I would agree with Jonathan that it is sap oozing from a wound, a filmy yeast-like fungus usually forms a skin resulting in these 'baloons' rather than a trickle.
It is worth keeping an eye on sap flows from trees as a number of rarely recorded insects are associated with these, feeding on the sap itself or the associated fungus. I will post an image (later) of a nitidulid beetle that I found a couple of weeks ago at a similar sap flow.
Thank you for this. It was the colour that confused me - most of the sap flows from these trees (we have 2) are clear and ambered coloured.
I'll check them out - this mild weather had brought out all sorts of insects and there are quite a few places on the trees where sap is seeping.
To me it looks more like resin than sap. Cherry trees and conifers have special cells that produce this resin to close wounds. In the past, resin from trees has hardened and formed amber.
How hard is it? If you cut a ball, does it have some softer, even liquid resin in the inside?
If they are hard all over, it means that the resin has hardened, so they are not fresh. In fact, they are technically not resin, but gum balls. Resin is produced by the conifer trees. This substance is produced by epithelial cells around resin ducts found mainly in the xylem, but also in the cambium and the bark. Some species that don’t have resin ducts can still produce traumatic resin ducts when needed.
Other trees produce gum. Damson trees and Cherry trees are related species, belonging to the genus Prunus, so they probably have the same ability to produce the protective gum. In this case, your tree has produced gum that has oozed from a cut in the living tissues of the trunk, or has been produced as a response to other traumatic factor.
I’ve found two other images of Damson (or Plum) tree gum:
So the resin and the gum are different from sap, having no trophic function. Here’s what Wikipedia has to say about this:
“Sap is a fluid transported in xylem cells (tracheids or vessel elements) or phloem sieve tube elements of a plant. It transports water and nutrients throughout the plant. Fluid found in the vacuoles of other cells is sometimes referred to as "cell sap". Other liquid compounds found in plants or exuded by plants, such as latex, resins or mucilage, are sometimes incorrectly referred to as sap.”
I hope this helps. Thanks for sharing this!