
xylem
feed on the plant juices. To obtain this juice an aphid
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bark
pith pierces the plant tissues with its long needle like organ
“proboscis”. It can be shown when a feeding aphid is
fig-19: Aphid extracting food
material from plant killed and the stem carefully sectioned, the proboscis
only penetrates upto a phloem sieve tube. This
proboscis also provides a ready-made means of obtaining the juice for
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analysis! The experiment can be done in this way. An aphid is killed while
in the act of feeding and the body is then carefully cut away, leaving the
hollow proboscis still inserted into the phloem. It is found that because
the contents of the phloem sieve tubes are under slight pressure the fluid
slowly exudes from the cut end of the proboscis in the form of drops;
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these drops are then collected and analysed. The fluid is found to contain
sugars and amino acids.
Not surprisingly, aphids absorb so much sugar from the phloem that
they cannot assimilate all of it and it excretes out of the body as a sticky
syrup called honey dew. Leaves which have been attacked by aphids often
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feel sticky as a result of honey dew.
You may notice that sometimes barks of the
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tree damaged more than a half, even then the tree
some growth is alive. How is this possible?
roots Further experiments to illustrate the
conduction of sugars by the phloem have been
no growth
done by removing a ring of bark from a shoot to
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expose the wood. Remove all tissues from the
fig-20: Removing ring of bark center outwards, including the phloem. After a
(Ringing experiment) few days, when the tissues above and below the
ring were analyzed it was shown that food had accumulated above the ring,