Evolution of Anthophytes: Flowers and Fruit and Bugs, oh my. Two synapomorphies that unite all anthophytes and separate them from the other plant lineages are

The anatomy of both these structures can tell you not only about their natural history, but also about their evolutionary relationships.


Evolution of the Flower The flowers of a given plant family generally have specific characteristics that set them apart from other families, but these characters tend not to vary much within a family. The earliest flowers did not have distinct sepals and petals, and probably were not showy.

As insects discovered the riches of pollen, a coevolutionary marvel took place. The undifferentiated leaves surrounding the reproductive parts of some flowers changed in response to selective pressure, becoming different from the sepals, and attractive to pollinators. Although in some groups petals are believed to have derived from sepals (e.g., in water lilies and magnolias, the sepals gradually "segue" into showy petals), in most angiosperms they are derived from stamens that lost their sporangia and became sterile. (Remember Archaefructus!) How can we tell?

Early angiosperm stamens were diverse in structure and purpose. The carpels, on the otehr hand, tended to be rather uniform and unspecialized, perhaps constrained in morphology by natural selection, as they were the carriers of the next generation.

Trends in Flower Evolution
As insect pollinators drove the earliest evolutionary radiations of angiosperm flowers, four major trends took place:

Success Stories: Asteraceae and Orchidaceae The two largest plant families also show the greatest degree of flower specialization.

Asteraceae With more than 22,000 species, this is the largest of all eudicot families, and each species tends to be numerous in its given habitat. Asteraceae (Daisy Family) is characaterized by composite flowers : they are bunched together in a tight head.
Each of the fertile, actinomorphic flowers in a composite flower has

These are surrounded by the ring of sterile ray flowers that are zygomorphic. Because the flowers in the inflorescence open from the margins to the center, different pollinators will visit, and the resulting seeds will have greater potential genetic diversity than if all of them opened at once, and were pollinated by the same pollinator.

Orchidaceae This monocot family is the most diverse of all plant families, with at least 24,000 species. Unlike the asters, however, these tend not to be individually numerous, and some are extremely rare.

The orchid flower has

Orchids are zygomorphic, and--as you already know--can be very strange in appearance.

Orchid culture is practically a cult, with breeders seeking the rarest and most bizarre orchids for culture. The genus Vanilla is commercially cultivated to produce the famous flavoring agent.


Pollination: A Driving Force in Anthophyte Evolution Not all anthophytes have large, showy flowers. Species pollinated by wind usually have small, inconspicuous flowers, leading many people to believe that they lack flowers altogether. For example, here are the male and female flowers of the wind-pollinated oak (Quercus sp.):

Wind pollination is most effective when the plants in question are numerous and not too far apart (as in a forest or grassland).

Flowers pollinated by animals have the comparative luxury of being able to be farther apart, as long as their mobile and/or volant (flying) pollinators have a long distance range. This promotes not only outcrossing, but the freedom to colonize outlying areas that might otherwise be out of the reach of wind-carried pollen.

Conspicuous flowers are the specialty of species whose pollen is distributed by animals such as insects, birds or mammals. These generally have flower structure and colors modified to attract their own particular pollinator(s). And of these, insects have been the most common, and perhaps one of the most important forces for natural selection and coevolution of plant and pollinator.

What are the advantages of being pollinated by an animal?

The bisexual flower was a major advantage when animal pollination first took hold: it meant that each pollinator visit was doubly efficient. The pollinator could deliver and collect pollen in a single visit.

Some flowers are pollinator generalists, whereas others have coevolved such specificity that if either member of the coevolutionary pollination "team" were to go extinct, so would the other.

Of prime importance in specializing to attract a particular type of pollinator are

Flower Color Flower color is produced mainly by two groups of compounds, carotenoids and flavonoids.

  • Carotenoids - yellow to orange pigments, oil soluble, stored in plastids. These first evolved as accessory pigments for photosynthesis, but have evolved more functions as pigments for flowers and fruits.

  • Flavonoids - ranging in color from pale ivory to blue to purple and red, these are water-soluble ring compounds stored in the central vacuole. One class of flavonoids, the anthocyanins, are red (phycoerythrin) or blue (phycocyanin), and are especially important in the coloration and patterns in many species of flowers.

    You can often tell the type of pollinator a plant has evolved to attract by the nature of its flowers...

    Beetles
    Magnolias and many other archaic flowering plants are pollinated by beetles, which were probably among the first animal pollinators. Originally making their way as herbivores and scavengers, beetles move primarily by walking and munching. Hence, beetle-pollinated flowers tend to produce copious pollen, some of which will be sacrificed as a meal for the beetle while the rest sticks to its legs and body and is transported to another flower.

    Although beetles have good vision and diurnal species can see a wide range of colors, they tend to be attracted more to odors than to color or shape. Beetle-pollinated flowers tend to have very strong scents, ranging from sweet and fruity to rotting flesh.

    Because many beetles are nocturnal, some of their night-blooming flowers are white--easily spotted at night.

    Flies
    Flies have good vision and can see color, but they, too, are more drawn to scent than color. Flies move about by flying, and because they have relatively short tongues, the flowers that use them as pollinators tend to be shallow, with easy to reach nectar.

    Fly flowers tend to be pale, and may have a sweet scent. (Male mosquitoes are important pollinators: they don't suck blood!). Others putrid scent, reminiscent of rotting flesh, and may be dark and hirsute, since blowflies and other carrion-eating flies are attracted to dark red, purple, and maroon (Now why, do you suppose?).

    Although flies first appear in the fossil record about 200 mya, their diversity exploded when they began to pollinate anthophyts, about 100 mya.

    Bees
    Bees are the most important pollinators in the world, and almost all of the 20,000 bee species feed on nectar and pollinate flowers.

    A bee flower usually has a somewhat fused corolla. It is usually not so deep that the bee's short tongue can't reach the nectaries, but just deep enough to require the bee to crawl inside and smear pollen all over its body.

  • a honey bee will visit 50-100 flowers on a single foray.
  • a worker honey bee can carry more than half her weight in nectar and pollen payload
  • honey is processed nectar: aged, oxidized, and regurgitated with a lot of bee spit enzymes
  • the compound eye of the bee is conducive to short wavelengths, and they cannot see beyond orange. Bee flowers tend to be white, yellow, blue, violet, and may have ultraviolet nectar guides.

    Lepidopterans
    Butterflies and moths are important pollinators. The former are diurnal, and the latter are primarily nocturnal. Both butterflies and moths sip nectar through a long, flexible tube called a proboscis, which is coiled when the insect is not feeding. Before they even uncoil it, they sample the flower with chemoreceptors on their feet.

    Butterflies are highly visual, and can see the entire spectrum visible to humans, as well as UV. They are particularly attracted to reds, oranges and pinks. Butterfly flowers are often orange, red or dark pink with very long, fused corollas that prevent other pollinators from stealing nectar. (Butterflies will also feed on shallow flowers; they're not picky.)

    Moths are also visual, but they rely largely on chemoreception to not only find mates, but food. Moth flowers are usually night-blooming, pale in color (usually white), and have a strong, sweet smell. Like butterfly flowers, they tend to be tubular.

    Birds
    Very few species of birds have much of a sense of smell. Flowers evolved to attract them rely on color and shape. Bird flowers are usually very brightly colored, and may be ornate. They tend to produce copious, watery nectar (birds have a high liquid requirement!), and are large and heavy, to be able to support the weight of a bird. They tend to extend beyond the leaves of the plant, to allow easier access for the bird.

    Few birds hover, so bird-pollinated flowers often have perches or landing pads (heavy, fleshy petals or bracts) that facilitate the bird's feeding and getting dusted with nectar.

    Hummingbird flowers are usually highly specialized for these unique pollinators. The corolla tends to be deep, fused, and hanging so that only a hovering animal can reach the nectar. They tend to be spaced in such a way that the bird's wings don't damage adjacent flowers as it feeds on one.

    Mammals
    Bats find food by smell and/or sight. Many cactus are pollinated by bats; they hav large, showy white flowers with sweet fragrance and LOTS of pollen and nectar to feed their high-metabolism pollinators.

    The flowers tend to be large enough to accomodate a bat's face or even its entire head and, like bird flowers, they tend to extend beyond the leaves for easier access for a large pollinator.

  • Some flowers are pollinated by birds during the day, and bats at night.
  • Bats not only pollinate, but are major seed dispersers.
  • Different species of bats are specialized to feed on pollen, nectar, fruit.
  • Bats have reasonably good vision, but nocturnal species can't see colors.

    Some flowers, such as certain species of Proteus are pollinated by rodents, and hang their flowers close to the ground for easier access.

    In Australia, pollen- and nectar-feeding marsupials such as the honey opossum and sugar glider are major pollinators of various species of Myrtaceae (Eucalyptus family).


    Flowers evolved to attract large pollinators with high energy requirements (e.g., bats, birds), must provide a large quantity of nectar and/or pollen. But they also must evolve means by which to exclude smaller pollinators who might enter the flower and fill up all in one go. No visit to another flower means no pollination!



    The End Result: Fruit A fruit is a mature ovulary containing seeds.

    The propagule of an anthophyte is composed of tissue from two different generations, grandma sporophyte (the fruit and seed coat) and grandchild sporophyte (the seed, comprised of a diploid sporophyte embryo and its "sibling," the triploid endosperm). The short-lived 8-nucleate embryo sac gametophyte is pretty much gone by the time the fruit is ripe enough for you to eat.

    A fruit is a three-layered structure. The outermost layer is known as the exocarp, the middle layer is the mesocarp and the inner layer surrounding the seed is the endocarp. Taken together, these three layers comprise the pericarp.

    Fruits may be

    A simple fruit is formed from a single flower with a single pistil. Remember, though, that a pistil may have one or many carpels per receptacle. Common examples of simple fruits are blueberries, citrus, tomatoes and melons.

    An aggregate fruit is formed from a single flower with many pistils borne on the same receptacle. That is, there's only one peduncle per fruit. Common examples are strawberries (an aggregate of achenes), raspberries (an aggregate of tiny drupes), and corn (an aggregate of grains).

    A multiple fruit is formed from a cluster of individual flowers in a single inflorescence. Each "fruitlet" has its own receptacle and its own pedicel. Common examples are pineapples, bananas, kiwi fruits. All of these have fused fruitlets, which appear to comprise a single, fleshy fruit. But they're actually a cluster of fused fruit.

    Accessory Fruit An accessory fruit is one in which the fleshy pericarp is composed of floral tissue in addition to the walls of the ovularies. For example, the pistil of an apple blossom (an epigynous flower) develops into the papery apple "core" and a bit of adjacent tissue. The fleshy, edible part of the fruit is derived from the receptacle, which has grown up to surround the pistil.

    Similarly, the "seeds" on the outside of a strawberry are the actual fruits. The fleshy, sugar-filled "fruit" designed to attract seed dispersers such as birds, monkeys or yourself is actually the receptacle that has evolved the function of attracting animal seed dispersers.


    Types of Fruits Fruits may be fleshy or dry at maturity (i.e., when the seeds are ready to be dispersed).

    A fleshy fruit is defined as one in which the mesocarp is moist and pulpy at seed maturity. Three basic types are the

    A dry fruit is defined as one in which the mesocarp is definitely dry at seed maturity.
  • Dry fruits may be either dehiscent or indehiscent.
  • A dehiscent fruit splits open at seed maturity to release the seeds.
  • An indehiscent fruit does not split open at seed maturity.

    Dry, dehiscent fruits include

    Dry, indehiscent fruits include

  • achenes (a single seed attached to its pericarp only at the base)
  • nuts (similar to the achene, but with a harder, thicker pericarp)
  • grain (a single seed tightly fused to its pericarp)
  • samara (a single-winged fruit)
  • schizocarp (unique to the parsley & carrot family, Apiaceae, these split into two fruits at seed maturity)

    Each of these fruit types may be modified by a number of dispersal mechanisms designed to carry the offspring as far away from the parent as possible. (Why is this a good idea?) Seeds may be dispersed by wind, water, animals or other mechanisms.

    Fleshy Fruits The Berry
    A vast number of superficially different fruits are botanically classified as berries. What makes a berry a berry? The entire pericarp is fleshy when the seeds are mature. There are three basic types of berries. . .

    (Surprise! Strawberries, raspberries, blackberries and mulberries are not berries at all! What are they?)

    The Drupe
    A drupe has a single seed enclosed in a hard endocarp, often called a "stone" or "pit." The rest of the pericarp is fleshy or pulpy. A drupe usually develops from a single-ovule flower with a superior ovary.

    A large number of fruits commonly called "nuts" are actually drupes, or parts of drupes, including

    The "shells" of these "nuts" is actually the stony endocarp.

    More easily recognizable as drupes are those whose exo- and mesocarps are edible, such as

    The "pit" of these fruits is actually the endocarp containing the seed. Crack open the stone of a peach and you'll find a seed that looks just like an almond. (Caution: cyanide!)

    Fruits such as raspberries, blackberries and mulberries are not berries, but aggregates of miniature drupes ("drupelets"). The crunchy nuggets in your raspberry jam are the "pits" of the drupelets that gave their lives for your toast.

    The Pome
    A pome is a fleshy fruits with a thin exocarp, fleshy mesocarp and somewhat papery or leathery endocarp that is shaped like a five-pointed star in cross section (it has five carpels). It develops from a flower with an inferior ovulary surrounded by the flower's receptacle (a epigynous flower).

    The the bulk of the pome is comprised of fleshy tissue derived from the receptacle, so it's an accessory fruit. Common pome fruits include

  • apple
  • pears
  • quinces Most pome-forming plants are members of Family Rosaceae, the Rose family. Next time you eat an apple (especially Golden Delicious), notice the faintly rose-like scent.


    Dry, Dehiscent Fruits The Pea Family (Family Fabaceae) is large, diverse and economically important. A great number of ecologically vital tropical rainforest trees are members of this family, commonly called "leguminous" plants or just "legumes."

    The family's common name is derived from the type of fruit produced by all its members. The pericarp is dry at seed maturity, and splits along two longitudinal seams to release the seeds. Examples include

    (You may note that many store-bought legumes, such as string beans, are green and fleshy. That's because they're harvested before the seeds are mature. If left in the field long enough, they would dry and split.) Peanuts are a bit unusual for legumes: once the flowers have been pollinated, the stems droop and the developing fruit actually are pulled underground by specialized stems. Rather than dehiscing, as most legumes do, the pericarp is broken open via bacterial action.

    The Capsule The capsule is the most common type of dry, dehiscent fruit. Capsules display a wide variety of morphologies but all consist of at least two carpels that split in various ways at maturity.

    Like the legume, the capsule is dry at maturity. Depending upon the species, some capsules split between carpels, others split through the carpel cavities and others form a caplike structure which pops off to release the mature seeds.

    Capsules may be smooth, or stubbled with stickers or long projections which may catch on fur and aid in seed dispersal. Mature capsules often rattle when touched or shaken by the wind.

    Some common plants that form capsules include


    Dry, Indehiscent Fruits The Achene
    An achene has a thin, dry pericarp attached only at the base to the single seed within. The pericarp is indehiscent: it does not split open at maturity to release its seed.

    Examples include

    All members of the Asteraceae form achenes. When you make a wish with a dandelion, you are blowing achenes all over the neighborhood.

    As mentioned before, a strawberry flower is actually an inflorescence of many small flowers, each of which matures into a tiny achene. The "seeds" on the outside of the strawberry are not seeds at all--they're the fruits! The "hair" on each "seed" is what's left of the stigma.

    After fertilization, the pedicels of the strawberry flowers fuse and become fleshy. As the seed within each achene matures, it produces a plant hormone (a gas known as ethylene) which causes the fused pedicels to ripen (become red and filled with sugar). You've seen strawberries that are red and ripe all over except for one little gnarled green area? That's because the ovules on the green area of the "berry" didn't get fertilized, don't mature and and so don't produce ethylene.

    A strawberry is not only an aggregate fruit (a cluster of achenes), but also an accessory fruit (made up of floral tissue in addition to the walls of the ovulary). The fleshy red stem bearing the achenes is designed to attract seed dispersers such as a bird or primate. The achenes pass rapidly through the digestive tract (at least the ones you didn't crush with your teeth. . . ) and are eventually deposited in a nice patch of organic fertilizer.

    The Nut
    A nut is a one-seeded fruit similar to an achene, but generally larger and with a much harder, thicker pericarp. It develops in a cup formed by fused bracts at the base of the fruit. (Imagine the "cap" of an acorn.) True nuts include

    Remember: many things commonly called "nuts" are not nuts in the botanical sense. Peanuts are (odd) legumes. Coconuts, walnuts, pecans, and almonds come from drupes. Brazil nuts are the seeds of a large capsule.

    The Grain
    Seventy five percent of the world's human population relies upon rice--a grain--as its major staple food source. Corn, another grain, supplies the United States with a multi-billion-dollar-a-year industry, with products ranging from the coating on the pages of your textbook to the high-fructose corn syrup in your cola to part of the gasoline in your gas tank. Wheat, oats, barley, rye are all grains. A grain is a one-seeded indehiscent fruit whose pericarp is completely fused to the seed. Because the pericarp and seed are so tightly united, you eat a lot of indigestible cellulose ("dietary fiber") when you eat whole grains.

    Grains are produced by members of the grass family (Poaceae). Another name for a grain is a caryopsis, a term used by those who wish to sound more learned at farm gatherings.