Beginning vegetable gardeners are often confounded by the
appearance of flowers on their summer squash plants that are followed by...no
fruit. Or even worse, they see a nascent zucchini behind the flower, but
apparently some terrible disease sets in and the baby squash, instead of
growing to its full, baseball-bat-sized glory, withers away.
It's time for floral anatomy. Understanding
and
identifying flowers help us understand more about the goings-on in our gardens,
whether we need to figure out fruit production, identify a plant, read a
botanical description or only want to appreciate the intricacies of the natural
world.
The first part of a flower's structure we see is often
what is covering the flower up. Flower buds may be covered with bracts, which
are leafy structures that open and remain under the flower or break away when
the petals expand. But sometimes the bracts are more showy, and take on the
appearance of flower parts.
The bright red (or white or pink, nowadays) parts of a
Christmas poinsettia (Euphorbia pulcherrima) are bracts, and a cluster of
flowers lie at the center. This is true of all the temperate euphorbias,
although in other species the bracts might not be quite as showy. But still, in
species such as Euphorbia characias, we can see the bract acting as a backdrop
or collar to the true flower.
Our native dogwood, Cornus nuttallii, and other treeish
dogwoods including C. kousa and C. florida also have showy bracts that take the
place of petals in overall appearance, while the true flowers are tiny.
Flower arrangement is a
broad category that helps us
divide and explain the way plants look when they are in bloom. Although some
flowers, such as Shirley poppies, appear singly, others occur in an
inflorescence, which is a cluster of flowers. Inflorescences come in a variety
of arrangements.
The stalk of an inflorescence is called the peduncle.
Flowers may be attached to the peduncle by a short stem of their own, which is
called a pedicel. Here are some commonly used terms for the different kinds of
arrangement of the inflorescence.
The flat-topped inflorescence of Angelica is called an
umbel; all of its flowers arise from a single point. It's an easy floral
arrangement to draw: one long line creates the peduncle, and at the top of the
line all the pedicels fan out. In a flat-topped umbel, the pedicels end up
being different lengths so that the top makes a horizontal plane.
Umbels can also be round; ornamental
onions, such as
Allium cristophii and chives (Allium schoenoprasum) have umbels. Some flowers,
such as yarrow (Achillea) are described as having "umbellike"
arrangements. A corymb can appear flat-topped, but the flowers actually grow
out of different points on the peduncle. The inflorescence of the European
elderberry, Sambucus nigra, although appearing to be a flat-topped umbel, is a
corymb.
Flowers that grow on a spike, such as Acanthus mollis,
are attached directly to the stem. A spike that has flowers attached by
pedicels is a raceme; garden penstemon, such as 'Sour Grapes' and 'Apple
Blossom', bloom on a raceme. But many inflorescences can appear-and be
described as-spikelike, because the pedicel is so short that it is almost
unnoticeable. These include hollyhocks, common foxglove (Digitalis purpurea)
and toadflax (Linaria purpurea).
Following close on the heels of a raceme is a panicle,
which has pedicels that are branched. Lilacs have panicles that, if they divide
once again, are called thryses.
The composite family (Asteraceae) is noted for its daisy
flowers. But one daisy flower contains quite a surprise when you look closely:
it is made up of lots of tiny flowers in the center surrounded by what are
called ray florets (and what we commonly call petals). This arrangement is
easiest to see in a giant sunflower, where with even a hand lens with low power
reveals each flower's position.
And speaking of parts of the flower, here they are:
petals, usually the showiest part of the flower; the collective term for petals
is the corolla. Sepals, the outer layers of a flower that often look like
petals; the collective term for sepals is calyx. The collective term for petals
and sepals together is perianth. The male part of the flower is the stamen,
which has a stalk (the filament) and an anther, where pollen is held. The
female part of the flower is the pistil, which comprises the stigma, where
pollen lands, and the style, which is a tube that leads to the ovary, where
seeds develop.
Now let's look at a real life example. Consider the
poppy: The slightly hairy sepals fall away as the showy petals open; in the
center is a ring of dark stamen that surround the pistil, which looks like a
disk.
When the petals and sepals can't be differentiated, the
structures are called tepals. Tulips and magnolias have tepals.
A flower that has all its parts
is called complete. If a
flower is missing something-no petals, for example, it is incomplete. A flower
that has both male and female parts is a perfect flower. If it has only one or
the other it is imperfect.
And now it's back to that zucchini. Squash plants produce
imperfect flowers: some flowers are male and some are female. Often, the first
flowers gardeners see in the season are the male flowers. They contain stamens
but no ovary. The female flowers do possess an ovary, and it looks like a small
squash just behind the flower.
If the female flower doesn't get pollinated, the nascent
fruit drops off. To the gardener, this looks suspiciously like a disease, but
in fact it's just a case of not having a male flower open at the same time for
the bees to transfer pollen.
There are other plants that produce staminate and
pistillate flowers, such as begonias. If a plant has complete flowers or if
both male and female flowers are on the same plant then that plant is
monecious; it needs only one plant to produce seeds, although you may get
better fertilization if you have more than one plant. That's why it's
recommended to cross-pollinate apples.
Other species have male and female flowers on entirely
separate plants; these species are dioecious. Two plants are needed for
reproduction to take place. Ginkgo biloba is dioeceous, and most cultivars sold
in nurseries are male selections, because the fruit of the Ginkgo is messy and
odoriferous. Other dioecious plants include Japanese holly (Ilex crenata),
Osmanthus heterophyllus, and kiwi (Actinidia).
The time is right to take these floral lessons out
into
the field. Buy a seven- or ten- powered hand lens that you can carry around in
your pocket while out in the garden or on a walk. It will come in handy as you
examine the flowers you see, and the more you look, the more you'll understand
the mysteries of flowers.
