Download Control of Growth

B2: Green Plants as Organisms
Control of Growth (H)
Control of growth
•
Plants must be able to detect the directions of gravity, of
moisture and of the greatest light intensity so that their
roots and shoots can grow in the direction which give the
plant the best chance of surviving. Thus plants are
sensitive to light, moisture and gravity.
•
Plant shoots grow towards light and away from gravity.
•
Plant roots grow towards gravity and towards moisture but
away from light.
Plant growth substances, e.g. auxins influence cell division and
elongation of cells at the root and shoot tips.
If a shoot is illuminated from one side only, auxin, produced in the tip,
tends to be more concentrated on the darker side so that greater cell
elongation is stimulated on the darker side. Thus the shoot tends to
grow towards the light.
Control of growth
Narcissus plants which
have been illuminated from
one side only for 24 hours.
Narcissus plants which have
been left on their side for 24
hours.
The shoots grow away from gravity
The shoots grow towards light
The responses of plant roots and shoots to light, gravity and moisture are
the result of unequal distribution of hormones, causing unequal growth
rates.
Control of growth
The hormones which control the processes of growth and reproduction in plants can be
used by humans to:
• produce large numbers of plants quickly by stimulating the growth of roots from cuttings;
• regulate the ripening of fruits on the plant and during transport to
customers;
• kill weeds by disrupting their normal growth patterns.
For example, broad leaf
plants, such as plantains
will absorb more growth
hormone (through the
leaves) than narrow leaf
plants, such as grass.
The hormones makes the
plantains ‘bolt’ (grow too big,
too quickly) and die, but will
not harm the grass.
Ethene gas, given
off by ripe fruits,
stimulates nearby
fruits to ripen.
Thus ethene can be
added to a
greenhouse or
added to boxes of
fruit to cause
ripening.
Cutting taken from
mature plant. Cut
end dipped into
rooting hormone
powder and
planted.
After a few days roots
form from the cut end.
Rooting hormone
powder
Control of growth
Try to complete the table, using the words ‘light’, or ‘gravity’ or ‘water’ or ‘hormone’.
Factor
Plant activity
Hormone.
Regulates the ripening of fruits.
Light.
Stems grow towards it.
Hormone.
Stimulates the root growth of cuttings.
Gravity.
Stems grow against the direction of it.
Water.
Plants wilt when too much is lost through
stomata.
Water.
Roots grow to where there is more of it.
Which two of the following are correct plant responses?
A. Roots grow in the direction of the force of gravity.
B. Shoots grow towards moisture.
C. Shoots grow in the direction of the force of gravity.
D. Shoots grow towards light.
E. Roots grow away from moisture.
Answer: A and D.