Knopper galls develop as a chemically induced distortion of growing acorns on pedunculate oak trees, and the gall  (pertains to a kind of swelling growth on the external tissues of plants) produced to reduce the fecundity of the oak host, making this gall potentially more of a threat to the reproductive ability of the tree.
Knopper galls develop as a chemically induced distortion of growing acorns on pedunculate oak (Quercus robur L.) trees, caused by gall wasps, which lay eggs in buds with their ovipositor. The gall thus produced can greatly reduce the fecundity of the oak host, making this gall potentially more of a threat to the reproductive ability of the tree than those that develop on leaves, buds, stems, etc. Galls (from Latin galla, 'oak-apple') or cecidia (from Greek kekidion, anything gushing out) are a kind of swelling growth on the external tissues of plants, fungi, or animals. 
