Angiosperms (flowering plants) have seeds enclosed in an ovary.
Flowers include ovaries and surrounding structures.
Ripened ovaries
make up a fruit. Roots, stems, and leaves reach maximum complexity
and diversity within the angiosperms. Angiosperms are divided into
"dicots" (Dicotyledonae) and "monocots" (Monocotyledonae).
Angiosperms: Flowering plants, with seeds enclosed in a protective
ovary.
Many flowers are fertilized by insects.
Angiosperms probably
evolved in response to selection by insects.
Flower: An ovary, together with surrounding reproductive structures:
Flower structure
- Petals and sepals
surround and protect the other flower parts,
especially in undeveloped flowers. In many species, these parts may have
colors or odors that attract insects or other species that pollinate
the plant or disperse its seeds.
- Male flower parts include stamens, each made of a
filament and a
pollen-producing anther. Pollen grains
contain male gametophytes.
- Female flower parts include stigma (whose sticky surface
catches pollen), style (a stalk-like part supporting the stigma),
and ovary, enclosing one or more ovules within modified
leaves called carpels. Each ovule develops into an 8-celled
gametophyte containing 1 egg, 2 polar nuclei, and 5 other cells.
- "Complete" flowers have all the parts listed above, but many
species have separate male flowers and female flowers.
Fertilization: First, a thin, long pollen tube grows from the
pollen grain down the style. Two nuclei (tube nucleus and generative
nucleus migrate down the pollen tube as it grows. The generative nucleus
then divides into two sperm cells: one fertilizes the egg (syngamy
or true fertilization; the other fuses with the 2 polar nuclei to form
a triploid (3N) endosperm, containing stored food that supports the
embryo later. This so-called double fertilization is characteristic
of all angiosperms. After fertilization, the new embryo and its endosperm
and protective coverings make up a seed.
Fruits: One or more ripened ovaries together constitute a fruit,
enclosing one to many seeds. Many fruits are eaten (and their seeds dispersed)
by animals; other seeds are dispersed by wind, etc.
Roots: Absorptive parts, usually underground. They contain, from the
outside inward:
- Epidermis, from which absorptive root hairs develop.
- Cortex, often a thick layer.
- Endodermis, containing a waterproof Casparian srtip.
- Vascular bundles, with xylem, phloem, and cambium surrounded
by a pericycle.
Stems: Supportive structures which contain, from the ouside inwatrd:
- Epidermis
- Cortex (bark)
- Vascular bundles, each containing phloem and xylem,
along with some persistently embryonic tissue called cambium.
- Pith (not always present) in the innermost parts of the stem.
Leaves: The major photosynthetic organs, described
here
Class Dicotyledonae ("dicots", Magnoliophyta): Angiosperms with a
seed containing two "seed leaves" (cotyledons) in which food is
stored. (The two halves of a dried peanut are a familiar example.)
- Flower parts are usually in multiples of 4 or 5.
- Veins in leaves and petals branch to form net-like patterns.
- Vascular bundles usually arranged in a circular ring.
- Includes the majority of angiosperms: buttercups (considered
primitive), roses, beans (and other legumes), daisies, oaks, maples,
apples, oranges, peaches, melons, and many others.
Class Monocotyledonae ("monocots," Liliophyta): Angiosperms with a
seed containing only one "seed leaf" (cotyledon) in which food is stored
(such as a kernel of corn, which cannot be divided into halves).
- Flower parts are usually in multiples of 3 or 6.
- Leaves and petals have parallel veins that branch only occasionally.
- Vascular bundles usually scattered throughout cross-section of stem.
- Includes the more advanced angiosperms: orchids, lilies, palms,
and grasses (including wheat, corn, rice, and other cereal grains).
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