Cheat!

As anybody who's followed my journal will know,  I'm fascinated by anything in nature that's odd, aberrant or bizarre, and this is one curiosity that most people probably aren't aware of. 
If you look closely into blackthorn bushes  at this time of year - perhaps to see how the sloes are getting along - you might notice in some of them that there are two different sorts of 'berries'. One type are small, dark green  and round - they are the incipient sloes that will turn dark purple and can be used to flavour gin; the other are paler and oval in shape - they look very much like green olives here but can elongate into a curly banana shape too.They are fakes!

Taphrina pruni is a fungus that parasitises the developing fruit and creates these 'monstrous growths'  to spread  its spores which appear in the form of a white mould on the surface. Blown by the wind they can then infect other trees, including plums and damsons. The galls are totally inedible and eventually turn brown and shrivel up. There's no cure but the fungus doesn't harm the tree which carries on bearing its own fruit alongside. 

I took one of the pocket plums home and cut it in half (inset). You can see the hollow pocket inside and the empty case of what would have become the stone.

A similar fungus causes the  witch's brooms often to be seen on birch trees. Personally I don't find galls unsightly or alarming, rather another extraordinary manifestation of the interwoven fabric of life.

Some excellent pics of the gall can be seen here.