I suspect they are "Galls". A response by the plant to a foreignsubstance,frequently but not always,introduced by insects. The growths are the plants tissues and an attempt to isolate the invading substance.

They are galls, inside will be the larvae of some type of small insect like an aphid or wasp. The insects introduce chemicals to the plant causing them to distort.

I agree to the comment of "adam h".

Combine "2:14 anon" + "adam h" to get your answer. Galls are not always caused by insects. Often microbes can cause them.See http://www.jmu.edu/biology/k12/galls/batessx.htm

These are leaf galls. Leaf galls are often caused by insects such as aphids, midges or mites. I know most about gall-forming aphids, since they infest the red elms near my house. Here is how an aphid leaf gall is formed. As the mother aphid sucks sap from the leaf, she leaves behind a chemical called a morphogen that causes the leaf cells to change shape and form a pouch. An aphid eating on the upper surface of the leaf will cause a gall to form on the underside, and vice versa. Since your galls are on the top side of the leaf, this could explain why you may not have seen the insects.The gall pouch eventually encloses the mother-to-be, who deposits eggs inside the gall. The eggs hatch into nymphs, which mature inside the gall. After 3 weeks or so the gall splits open and releases them. During this time, the color of the gall also changes from green to red. The red color must make the galls more visible to birds, who love to snack on them -- a high protein snack in a small package! I have seen lots of gall-infested leaves with holes in them where galls used to be. I have observed that elm gall aphids don't kill their host tree, even the leaf they infest. If the galls survive to maturity, they just dry up, leaving a small brown spot.If you want to see for yourself what kind of insect is causing the galls you found, pick a leaf or two from the affected tree to take home. Carefully cut a gall in half with an X-acto knife or razor blade, and view the contents through a magnifying glass. A dissecting microscope works even better, if you have access to one. A good reference book for British plant galls is British Plant Galls: Identification of galls on plants and fungi by Margaret Redfern and Peter Shirley, Field Studies, 10, (2002) 207-531.