Flowers
and Fruits, pt. 7: (usually) Many-seeded Dehiscent Dry Fruits
Many dry fruits, when mature, split open and release their (usually
numerous) seeds. They are categorized mainly on the basis of
the number of carpels, and how they split open.
Below, wild senna. This prairie wildflower is a member of the pea
(bean,
pulse, or legume family), the fabulous Fabaceae. Its characteristic
fruit type is a farily simple one, the LEGUME, a dry fruit derived from
a unicarpellate gynoecium that splits along 2 lines or
sutures.
MOUSEOVER the image
to see dandelion in flower.
Wild senna fruits are
legumes.
Another unicarpellate
dehiscent fruit is the FOLLICLE. It splits along
one line (suture). The best example of a follicle, in fact the most
magnificent thing that Gog ever created (that's not a typo) is the
milkweed "pod." ("Pod," by the way, is not any particular type of
fruit.)
Below, Sullivant's milkweed. This is a fairly rare prairie species that
is at the eastern edge of its range in Ohio (even though it was
discovered here).
MOUSEOVER
the image to see Sullivant's milkweed in flower
(and pollinium circled).
Sullivant's milkweed follicle.
Milkweed
flower anatomy and associated pollination ecology are special. The
pollen, instead of being distributed as individual grains, is
carried instead in bulk, as packets consisting of an entire stamen's
worth
of pollen (two half-anthers each from adjacent stamens,
actually). Called "pollinia," the pollen packets are snagged
inadvertently by bees or butterflies merrily sipping sweet nectar from
the flower's special bowl-like "hoods" and then carried
saddlebag-like to a subsequently visited flower. Here's a little video
of honeybees enjoying Sullivant's milkweed.
Honeybees
visting Sullivant's milkweed.
Killdeer Plains Wildlife Area, Marion County, Ohio. July, 2008.
Pollination occurs when
a pollinium haphazardly dragged upwards across a flower happens to slip
into a downward-facing groove, the stigmatic chamber.
Once inside the chamber , the pollen grains grow, developing into
hair-thin sperm-delivering tubes, each of which makes
its way into an ovule within an ovary. Thus it is
possible (and believed typical) for all of the nearly 100 seeds in a
fruit to be sired by the same male plant, i.e., the seeds
in are "full sibs" (a fairly unusual circumstance, as flowers
typically receive mixtures of pollen from many sources).
Below, some details of milkweed pollination.
Left: milkweed
flower with pollinium inserted into the stigmatic groove.
Right: cross-section of pollinated milkweed flower.
As shown in the
cross-section above, each milkweed flower has two ovaries. There are 5
stigmatic chambers; it is believed that 3 of them serve one ovary, and
two serve another (perhaps with a little cross-over). Thus is is
possible for a flower to have both ovaries pollinated and, if there are
ample resources to support it, both ovaries may develop into
follicles. Two fruits from one flower!
Below, an example of such "twin" fruits (of a different species, common
milkweed, Asclepias syriaca). Note that, on the
left, the two follicles are attached to a single pedicel (flower stalk)
whereas on the right, the two fruits are simply from separate flowers
within the same umbel (flower cluster).
Set
of pairs of common milkweed fruits resulting from simultaneous
fertilization of two ovaries of
(left) a single flower, and (right) two separate
flowers.
The most widespread dry fruit is the CAPSULE.
Capsules are derived from a syncarpus gynoecium (more than one carpel)
and they split open to release their seeds. Capsules can split in various
ways
and are often sub-categorized on that basis (picky-picky!).
Below, royal catchfly. This, one of our very few truly red-flowered
plants, is a rare species in Ohio that nonetheless grows in fairly
disturbed prairies. It is an example of a septicidal
capsule, i.e., one that splits along the plane of carpel union.
(Other ways a capsule might split are longitudinally, or by pores at the top.)
MOOSEOVER
the image to see royal catchfly in flower.
Royal catchfly capsules.
A SILIQUE is a special capsule found only in the
mustard family (Brassicaceae). It doesn't look so special, the distinction
is internal. There's a papery partititon inside that divides the fruit
into two chambers that runs across the two carpels of which the
fruit is composed. (Usually a partion separates the carpels.)
Below,
black mustard. This common weed is actually the species from which the
condiment is derived.
MOUSEOVER
the image to see black mustard flower.
A mustard family fruit is a modified capsule called a SILIQUE .
