Common physical defences against herbivory are thorns, barbs, spikes, spiny leaves, fibrous, inedible tissue, hairy leaves, stings and trichomes (stinging hairs).
- Chemical defences are:
- Tannins are phenols that give the leaf a bitter taste. They are toxic to insects as they inhibit salivary enzymes e.g. tea, red wine.
- Alkaloids are bitter tasting nitrogenous compounds. They may act as drugs and be toxic e.g. caffeine, nicotine, morphine and cocaine. Caffeine prevents the germination of other seeds.
- Terpenoids are toxic to insects and fungi. Pyrethrin produced by chrysanthemums is an insect neurotoxin. Citronella is an insect repellent.
