I don't know if it occurs in any other family, but orchids don't have any
cotyledons at all. The seeds are extremely tiny, and were evolved to be wind
dispersed. There may be 3 million seeds in one orchid capsule. Orchid seeds
don't have any endosperm. The only food they contain may be a couple of fat
cells. When an orchid seed finds the right location & starts to germinate, it
is invaded by a mycorrhiza, usually Rhizoctonia. The fungus provides the sugar
needed by the baby seedling, & in the wild, the orchid plant has the mycorrhiza
for the rest of its life. In cultivation, orchid seeds are germinated in
sterile laboratory flasks on nutrient rich agar.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)