Oak marbles

On a walk round the old Upton Colliery site near Wakefield last week we saw a little oak tree covered in masses of oak marbles, a type of gall. I'd never seen a tree with so many on it before, so I thought I'd share this photo with you....Oak Marbles

I've always known these as oak apples, but it turns out that term actually refers to a different type of gall (now I know why latin names are so useful...). A bit of digging on the British Plant Gall Society website (again! I seem to have spent quite a lot of time there recently!) told me that they are caused by Andricus kollari, and the Wikipedia entry reckons this is a wasp that was introduced to Britain in the 1830s from the Middle East and that the galls they cause contain high levels of tannin which made them useful for dying cloth and making ink!