Control of Growth (H)

1
Control of Growth (H)
B2 Green Plants as Organisms
2
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.
3
Control of growth
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.
4
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.
5
Control of growth
Try to complete the table, using the words
light, or gravity or water or hormone.
Factor Plant activity
Regulates the ripening of fruits.
Stems grow towards it.
Stimulates the root growth of cuttings.
Stems grow against the direction of it.
Plants wilt when too much is lost through stomata.
Roots grow to where there is more of it.
Hormone.
Light.
Hormone.
Gravity.
Water.
Water.
Answer A and D.