At this time of the year I gather acorns to put out to attract bird, particularly Jays, and Red Squirrels. So let's look at acorns a bit. There are two native species of Oak in the UK, The English Oak, Quercus robur and the Sessile Oak, Quercus petraea. As well as different leaves their acorns also differ. The English Oak has long thin acorns whilst the Sessile has more spherical ones. See picture. Sometimes the acorns are attacked by a parasitic gall wasp which results in a highly deformed acorn like the right hand side of the picture. Hard to believe but the the wasp was introduced to Britain.