Early Flowers

Characteristics of early flowers

1.     Presence or absence of petals, and, when present, whether separate or united.

a.     Separate petals seem to have come first, and from this condition the derived condition of apetaly and sympetaly have been evolved.

2.     Number of parts

a.     It is believed that primitive flowers had indefinite number of parts, the stamens and carpels often being numerous. The general trend of evolution has been toward fewer parts.

3.     Union of parts

a.     Separate parts are generally considered to be ancestral to united parts: separate petals preceded united petals, and separate carpels preceded united carpels.

b.     The condition in which the carpels are separated is apocarpous; the condition of united carpels is syncarpous.

c.      The apocarpous condition, with more than one carpel, can only occur in hypogynous or perigynous flowers, all epigynous flowers being syncarpous or else having only a single carpel.

d.     The earliest gynoecium had free carpels, and the first fruits derived from them were apparently follicles or nutlets. Fossils with fused carpels are not as old as those with free carpels. The first gynoecia with fused carpels may have developed into capsules. Fused carpels in more recent fossils show the development of nuts, drupes, berries, and pods.

4.     Some early fossils of flowers from the Cretaceous period have floral parts arranged in a spiral on their axis, as in the flowers and fruits of the modern magnolia.

5.     All of the first flowers displayed a radial symmetry, like poppies and buttercups.

a. Their petals, when present, were free (unattached to one another). Flowers with distinct bilateral symmetry, modern violets or snapdragons, and flowers with fused petals, such as the cape honeysuckle, did not appear until the Paleocene period, less the 65 million years ago.

6.     Other fossils have some of all their floral parts arranged in circles or whorls, around the floral axis, which is the most common arrangement of floral parts among extant angiosperms.

7.     The earliest gynoecium had free carpels, and the first fruits derived from them were apparently follicles or nutlets. Fossils with fused carpels are not as old as those with free carpels. The first gynoecia with fused carpels may have developed into capsules. Fused carpels in more recent fossils show the development of nuts, drupes, berries, and pods.

When and from whom did flowering plants evolve?

1.     Although the first fossils of angiosperms are no older than 135 million years, the angiosperms probably arose much earlier. Indirect evidence from the possible ancestors of angiosperms indicates that they may have originated as long as 200 million years ago. Angiosperm fossils of that age are unknown, probably because they evolved in dry, uplands that were not conducive to fossilization. (Location of origin is still controversial and unclear).

a.     Most discovered plant fossils appear to have been associated with wet lowland areas where organic decomposition could be inhibited by silt and mud, and fossils left due to pressures borne by sedimentation.

b.     Stebbens proposed that the alternation of a wet and dry season (rainy periods followed by periods of calm) provided an opportune time for flowering and insect pollination, as well as promoting selection for protective seed structures such as closed carpels. Stebbens thought that these conditions may have first prevailed in semi-arid mountainous regions with annual droughts: e.g. present day South Africa, Ethiopia, Ecuador, and Mexico.

c.      Peter Raven (Missouri Botanical Garden) suggests that tropical lowland conditions with their large insect populations are more favorable for plants dependent on insect pollination than on wind pollination.

2.     One hypothesis of angiosperm derivation cites the extinct cycadeoids (seed ferns) as its ancestor. Seed ferns were prominent in the Carboniferous period, but few persisted into the Mesozoic period.

a.     This has to do with the origin of the carpel.

b.     The carpel is believed to have developed from the cupule of a seed fern. According to this hypothesis, cupule tissue surrounding the seeds fused to form a closed carpel.

c.      Cycadeoids aere once considered to be ancestors of angiosperms because the microsporangia and ovules of cycadeoids occur in the same cone. Such an arrangement simulates a perfect flower – flowers with both stamens and carpels on the same receptacle.

How did angiosperms evolve?

1.     Any discussion of how angiosperms evolved must include a topic we’ve touched upon briefly before in our discussion of flowers: the role of insects.

2.     Early in seed-plant evolution, insects became pollen carriers as they searched for food. In turn, plants evolved floral nectar and odors for attracting insects to carry pollen. The earliest, unequivocal angiosperm nectaries are from the late Cretaceous period, but they probably evolved earlier than that.

3.     Earliest pollinating insects were probably beetles. Cycadeoids were already specialized for pollination by beetles long before the appearance of angiosperms.

4.     Insect pollination was not associated with rapid diversification of angiosperms until the appearance of specialized lepidopterans (butterflies and moths) and hymenopterans (bees) during the late Cretaceous, early Tertiary periods.

5.     The rise to dominance of angiosperms in the Tertiary seems to have been greatly influenced by adaptations for pollination by increasing diversity of flying insects.

What features account for the evolutionary success of angiosperms? Why are the flowering plants so successful in terms of their ecological dominance and in terms of their great number of species (diversity)?

1.     Reproductive structures and processes.

a.     Seed production – primary means of reproduction and dispersal; an adaptation shared with gymnosperms.

b.     Flower – with its composition of essential and inessential parts, the flower lures insects, birds, bats to itself, and in the process has dramatically increased the diversity of flowering plants. Co-evolution (mutual adaptation) with insects. Insures cross-pollination with members of the same species by utilizing only a relatively small amount of pollen compared to the large amounts of pollen necessary in random wing pollination. As a result, angiosperm flowers, derived from leaves modified into sepals, petals, and related structures, are amongst the most intricate and attractive organs that veer appeared in plants.

c.      Closed carpels – allow seeds to develop enclosed within a fruit protecting seeds from desiccation as they grow and mature, and aids in the dispersal of seeds.

d.     Double fertilization – which results in the production of endosperm, a nutritive tissue that feeds the developing embryo.

2.     Vascular system

a.     Flowering plants possess very efficient water conducting cells, called vessel elements, in their xylem, in addition to tracheids. (Gymnosperms xylem consists on tracheids exclusively).

3.     Leaves

a.     The leaves of flowering plants, with their broad, expanded blades, are structured for maximum efficiency in photosynthesis.

b.     Abscission of these leaves during cold or dry spells reduces water loss and thus has enabled some flowering plants to expand into habitats that would otherwise be too harsh for survival.

c.      The stems and roots of flowering plants are often modified for storage, as we have discussed.

Gee Whiz of flowering plants

1. Flowering plants represent an extraordinary diverse group of life forms. Although they all produce flowers and seed-bearing fruits,they come in an astonishing variety of shapes and sizes. Flowering plants range in size from minute wolffia plants (Wolffia globosa) less than a millimeter (1/25th of an inch) long to giant Australian eucalyptus trees (Eucalyptus regnans) over 300 feet (100 m) tall. Although the current record-holder for tallest tree is actually a 367 foot (112 m) conifer called the California redwood (Sequoia sempervirens), a fallen Eucalyptus regnans measuring 18 feet (5.4 m) in diameter and 435 feet (133 m) tall was reported from

Australia in 1872. According to Stan Kelly (Eucalypts Volume 1, 1977), the tallest E. regnans currently standing is 322 feet (98 m).

The duckweed family (Lemnaceae) includes the undisputed smallest and fastest reproducing flowering plants in the world. In fact,

the genus Wolffia is Mr. Wolffia's (the editor of WAYNE'S WORD) favorite plant group. Imagine flowering wolffia plants (genus

Wolffia) that weigh only 150 micrograms (1/190,000 of an ounce), or the approximate weight of two ordinary grains of table salt.

Individual wolffia plants are 165,000 times shorter than the tallest Australian eucalyptus tree and 7 trillion times lighter than the most

massive giant sequoia tree. They are carried from pond to pond on the feet of water fowl (tucked neatly under the ducks' bodies

during flight), and there are records of wolffia plant bodies being carried by a tornado. They have even been reported in the water of

melted hailstones. One wolffia plant is small enough to slip through the eye of an ordinary sewing needle, and at least 5,000 plants

could be packed into a thimble. Each plant produces a microscopic flower inside a small cavity that develops on the upper side of the

plant body. The minute flower consists of a single pistil and stamen. A bouquet of one dozen plants in full bloom will easily fit on the

head of a pin. After pollination the ovary develops into a tiny one-seeded fruit called a utricle, which also holds the record for the

world's smallest fruit.

The common connecting link between all flowering plants is that they produce sexual reproductive organs called flowers. A flower

is composed of 4 major parts: petals, sepals, stamens and one or more pistils (see above diagram). The number, shape, size, and

arrangement of these floral parts varies considerably with different plant families, and is reflected in the tremendous diversity of

flowering plants. According to the foremost authority on angiosperm diversity and phylogeny Dr. R.F. Thorne (The Botanical

Review Volume 58: 225-348, 1992), the total number of different flowering plant families is 437. This great number of families is

subdivided into 400 subfamilies, 12,650 genera and 233,885 species. For example: The grass family (Poaceae) has 3 stamens and

one pistil, but no petals or sepals. The duckweed family (Lemnaceae) has one stamen and one pistil, and no petals or sepals. The

cactus family (Cactaceae) has numerous stamens, one pistil, and numerous petals and sepals. The buttercup family (Ranunculaceae)

has many stamens, many pistils, many petals and many sepals. Families with many separate floral parts (such as the Magnoliaceae

and Ranunculaceae) are considered more primitive. Families with fewer floral parts which are fused together and irregularly shaped

(such as the Orchidaceae) are considered more advanced in the evolution of flowering plants. Flowers without colorful petals and

sepals are typically wind-pollinated and do not need showy parts to attract insects. Colorful, insect-pollinated flowers also produce

sweet (sugary), fragrant nectar in nectar glands at the base of the blossom, which further entices insects to visit them.