Page 99 - NCERT Science Class 10 English Medium

pumping system. To avoid this, the blood has platelet cells which circulate
around the body and plug these leaks by helping to clot the blood at
these points of injury.

Lymph
There is another type of fluid also involved in transportation. This is
called lymph or tissue fluid. Through the pores present in the walls of
capillaries some amount of plasma, proteins and blood cells escape into
intercellular spaces in the tissues to form the tissue fluid or lymph. It is
similar to the plasma of blood but colourless and contains less protein.
Lymph drains into lymphatic capillaries from the intercellular spaces,
which join to form large lymph vessels that finally open into larger veins.
Lymph carries digested and absorbed fat from intestine and drains excess
fluid from extra cellular space back into the blood.

5.4.2 Transportation in Plants

We have discussed earlier how plants take in simple compounds such
as CO and photosynthesise energy stored in their chlorophyll-containing
2
organs, namely leaves. The other kinds of raw materials needed for
building plant bodies will also have to be taken up separately. For plants,
the soil is the nearest and richest source of raw materials like nitrogen,
phosphorus and other minerals. The absorption of these substances
therefore occurs through the part in contact with the soil, namely roots.
If the distances between soil-contacting organs and chlorophyll-
containing organs are small, energy and raw materials can easily diffuse
to all parts of the plant body. But if these distances become large because
of changes in plant body design, diffusion processes will not be sufficient
to provide raw material in leaves and energy in roots. A proper system of
transportation is therefore essential in such situations.
Energy needs differ between different body designs. Plants do not
move, and plant bodies have a large proportion of dead cells in many
tissues. As a result, plants have low energy needs, and can use relatively
slow transport systems. The distances over which transport systems
have to operate, however, can be very large in plants such as very tall
trees.
Plant transport systems will move energy stores from leaves and raw
materials from roots. These two pathways are constructed as
independently organised conducting tubes. One, the xylem moves water
and minerals obtained from the soil. The other, phloem transports
products of photosynthesis from the leaves where they are synthesised
to other parts of the plant. We have studied the structure of these tissues
in detail in Class IX.

Transport of water
In xylem tissue, vessels and tracheids of the roots, stems and leaves are
interconnected to form a continuous system of water-conducting
channels reaching all parts of the plant. At the roots, cells in contact
with the soil actively take up ions. This creates a difference in the
concentration of these ions between the root and the soil. Water, therefore,

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