I have only just become familiar with Galls and what they actually are. I thought I would try and explain these photographs instead of just saying “these are Galls” I think this is right, I am sure someone will correct me if it isn’t!
The large growths on the undersides of these oak leaves are Cherry Galls. They are made by the Cynips quercusfolii wasps (Cynipidae family) and each gall contains a grub, a parthenogenetic female (they produce offspring without mating). They will emerge in midwinter as a gall wasps. Then will lay their own eggs on or close to an Oak tree trunk in the Spring, which become tiny purple “sexual generation” (male + female) galls which mate to produce these large cherry galls again – full circle!
In the second photo , there are some small growths on the same leaf as the cherry gall, these are Spangle Galls, made by Neuroterus quercusbaccarum, another Gall wasp. In spring wasps barely 3mm long will hatch from these, they will all be parthenogenetic females. They will then lay their own eggs in oak male catkins (flowers), when they hatch the plant tissue will swell around larvae, this is what the larvae feed on, as the larvae grow so does the gall around it. Male and female wasps emerge from these, they will mate, lay eggs on the leave and the whole process starts again.
There all lots of types of oak gall as well as galls on other plants like roses, willow and brambles, which I am yet to see myself – or probably have seen but not realised what they were. I think it would be good to see more images here of different types – so if anyone has any……………. I’d love to know what to look out for in future.