The family Orchidaceae is one of the two largest families of flowering
plants; the other, Asteraceae (Compositae), is sometimes stated to be the
larger, but this point has been hotly debated over the years. No
clear resolution is in sight, so my first sentence is one that systematists
on both sides of the issue can agree. Among monocotyledons,
the orchids (with perhaps as many as 25,000 species) have about
twice the number of species as the next largest family, Poaceae
(Graminae). Orchids are particularly diverse in the wet tropics worldwide,
and they are absent only from the polar regions and the driest
of deserts. Their highly modified flowers are, in the minds of most
people, their hallmark, but the best and only truly universal character
for the family is their unusual early developmental (postgermination)
stage, the protocorm, which is the point at which
orchids form their linkage with a symbiotic fungus.