From Seed to Seed:
Plant Science for K-8 Educators


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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.

      The Photoperiodic Response

    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.

      The Role of Photoperiodism

    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.

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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