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Speaking of flowers...
Representing innocence, flowers-in all of their beauty-have been bestowed
with symbolic meaning and revered for their mystical powers in every culture.
And no wonder-close your eyes for a moment and picture the ethereal blue
of a morning glory, the sensuous curves of a rosebud, the alluring scent
of the gardenia. What mystery and magic they hold for us!
But, alas, this is a course in botany, not poetry. While we might like to
think that those beautiful colors and fragrances are there to please us,
the sole function of the flower
is reproduction. These modified shoots have evolved over millennia
to attract pollinators,
or otherwise ensure the mechanics of fertilization. The allure that they
hold for us is merely incidental.
Flowers have adapted some very clever ways to attract pollinators. We're
all familiar with brightly colored flowers that promise  sweet
nectar to foraging bees. In the process of gathering nectar, the bees inadvertently
pick up pollen and carry it to the next flower, fertilizing it. But consider
also the hammer orchid, whose flower evolved to resemble a female wasp to
attract its pollinator-you guessed it, a male wasp. Or skunk cabbage, whose
strong odor attracts pollinating beetles. And then there are beautiful color
patterns on foxglove and iris-like the lights on an airport runway, these
patterns guide pollinators to the flowers' nectar (and pollen).
Although there is an almost endless variety of flower shapes, most flowers
do have some features in common. As we have already said, flowers are specialized
shoots, bearing special "leaves" (petals), designed solely for reproduction.
Many flowers are composed of four parts that are arranged in circles or
whorls.
 The
petals represent one of the sterile whorls and, as we have already mentioned,
are usually brightly colored. The sepals compose the other sterile whorl
and are commonly green and thick. They are located below the petals. The
sepals cover and protect the flower parts when the flower is a bud. Both
the petals and the sepals are leaflike in structure.
The stamens and pistils represent the two fertile whorls. The stamen
is the male part of the flower and is composed of the anther
and the filament. The pistil
is the female part and consists of the stigma,
style, and ovary.
We will learn much more about the structure and function of these parts
in the context of sexual reproduction.
 
The majority of flowers are perfect,
which in this context means they contain both stamens and pistils. However,
some are missing one or the other of these fertile whorls and are said to
be imperfect. In fact, any one
of the four floral whorls can be missing from a flower. This makes the difference
between complete (all whorls
present) and incomplete (lacking
a whorl) flowers.
Plants that have evolved to be pollinated by the wind usually have relatively
non-showy flowers. Think of grass flowers-those fluffy or spiky heads-or
the flowers on many trees, such as birch or walnut. Although sometimes deemed
"insignificant" by gardeners, these flowers too carry the responsibility
for continuing the species.
Can plants tell time?
How do plants know when it is time to produce flowers? Why is it
that some bulbs need several weeks of chilling before you can force
them to flower?
As you have probably observed, many plants have a specific  bloom
period that may last weeks or even months, but that is consistent
from year to year. Daffodils bloom in the spring, garden phlox in
midsummer, and certain asters in the fall.
It's tempting to say that it just takes longer for aster flowers
to develop than for phlox flowers to develop. But compare the low-growing,
spring-blooming Phlox subulata with the taller, summer-blooming
garden phlox, Phlox paniculata. Here are two plants with
similar flowers that never-theless bloom months apart.
For many plants, especially those native to temperate regions, the
onset of flowering is closely tied to day length. This phenomenon
is known as photoperiodism.
The discovery of photoperiodism. Photoperiodism
is a fairly recent discovery. Scientists first linked the onset
of flowering to day length in the 1920s while experimenting with
soybeans and tobacco. During one experiment, plots of soybeans were
planted at two-week intervals throughout the spring and early summer.
Surprisingly, all of the plants flowered at approximately the same
time, no matter what their age. Based on this result, scientists
postulated that an environmental factor was triggering the flowering.
Further experiments on tobacco also supported this explanation.
Most tobacco plants flower during the summer. However, around 1920,
a mutant appeared in a field of tobacco growing near Washington,
DC. The plant had unusually large leaves and grew to an enormous
height without ever flowering. This new variety was named "Maryland
Mammoth," and was the subject of several experiments by two researchers
from the USDA, W. W. Garner and H. A. Allard. The researchers took
cuttings of this new variety and grew them in a greenhouse, where
they would be protected from frost. These cuttings flowered in December-even
though at that time they were only half as tall as the field-grown
specimen. Plants grown from this mutant's seed also flowered in
the winter.
Based on these and other experiments, scientists concluded that
the flowering was related to day length, or the number of hours
of light the plants received. They termed this phenomenon photoperiodism,
and categorized plants as long-day, short-day, or day-neutral.
Eventually, scientists discovered that it was actually the hours
of uninterrupted darkness that triggered flowering, rather
than the hours of light. Experiments showed that even brief flashes
of light during the dark period of the cycle could interfere with
flower development-a discovery that we use to our advantage when
growing some plants, such as poinsettias. Despite this new understanding
of photoperiodism, the terms long- and short-day, which refer to
hours of light rather than darkness, are still commonly used.
So, back to our original question: Can plants tell time? We've seen
that they are able to measure, and respond to, the relative lengths
of day and night. So they must have some capacity to "count" the
number of hours. Scientists have determined that a plant's leaves
are responsible for doing the counting. Leaves contain a light-sensitive
protein pigment called phytochrome.
This pigment occurs in two stable forms, one sensitive to visible
red light, one sensitive to far-red light (that on the edge of the
visible spectrum). A pigment molecule can convert from one form
to the other, depending on the type of light it receives. In total
darkness, however, the far-red sensitive form slowly reverts to
the other form. The length of total darkness, then, determines the
ratio of the two forms. This is how plants "count" the number of
hours of darkness. And it is because of phytochrome's ability to
convert from one form to another that the plant is able to detect
when any type of light breaks the dark period.
Only when a plant's darkness requirement is met will the leaves
release certain plant
growth regulators.
The substances travel from the leaves through the stem to the apical
buds, stimulating some of those buds to switch from leaf to flower
production.
We have already learned that flowers are the reproductive structures
responsible for producing seeds. Now let's think about why the
timing of flowering might be important to a plant. Here are a
few possibilities:
•Timing must be such that other plants of the same species are
flowering at the same time, encouraging cross-pollination.
•The plant should flower when its pollinators are active.
•The plant should flower early enough in the season for seeds
to ripen and disperse before the cold weather sets in.
It's easy to see that, as they say, timing is everything. Photoperiodism
is a way that plants can "tell time," and ensure that flowering
occurs on schedule. This awareness, or measurement, of day length
is also common in the animal kingdom. For example, animals such
as deer mate in the fall so that their gestation period lasts through
the winter and the young are born in the spring. This way, the young
have all summer to mature before the cold weather returns. Scientists
believe that this timing is in response to photoperiod-rather than
being based on weather conditions or other environmental factors.
Photoperiodism also influences the timing of animal migrations.
So what about those plants that don't employ photoperiodism to determine
when to flower? Another factor that influences the onset of flowering
is vernalization.
Did you ever wonder where cabbage or carrot seeds come from? Ok,
you might be saying, "cabbage and carrot flowers, of course!" But
have you ever seen these plants in flower at the end of the growing
season? No. In some plants, another environmental factor (besides
day length) affects flower initiation. Many plants require a period
of chilling before they will flower. Biennial plants, including
cabbage and carrots, grow only foliage in the first year, overwinter,
and flower in the second season. For these plants, winter chilling
is critical to flower initiation. This phenomenon is called vernalization,
which can be defined as the promotion of flowering due to exposure
to low temperatures or chilling.
Vegetable gardeners take advantage of plants that require vernalization
 when
they grow biennial vegetables such as beet, turnip, carrot, kale,
and cabbage. These crops are harvested during the first growing
season, when the plants have grown only foliage and, in the case
of root crops, stored large carbohydrate reserves in their roots.
Were these plants allowed to overwinter, they would begin to grow
again the following spring, and produce flowers and seed sometime
during the growing season.
What is the purpose of vernalization? Again, as with photoperiodism,
the need for a certain period of chilling guarantees that plants
in temperate regions will flower at the appropriate time.
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And now, at long last, we are ready to talk about what happens in those
beautiful structures that we call flowers.
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