Plants are not totally helpless against pests – they have their own defence mechanisms. The most obvious ones are thorny leaves or stems to deter grazing animals. A scaled-down versions of these are hairy plant stems, for instance, some plant hairs can be quite sharp and cause skin irritation.
Ragwort is posionous
Other plants, such as butterfly flower and petunia, have sticky plant hairs that act as miniature fly traps, making difficult the passage of greenflies and other small insects. Some plants rely on being unpalatable or poisonous. For example, plants with milky sap do not seem to suffer so much from greenflies, and the well-known insecticides – derris, pyrethrum, nicotine and quassia are all of plant origin.
Even common plants have ways of dissuading feeding animals – rhubarb is a strong laxative, the leaves containing large quantities of poisonous oxalic acid. Ragwort, bracken and yew are poisonous to grazing animals. Laburnum, daphne, cotoneaster, laurel and monkshood are examples of cultivated plants that are poisonous to a greater or lesser degree.