Pterocarpus angolensis, or wild teak, looks like a perfectly normal tree until it's wounded. When you cut into it, it dribbles long trails of dark-red liquid down its trunk. Wild teak has come to be known as Bloodwood, for obvious reasons.
Tannins cause the dark red color of the sap. Regular plants have parts – leaves, skins, sap – that are about 12-20% tannins. Bloodwood has sap that is 77% tannins.
Tannins are the chemicals that make your mouth feel a little like it was washed with cleanser after you eat grapes or drink wine. They are polyphenols, which means they are made up of many phenols. If that doesn't clear anything up for you, phenols are otherwise known as carbolic acid, which explains the astringent taste of tannin-rich foods. Tannins can bind with or precipitate proteins, and scientists think that they form part of the plant's defense mechanism. When an animal bites into the plant, the tannins, which are usually stored in only certain parts of the plant, are let loose. Not only are they supposed to taste bad to animals, their ability to bind to nutrients like proteins (and sometimes carbohydrates and starches) reduces the animal's ability to digest its food. Tannins can make food not worth eating.
Which isn't to say that we don't eat them anyway. Tannins leach into wine from the oak barrels it's stored in, and humans have developed a taste for them. Sometimes wine-makers add powdered tannins to wine to simulate the flavor.
Does the bloodwood's copious amount of tannins reduce its consumption? Perhaps by animals, but not by humans. The timber is sought after in its native Africa, and around the world. Because of the sap's resemblance to blood, it's even become a folk-remedy for diseases of the blood.
Top Image: Trees of Bangalore, via Wikimedia Commons.