Wounded plants bleed, just like animals do. This is a common defense mechanism in tropical trees – secretion of compounds that either seal a wound or deter further attack. These ‘secondary compounds’ are only necessary in a world with enemies – they are expensive investments and involve the evolution of biochemical pathways that have no positive impact on growth or reproduction. Caffeine and nicotine are two well-known examples of plant-produced chemicals whose purpose is mainly to deter attack. Here are a few examples of how plants bleed to defend themselves.
First, plants often secrete a latex when their stems or leaves are cut. This latex is a milky fluid comprised of many different types of chemicals that can be sticky, poisonous, or both. Latex can come in many different colors, and may appear to either ooze slowly out of a stem or spurt out immediately – thus, scientists can use latex as an important character for identifying an unknown plant. Here is the latex from Manilkara bidentata, a common tree that grows here. It is thick and very sticky – even after several hand-washes it will remain attached to your skin.
Some plants also exude resins, to seal themselves after existing damage and to prevent future infection. Pines commonly produce resins. Here is an example of the solid white resin of Dacryodes excelsa, one of the most common species here. This resin can be collected and used for sealing wood or making candles!
Many plant defenses are also dangerous to people. The examples I showed here are safe to touch, but many are not. For example, the latex of Euphorbia cooperi is used as a fish poison and can easily cause permanent blindess in humans. It is dangerous to cut open the stem of an unknown plant in some parts of the world!