Acorn
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The acorn, or oaknut, is the nut of the oaks and their close relatives. It usually contains a seedling surrounded by two cotyledons, enclosed in a tough shell known as the pericarp, and borne in a cup-shaped cupule. Acorns are 1–6 cm long and 0.8–4 cm on the fat side. Acorns take between 5 and 24 months to mature; see the list of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.
Acorns from small to large of the Willow Oak, Q. phellos (very small, at center); the Southern Red Oak, Q. falcata; the White Oak, Q. alba; the Scarlet Oak, Q. coccinea; from southern Greenville County, SC, USA. Scale bar at upper right is 1 centimetre (0.39 in).
Diagram of the anatomy of an acorn: A.) Cupule B.) Pericarp (fruit wall) C.) Seed coat (testa) D.) Cotyledons (2) E.) Plumule F.) Radicle G.) Remains of style. Together D., E., and F. make up the embryo.
Ponies eating acorns. Acorns can cause painful death in equines, especially if eaten to excess amounts.
Cotyledon
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A cotyledon is a "seed leaf" - a significant part of the embryo within the seed of a plant, and is formally defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed." Botanists use the number of cotyledons present as one characteristic to classify the flowering plants (angiosperms): species with one cotyledon are called monocotyledonous ("monocots"); plants with two embryonic leaves are termed dicotyledonous ("dicots").
Cotyledon from a Judas-tree (Cercis siliquastrum, a dicot) seedling
Comparison of a monocot and dicot sprouting. The visible part of the monocot plant (left) is actually the first true leaf produced from the meristem; the cotyledon itself remains within the seed
Peanut seeds split in half, showing the embryos with cotyledons and primordial root
Two-week-old Douglas fir (a conifer) with seven cotyledons