PLANT GROWTH SUBSTANCES- LIFE SCIENCE- GRADE 12 (PAPER 1)
Human beings grow and function due to the production of hormones. Similarly, the growth of
plants are also regulated and controlled by ‘plant hormones’ otherwise known as plant growth
substances.
A hormone is a chemical substance that is produced by living organisms, in small amounts,
by a specific part of the organism, and is then transported to all parts of the organism, in order
to regulate cell activity. It is important to note, that hormones may either (1) stimulate, which
is to promote or (2) inhibit, which is to stop, processes from occurring.
Plant hormones are produced by tissues of certain part of the plant. There are 5 types of plant
hormones; auxins, gibberlins, abscisic acid, cytokinins, and ethene.
There are 3 important hormones, for us to note, under this topic:
1. Auxins ***
2. Gibberlins
3. Abscissic Acid
Each of which will now be discussed down below.
Auxins (I.A.A)
Auxin is a plant hormone which controls the growth of the plant. An auxin may decide to
either promote or inhibit/ retard the growth of the plant. Auxins are located in the root tip of
the plant and the growing point of the stem. Auxins can also be found in the buds of the
plant.
If there is a high concentration of auxins that is found in the plant, the growth of the stems
is promoted and the growth of the roots is retarded/ inhibited. High levels of auxins in the
roots therefore inhibits (stops) cell division and the elongation the cell. However, in the
stems, high levels of auxins promotes cell division and cell elongation.
What do Auxins do?
Auxins do 3 important things:
1. Auxins promote growth of plant tissues and organs. Therefore, auxins allow buds to
grow into flowers, as a basic example.
2. Auxins are in control of apical dominance. The apical bud of the plant creates the
auxins, this then limits the growth of the later buds of the plant, in other words, the
plant continues to grow upwards and taller instead of sidewards. Think about your
gardener, he trims or prunes the hedges, he cuts of the upper parts of hedges, correct?
By doing this, the apical buds of the hedges are removed, and no auxins are produced.
Therefore, in the absence of auxins, the hedges grow thicker and sideways, since there
are no auxins to prevent the side way development of the hedges.
3. The most important thing that auxins do, is control tropic responses in plants. What
are tropic responses you may ask; well, this will be discussed below….
Tropic responses, referred to as Tropisms refers to the growth response by a part of the
plant to an external stimulus. These external stimuli are the light, gravity, and water.
Therefore, tropism analyses how a plant grows when that plant is in connection with either,
light, gravity, or water.
1
