List the means of passive protection in plants. Give examples.

Many plants have passive protection against animal ingestion. For example, the skin cells of the leaves and stalks of nettles carry burning hairs containing formic acid on their surface. The walls of the hair are saturated with lime, and the end with silica. Such a hair, if touched, easily breaks off and scratches the surface of the skin; a burning fluid gets into the scratch, causing redness and itching in humans and animals. Some plants are protected from animal eating by thorns – outgrowths of the skin of the stem, as in blackberries and rose hips, or thorns – modified shoots, like hawthorn and thorns.