Eggs?

[Lesinlondon]

Hmmm, well maybe not? but these looked to be fastened to the leaf, as if by some insect? Just a bit baffled.

[Kayleigh]

They look like some sort of leaf gall..
The experts will let you know soon..

[jaguarondi]

I think they're galls acused by eriophyid mites - quite common on sycamore and other maples. But other plants also affected.

[Lesinlondon]

Ahhh. I thought they couldn't be eggs, that colour. Gall, I had to look that up and learn that "The gall itself is the plant’s response to the irritation", ie being eaten by insects. That's fascinating. Thanks!

[poschiavanus]

Rounded tops, presumably on Sycamore, should be Aceria cephaloneus. As others have said a common mite gall.

I don't think the definition is quite right: usually (always?) the gall-maker is suborning normal processes of plant development - in this case making a leaf - to its own ends. A crude analogy is if you're adding an extension someone comes along takes your bricks and builds the kind of extension they fancy attached to your house using your workmen. The gall is not a reaction by the plant it is 'deliberately' created by the gall-maker. This is why they are sufficiently similar to enable many to be identified based on the appearance of the gall.

[Lesinlondon]

Thanks for that explanation. Would you agree with this, found on Flickr -

shane58 says: Technically, galls are formed by the plant as a consequence of something living within and influencing the tissues for it's own benefit . What an amazing world we live in! This provides food and habitat (i.e. a home) for the gall causer which is usually a mite, a wasp (or maybe a moth) or a fungal growth. I think galls mainly affect the actively growing tissue as this is most 'pliable'. I'm no expert by the way just quoting the excellent book by Redfern and Shirley isbn 1 85153 214 5.