Physical vs Chemical Defence

There are several physical defences that a plant may have, including:

• Bark - a lot of plants, especially trees, contain an outer skin called bark. This bark is made of cellulose, which is thick and waxy. It is placed on each plant cell wall, to make it resistant to diseases.
• Thorns & hairs - much like our skin, a plant can have hairs on it, which allow the removal of dirt or movement of pathogens away from its orifices. Thorns, such as those on a bramble or rose, also help to protect it. They come in different forms, such as thick ones on a rose, or thin ones on a cactus. They stop the plant from being eaten, or from other pests that may try and interfere with them.
• Drooping leaves - the Mimosa pudica, itself a spiny plant, has another mechanism to add to it's bow. When in contact with an insect (or for instance when you go to touch the plant leaves), it will curl up its leaves and point it's stem to the floor. This in turn will make the insect drop to the ground, instead of landing on the plant.

Chemical Defences

There are several chemical defences a plant has that allows for it to defend itself. Firstly, there are metabolites*1 such as hydrocarbon isoprene, which is a volatile gas that is emitted when photosynthesis occurs. This coats the plant and prevents damage happening. This in turn prevents any pathogens attacking the plants.

Another chemical that plants have are essential oils. These are the smell a flower produces, or that the plant itself produces, and can act as a toxin toward insects. Mint plants (like Mentha viridis - spearmint) and chrysanthemums produce pyrethrins, which acts like neurotoxins for insects, and are also the basis of some insecticides, which effectively kill off insects.

Here is a YouTube video from Fuse Schools to watch:

Aspirin from Plants

Back in the time before modern medicine, medicines were mainly plant based. To relieve a headache nowadays, you would probably use paracetamol or ibuprofen. But back then (around the 1800s), people would use Aspirin.

This was originally made from a plant called White Willow (Salix alba). From the bark in all Salix species, Aspirin is derived from salicin, which is a compound. People used to chew the bark of the tree for pain relief, before being harvested or replicated for use in medicines.

Not only that, there were other uses for the Aspirin. It was used to treat diarrhoea, help reduce inflammation in joints, and for sore throats. It could also stop wounds from bleeding.

Today, Aspirin is still used, but it is synthetically made, and is also not used as often as the likes of paracetamol or ibuprofen.