Leaf galls can frequently be spotted on oak trees. These galls are caused by a species of parasitic wasp, known (logically enough) as the gall wasp or gallfly. In the center of the gall, were you to cut it open carefully, you would find the larvae of one of these wasps.
The gall wasp injects its egg into the flesh of the leaf. As the larvae hatches and grows, the gall grows around it, like a sort of leaf tumor. The mechanism by which this gall grows is unknown. The larvae grows fat and healthy, eating the gall flesh of the leaf.
Of course, other parasitic wasps have learned about this trick. In fact, most galls contain both the parasitic larvae of a gall wasp, and the parasitic larvae of a wasp which feeds on the parasitic larvae of the gall wasp. (It's like an Escher drawing! But with parasitic wasps.)
Another interesting fact is that many gall wasps practice parthenogenesis, which is where the female reproduces without any interaction with a male. Some species practice blended methods (sometimes reproducing sexually, and other times using parthenogenesis) and other species alternate generations (with one generation reproducing sexually, and the next generation reproducing through parthenogenesis).
Rose Bedeguar Gall
Galls can be produced from other irritants, and in other shapes. One well-known type of gall is called a Rose bedeguar gall, or Moss gall. These form on certain species of rose bush, and are caused by a particular species of gall wasp.
The bedeguar gall is caused by a gall which forms at a leaf bud, causing the leaf to grow out in a distinctive and bizarre pincushion shape. Rose bedeguar galls have captured people's attention and imagination for centuries, and feature both in English and Persian mythology.
Oak Apples and Oak Marbles
One particular species of gall wasp, when it parasitizes the leaves of one particular species of oak tree, creates a big hard lump the size and shape of a small apple. A slightly different species of wasp creates a smaller gall, called an "oak marble."
Oak marbles are, like many galls, filled with concentrated chemicals from the tree itself. In the case of the oak gall, the concentration of tannic acid allowed them to be used in one of the earliest forms of ink. Iron gall ink is complicated and painstaking to make, and is one of the original inks used by the earliest writers in medieval times, from the 12th century until the early 20th century.
Cedar-Apple Rust (Pictured)
This is an example of a gall caused by a parasitic fungus. The fungus infects the host plant (often a juniper) and causes it to grow "orange tentacle-like spore tubes" which "have a jelly-like consistency when wet."
It's worth noting that it's not the fungus itself which grows this way. It is the fungus which causes the tree branches to grow freakish jelly spore tentacles. (Aren't you glad you're not a tree?)
Photo credit: Flickr/photoholic1