Plant Responses To The Environment

Question 1

Plant growth substances

Answer 1

Auxins, gibberellins and abscisic acid are some examples of plant growth substances that may be considered to be hormones. However, they are not true hormones because they may bring about an effect in that part of the plant body where they are produced.

Question 2

Functions of Auxins

Answer 2

Bring about a bending reaction in plants known as tropism. Geotropism and phototropism are examples of tropisms.
Promote cell division.
Responsible for cell enlargement.
Responsible for optical dominance. Auxins in the optical bud will promote vertical growth and inhibit the growth of natural branches. If the optical bud is removed then the growth of bilateral branches is promoted.
Promote root development.

Question 3

Functions of Gibberellins

Answer 3

Bring about elongation of the internodes of stems.
Stimulates root growth.
Promote development of flowers.
Appear to promote sprouting of buds which have been dominant.
Increases fruit size.

Question 4

Functions of Abscisic Acid

Answer 4

Brings about dormancy of seeds by slowing down germination, and dormancy of optical buds.
Induces flowering in some plants.
Helps in the closing of stomata during periods of water shortage or drought.
Promotes the ageing of leaves.

Question 5

Phototropism & Geotropism

Answer 5

Auxins are plant growth substances produced at the tips of roots and shoots. The most well-known Auxin is indole-acetic acid (IAA). Auxins bring about bending reactions in plants referred to as tropisms.

Question 6

What is a tropism

Answer 6

A tropism is a growth movement or bending reaction of a plant (or part of a plant) in response to an external stimulus. The direction of growth or bending of the plant depends on the direction of the stimulus. When the external stimulus is light, this is known as phototropism. If the external stimulus is gravity, this is known as geotropism

Question 7

Phototropism

Answer 7

Phototropism is the growth movement of plant organs in response to the stimulus of light. Stems are positively phototropic i.e. they grow towards light. Roots are negatively phototropic i.e. they grow away from light.

Question 8

Geotropism

Answer 8

Geotropism is the growth movement of plant organs in response to the stimulus of gravity. Roots are positively geotropic while stems are negatively geotropic.

Question 9

Role of Auxins in phototropism

Answer 9

Auxin are produced at the tip of the stem from where they move downwards evenly.
The even distribution of auxins brings about equal growth on all sides of the stem.
The stem grows straight upwards.
When stems are exposed to unilateral light (light from one side) the brightly-lit side suffers from a shortage of auxins probably because the auxins are destroyed by the light or because they move to the darker side.
A high concentration of auxins in stems promotes growth.
An uneven distribution of auxins uneven growth of the stem with the darker side growing faster.
The stem thus bends towards the light i.e. stems are positively phototropic.

Question 10

Role of Auxins in geotropism

Answer 10

Auxins are produced at the tip of the root from where they move upwards evenly.
The even distribution of Auxins brings about equal growth on all sides of the root.
Therefore, the root grows straight downwards.
When a root is placed horizontally the auxins accumulate on the lower side probably because of gravity.
A high concentration of auxins in roots inhibits growth.
Thus uneven distribution of auxins causes uneven growth of the root with the upper side growing faster.
The root thus bends downwards i.e. roots are positively geotropic.

Question 11

Weed Control by using Growth Hormone

Answer 11

Hormone weed-killers are auxin-based selective herbicide, which are pesticides that kill only one specific unwanted plant as long as they are used in the appropriate dosage. Hormone weed-killers target and kill weeds specifically. Hormone weed-killers is made by chemical synthesis.

Question 12

The advantage of hormone weed-killers

Answer 12

The advantage of hormone weed-killers is that one no longer needs to handpick annoying unwanted weeds in the garden. The weed-killers are not toxic for animals or humans, so they are child- and pet-friendly.

Question 13

the disadvantage of hormone weed-killers

Answer 13

Hormone weed-killers target weeds, but unfortunately, plants that are similar to weeds in terms of being broad-leaved can be damaged. The symptoms that occur in broad-leaved plants contaminated by hormone weed-killers include distorted or or twisted leaf stalks, swollen stems and worts or galls on the plants. When used in high quantities, hormone weed-killers caused reduced plant growth and can cause plant death. Thus, Its important to apply hormone weed-killers in quantities suggested by the producer.

Question 14

Plant Defense Mechanisms (Thorns)

Answer 14

Many plants have thorns on their stems and leaves. These thorns serve as a form of protection. Since plants are unable to move or escape from threats such as herbivores, they develop thorns as a means to keep from attack. The thorns are evolutionary developments that allow for group defense when plants grow in bunches. Thorns also allow for plants to spread to other areas as vegetations can become stuck in predators fur and skin.

Question 15

Plants that developed thorns

Answer 15

Plants that have developed thorns include pomegranates and lemons. Cacti and roses also grow thorns on their stalks and stems.