Acorns start out as a few cells at the tip of the branch.
The end of each twig can have flowers of both sexes and of course, new leaves. Other cells at the tip of the branch elongate and swell into a tiny waterfall of yellow balls, each able to explode to release a cloud of pollen grains. At the same time the ovaries and pollen form, other bud cells divide and grow into crenulated sheaths that swell and stretch into the familiar leaves we recognize. There are ways that oaks avoid self-breeding. Pollen and female flowers appear on one tree at slightly different times, and apparently, pollen from one oak does not pollinate the flowers on the same oak.