_Spiranthes cernua_
_Flowers_--Small, white or yellowish, without a spur, fragrant, nodding
or spreading in 3 rows on a cylindrical, slightly twisted spike 4 or 5
in. long. Side sepals free, the upper ones arching, and united with
petals; the oblong, spreading lip crinkle-edged, and bearing minute,
hairy callosities at base. _Stem:_ 6 in. to 2 ft. tall, with several
pointed, wrapping bracts. _Leaves:_ From or near the base, linear,
almost grass-like.
_Preferred Habitat_--Low meadows, ditches, and swamps.
_Flowering Season_--July-October.
_Distribution_--Nova Scotia to the Gulf of Mexico, and westward to the
Mississippi.
This last orchid of the season, and perhaps the commonest of its
interesting tribe in the eastern United States, at least, bears flowers
that, however insignificant in size, are marvellous pieces of mechanism,
to which such men as Charles Darwin and Asa Gray have devoted hours of
study and, these two men particularly, much correspondence.
Just as a woodpecker begins at the bottom of a tree and taps his way
upward, so a bee begins at the lower and older flowers on a spike and
works up to the younger ones; a fact on which this little orchid, like
many another plant that arranges its blossoms in long racemes, depends.
Let us not note for the present what happens in the older flowers, but
begin our observations, with the help of a powerful lens, when the bee
has alighted on the spreading lip of a newly opened blossom toward the
top of the spire. As nectar is already secreted for her in its
receptacle, she thrusts her tongue through the channel provided to guide
it aright, and by the slight contact with the furrowed rostellum, it
splits, and releases a boat-shaped disk standing vertically on its stern
in the passage. Within the boat is an extremely sticky cement that
hardens almost instantly on exposure to the air. The splitting of the
rostellum, curiously enough, never happens without insect aid; but if a
bristle or needle be passed over it ever so lightly, a stream of sticky,
milky fluid exudes, hardens, and the boat-shaped disk, with pollen
masses attached, may be withdrawn on the bristle just as the bee removes
them with her tongue. Each pollinium consists of two leaves of pollen
united for about half their length in the middle with elastic threads.
As the pollinia are attached parallel to the disk, they stick parallel
on the bee’s tongue, yet she may fold up her proboscis under her head,
if she choose, without inconvenience from the pollen masses, or without
danger of loosening them. Now, having finished sucking the newly-opened
flowers at the top of the spike, away she flies to an older flower at
the bottom of another one. Here a marvellous thing has happened. The
passage which, when the flower first expanded, scarcely permitted a
bristle to pass, has now widened through the automatic downward
movement of the column in order to expose the stigmatic surfaces to
contact with the pollen masses brought by the bee. Without the bee’s
help this orchid, with a host of other flowers, must disappear from the
face of the earth. So very many species which have lost the power to
fertilize themselves now depend absolutely on these little pollen
carriers, it is safe to say that, should the bees perish, one half our
flora would be exterminated with them. On the slight downward movement
of the column in the ladies’ tresses, then, as well as on the bee’s
ministrations, the fertilization of the flower absolutely depends. “If
the stigma of the lowest flower has already been fully fertilized,” says
Darwin, “little or no pollen will be left on its dried surface; but on
the next succeeding flower, of which the stigma is adhesive, large
sheets of pollen will be left. Then as soon as the bee arrives near the
summit of the spike she will withdraw fresh pollinia, will fly to the
lower flowers on another plant, and fertilize them; and thus, as she
goes her rounds and adds to her store of honey, she continually
fertilizes fresh flowers and perpetuates the race of autumnal
spiranthes, which will yield honey to future generations of bees.”