The spittle is the secretion of the Spittlebug nymph. They attach themselves to a stem, gorge themselves on the sap, taking up excessive amounts of water and carbohydrates. They expel the excess via their anal area, thus providing themselves with a frothy 'cocoon' which protects the nymph from predation as well as insulating it from the elements.
I didn't wreck one of the frothy masses to find the nymph, deciding to leave them in peace. I will have another look for the insect now the wind has ceased here and the camera is back on the table!
(Click to enlarge)
UPDATE: I did a little exploration.