seeds split in half, showing the embryos with cotyledons and primordial root (a conifer) with seven cotyledons Cotyledons may be either
epigeal, expanding on the germination of the seed, throwing off the seed shell, rising above the ground, and perhaps becoming photosynthetic; or
hypogeal, not expanding, remaining below ground and not becoming photosynthetic. The latter is typically the case where the cotyledons act as a storage organ, as in many
nuts and
acorns. Hypogeal plants have (on average) significantly larger seeds than epigeal ones. They are also capable of surviving if the seedling is clipped off, as
meristem buds remain underground (with epigeal plants, the meristem is clipped off if the seedling is grazed). The tradeoff is whether the plant should produce a large number of small seeds, or a smaller number of seeds which are more likely to survive. The ultimate development of the epigeal habit is represented by a few plants, mostly in the family
Gesneriaceae in which the cotyledon persists for a lifetime. In
Streptocarpus wendlandii of
South Africa, one cotyledon grows to be up to in length and up to wide, the largest cotyledon of any dicot, and exceeded only by the monocot
Lodoicea. Adventitious flower clusters form along the midrib of the cotyledon. The second cotyledon is much smaller and ephemeral. Related plants may show a mixture of hypogeal and epigeal development, even within the same plant family. Groups which contain both hypogeal and epigeal species include, for example, the Southern Hemisphere conifer family
Araucariaceae, the pea family,
Fabaceae, and the genus
Lilium (see
Lily seed germination types). The frequently garden grown
common bean,
Phaseolus vulgaris, is epigeal, while the closely related
runner bean,
Phaseolus coccineus, is hypogeal. ==History==