What is the essence of chemical protection of plants from herbivorous animals, pathogenic bacteria and viruses?

What is the essence of chemical protection of plants from herbivorous animals, pathogenic bacteria and viruses? Give examples of such plants?

Plants also have chemical protection means. For example, in celandine and milkweed, milky juice is formed – a liquid containing chemicals that prevent animals from eating these plants. A similar role is played by resin (resin), which is filled with resin passages in the organs of some plants, for example, conifers. The resin makes their tissues less edible, prevents spores of parasitic fungi from getting into plants, as it seals openings that form on the bark during damage.
Some plants release phytoncides into the environment – volatile biologically active substances produced by cells of aboveground and underground organs. Phytoncides protect plants from pathogens. For example, volatile onions, garlic, horseradish, mustard inhibit the growth and development of many pathogens of infectious diseases.

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