Angiosperms
Flowering plants (Magnoliophyta) are the most successful of all plant groups
in terms of their diversity. The group includes more than 250,000 species, and
at least 12,000 genera.
This group is usually
referred to as angiosperms because, as their name implies, their seeds are
enclosed in a carpel (in a vessel). The carpel is the primary feature that
distinguishes angiosperms from gymnosperms.
Angiosperms live in all
terrestrial and aquatic habitats on earth. Except for conifer forests and moss-lichen
tundras, angiosperms dominate all the major
terrestrial zones of vegetation.
Terms
1.
Herbaceous –
a plant composed of soft, non-woody tissue.
2.
Herb – a
plant without a persistent above-ground woody stem, the stems dying back to the
ground at the end of the growing season.
3.
Annual – a
plant that completes its life cycle within a single year or growing season.
4.
Biennial – a
plant that completes its life cycle within two years, producing only vegetative
growth in the first year, and flowering the second.
5.
Perennial –
a plant that lives for a number of years.
6.
Dicotyledonous
plant – (Magnoliopsida) flowering plants that have
two cotyledons. Their flowering parts
are usually in multiples of 4s and 5s; their leaf venation is usually
dichotomous or digitate; roots usually maintain a
persistent taproot; their growth form is both herbaceous and woody; their
vascular bundles are usually arranged in a ring.
7.
Monocotyledonous plant – (Liliopsida) flowering plant with
only one cotyledon. Their flowering parts are usually arranged in multiples of
3s and 6s; their leaf venation is usually parallel to the midrid;
roots are often fibrous (adventitious); their growth form is mostly herbaceous;
their vascular bundles are usually scattered throughout the ground parenchyma,
not in a discernible ring.
8.
Monocot and dicots represent classes with in the division of Angiospermae.
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.
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
c.
Peter Raven (
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
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
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.