How do plants grow?

Growth hormones in plants

Auxins:

Plants do not have a sophisticated responsive system, like the nervous system in animals.  Therefore, they can't respond as quickly as animals.  They react to stimuli much more slowly.  Their reactions are controlled by hormones called AUXINS.

• Auxins are produced in the tips of root and shoot.  
• The Auxins dissolve in water and are then transported throughout the plant.  
• These then stimulate cell growth at the roots and shoots in the elongation region, which is just behind the tips.  
• Auxins also regulate flowering and ripening of fruit.  
• The effect of Auxins can be demonstrated by removing the tip of shoots.  If no Auxins available, then the shoot may stop growing.  
• Shoot tips also produce growth inhibitor for side shoots.  If the tips are decapitated, it results in a lot of side shoots, because there is a lack of inhibitor substance. Farmers/gardeners often clip the tops of hedges to promote bushier hedges.

Auxins control the way plants grow.  They promote growth in the shoots but in fact inhibit growth in the roots.

Shoot animation courtesy of science-resources.co.uk

Shoots:

• The shoot of the plant always grow towards light.  This is due to the fact that leaves need light to make the food needed for plants to grow.  See photosynthesis.  
• Growing towards the light is called PHOTOTROPISM.  'Photo' means light and 'tropism' means growth.  
• When a shoot tip is subjected to light, it provides more Auxins to the shaded side than the side facing the light.  This results in the shoot to grow faster on the shaded side and the tip bends towards the light.
• However, when shoot happens to grow on its side, the gravity leads to uneven allocation of Auxins in the tip.  The lower side ends up with more Auxins.  Hence the lower side grows faster, thus bending the shoot upwards.

Shoot light animation courtesy of science-resources.co.uk​

Shoot gravity animation courtesy of science-resources.co.uk​

Roots:

• Growth of roots is affected by GRAVITY.  
• They bend towards gravity.  Tick Growing towards gravity is known as GEOTROPISM.  'geo means gravity and 'tropism' means growth.  
• When a root grows on its side, more Auxins are found on the lower side.  However, extra Auxins in fact inhibit growth, thus causing the upper side to grow faster resulting it to bend downwards.  
• Root growth is also affected by MOISTURE.  
• The moist side of root will attracts more Auxins then the dry side.  The growth of the moist side is thus inhibited and causing the root to grow towards the moisture.

Root gravity animation courtesy of science-resources.co.uk​

Root gravity animation courtesy of science-resources.co.uk​

Using plant hormones:

Plant hormones are used commercially to produce food that is more convenient to us.  For example: 

• Growing Seedless Fruit - we can use synthetic growth hormones (Auxins) to trick plants into making fruit that we enjoy.  Normally, plants produce fruit when they pollinated by insects, with inconvenient seeds in the middle of the fruit.  If the plants don't get pollinated, the fruit and seed is not produced.  Using synthetic Auxins it is possible to over come this problem.  If Auxins are applied to un-pollinated flowers the plant will produce a seedless fruit, which is more convenient to eat.
• Ripening Fruit - Auxins can also be used to control the ripening of fruit.  This can be done whilst the fruit is still attached to the plant or during transport to the food stores.This is useful to the farmers for a number of reasons.  They can pick the fruit whilst it is still unripe.  Fruit is then easy to handle, and doesn't get damaged.  During transportation the fruit then can be sprayed with ripening hormones so that it is perfect as it reaches the supermarkets. 
• Making more Plants From Cuttings - For a plant to produce more young plants naturally, it can take many years. Cutting shoots off plants and dipping them in Auxins can speed up this process.  Gardeners use Auxins, known as rooting powder, to make new plants.
• Getting Rid of Weeds - Many of the weeds found on a lawn or a field have broad leaves compared to grass which has thin narrow leaves.  Selective weedkillers have been developed from Auxins which only affect the broad leaved plants.  The hormones accelerates the growth to an extent that it kills the weed whilst leaving the grass alone.