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The key points covered of this video include:

1. Responding to Changes in the Environment
2. Chemical Responses to External Stimuli
3. Tropisms
4. The Control of Plant Responses by Plant Hormones

Responding to Changes in the Environment

All multicellular organisms need to respond to changes in their environment in order to survive - these changes are called stimuli. A stimulus is detectable change in the external or internal environment of an organism that leads to a response from the organism. Organisms need to respond to both external and internal stimuli. This allows organisms to avoid harmful external changes whilst also maintaining an optimal internal environment for metabolic reactions. Plants respond to environmental changes by producing chemicals or altering their growth. Some growth responses are very rapid like the closing of a Venus fly trap and other growth responses are very slow, such as the growth of a shoot towards light.

Chemical Responses to External Stimuli

When plants respond to changes in their environment, it helps them survive longer so there is a greater chance of them reproducing. Plants respond to both living and non-living components in their external environment. Plants respond to both living and non-living components in their external environment. Non-living components, such as changes in the weather are called abiotic components. Living components, such as other organisms, are called biotic components. Some plants employ chemical defenses to prevent animals eating them and other microorganisms causing damage to the plant. Some plants release tannins which are toxic to microorganisms and herbivores. Some plants also contain alkaloids in the growing shoot tips and the peripheral layers of stems and roots. Alkaloids are derived from amino acids and taste bitter - therefore deterring animals from eating the plant. Pheromones are chemicals released by plants which can affect the behavior or physiology of other organisms.

Tropisms

Plants also respond to stimuli by growing towards or away from a particular external stimulus - this response is called a tropism. A tropism is a directional growth response in which the direction of the response is determined by the direction of the external stimulus. A phototropism is when part of a plant grows in a particular direction in response to a light stimulus. A geotropism is when a part of a plant grows in a particular direction in response to the force of gravity. If a plant responds towards a stimulus, we call this a positive trophic response. If a plant responds away from a stimulus, it is a negative trophic response. Positive geotropisms help the roots anchor the plant in the soil so that it is stable and does not fall over. A chemotropism is when a part of a plant grows in response to the location of chemicals. An example of a positive chemotropism is when a pollen tube grows down towards the ovary so fertilisation can occur. Some plant responses are non--directional - these are called nastic responses. For example, the plant Mimosa pudica responds to touch by suddenly folding its leaves. This response is called a thigmonastic response - a non-directional response to touch.

The Control of Plant Responses by Plant Growth Factors

Plants do not have a nervous system like animals - instead their responses are controlled by plant hormones. These plant hormones are called plant growth factors. Plant growth factors are produced by many plant tissues and can be transported to different parts of the plant. Plant growth factors can influence cell division, cell elongation or cell differentiation which all affect the growth of the plant.