Imagine waking up one morning with a similar growth coming out of your skin – maybe on some intimate part of it – and feel for this Tilia × europaea. It was the only specimen presenting such symptoms I could spot while walking around the Pfaueinsel in the middle of the Wannsee. It looked like a horrible disease to me: I hope it does not sound too insensitive if I admit that I have found its effects quite beautiful and curiously looking like tiny red peppers, so I took a pic.
The strange outgrowths one can find on stems, leaves and roots of plants are called galls – they come in an impressive variety of shapes, colours and sizes and are mostly caused by stuff like parasites, mites, fungi, bacteria. Another name of gall is cecidia, and there is a whole science studying them – cecidology – that you probably never heard about and that does not seem to be a highlight of science popularisation: there’s not even a Wikipedia dedicated page.
The galls you see in the picture above are commonly known as lime nail gall or bugle gall and are formed by a cutie mite called Eriophyes tiliae. As scary as such chemically induced rash may look, it seems that this sort of galls – as well as many others – does not affect the health of the plant, at least not to our knowledge. Evil galls that dramatically deform the natural shape of plants or cause damaging infestations have been indentified, but all in all they seem to be a minority.
Otherwise, some galls are useful for other purposes as they can be eaten by insects, and of course humans found a way to take advantage of them as well: Galls are full of tannic acid and resins that have been used in the production of permanent inks.