Grape Pearls.

Grape pearls, or sap balls, are tiny sap-filled droplets that are exuded from surface cells of rapidly growing grapevines.  Often mistaken for insect eggs, which is what I thought they were when I first saw them, grape pearls are a curiosity (to me at least) found on grapevine shoots, and the underside of leaves, in the spring only.  In fact, I had to look under several leaves before I found a node displaying these slightly opaque sap balls: they were much more evident a week or so ago, but I was too busy elsewhere to stop and take a photograph. 
The technical name for this phenomenon is guttation.  Grapevines experiencing rapid spring growth can exhibit guttation under high moisture conditions - diffusion pressure builds inside the plant because of high soil moisture and a low rate of transpiration due to high humidity.  The built up pressure is released by exuding water and minerals from specialised cells...voila, grape pearls.  Apparently, pearls occur on some grape varieties more than others, which probably explains why I have seen then mainly on the Orange muscat vines.