Page 100 - NCERT Science Class 10 English Medium

moves into the root from the soil to eliminate this difference. This means
that there is steady movement of water into root xylem, creating a column
of water that is steadily pushed upwards.
However, this pressure by itself is unlikely to be enough to move
water over the heights that we commonly see in plants. Plants use another
strategy to move water in the xylem upwards to the highest points of the
plant body.

Activity 5.85.8
Activity 5.8
Activity 5.8
Activity 5.8
Activity
n Take two small pots of approximately the same size and having
the same amount of soil. One should have a plant in it. Place a
stick of the same height as the plant in the other pot.
n Cover the soil in both pots with a plastic sheet so that moisture
cannot escape by evaporation.
n Cover both sets, one with the plant and the other with the stick,
with plastic sheets and place in bright sunlight for half an hour.
n Do you observe any difference in the two cases?

Provided that the plant has an adequate
supply of water, the water which is lost through the
stomata is replaced by water from the xylem vessels
in the leaf. In fact, evaporation of water molecules from
the cells of a leaf creates a suction which pulls water
from the xylem cells of roots. The loss of water in the
form of vapour from the aerial parts of the plant is
known as transpiration.
Thus, transpiration helps in the absorption and
upward movement of water and minerals dissolved
in it from roots to the leaves. It also helps in
temperature regulation. The effect of root pressure in
transport of water is more important at night. During
the day when the stomata are open, the transpiration
pull becomes the major driving force in the movement Figure 5.12
Figure 5.12
Figure 5.125.12
5.12
Figure
Figure
of water in the xylem. Movement of water during transpiration in a tree
Transport of food and other substances
So far we have discussed the transport of water and minerals in plants.
Now let us consider how the products of metabolic processes, particularly
photosynthesis, are moved from leaves, where they are formed, to other
parts of the plant. This transport of soluble products of photosynthesis
is called translocation and it occurs in the part of the vascular tissue
known as phloem. Besides the products of photosynthesis, the phloem
transports amino acids and other substances. These substances are
especially delivered to the storage organs of roots, fruits and seeds and
to growing organs. The translocation of food and other substances takes
place in the sieve tubes with the help of adjacent companion cells both
in upward and downward directions.
Unlike transport in xylem which can be largely explained by simple
physical forces, the translocation in phloem is achieved by utilising

Life Processes 95

2024-25