Transcript Angiosperms

Seed Plants – the Angiosperms – Flowering Plants
The angiosperms are seed plants, similar to gymnosperms, but with some important
evolutionary modifications. Flowers are reproductive organs derived from leaf-like
appendages. The relationship of the accessory flower organs, petals and sepals, is obvious.
The stamens and pistils can also be seen in development to originate from leaf-like
structures. In the flowering plant life cycle, the male gametophyte which develops within
the microspore wall into a pollen grain are even more reduced than in the gymnosperms.
Its movement to the ovule is often aided by appearance and scent, attracting pollinators. The
female gametophyte develops as the embryo sac, within an ovule, and within a new
structure: the ovary. In pollination the pollen grain germinates on the stigma of the pistil
and grows down the length of the style to the opening of the ovule. After fertilization, the
embryo sac and ovule develop into the seed. A second fertilization produces a nutritive
tissue, the endosperm, that surrounds the embryo. At maturity, the ovules, or seeds, are
protected within the ripened ovary wall to become a fruit. The fruit, fleshy or dry, aids in
dispersal.
Peduncle – flower stalk
Sepals – the lowermost or outermost whorls
of structures, which are usually leaflike and
protect the developing flower; the sepals
collectively constitute the calyx.
Petals – whorls of structures located inside
and usually above the sepals; the petals
collectively constitute the corolla.
Androecium – the male portion of the plant;
consists of stamens, each of which consist of
a filament atop which is located an anther;
inside the anthers are pollen grains which
produce the male gametes
Gynoecium – the females portion of the
plant; consist of one or more carpels, each
made up of an ovary, style, and stigma; the
ovary contains ovules that contain the female
gametes. The term pistil is sometimes used
to refer to an individual carpel or a group of
fused carpels.
More information about Angiosperms
Two classes of angiosperms
Monocots
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One cotyledon per embryo
Flower parts in sets of three
Parallel venation in leaves
Multiple rings of vascular bundles in
stem
Lack a true vascular cambium (lateral
meristem)
Dicots
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Two cotyledons per embryo
Flower parts in sets of 4 or 5
Reticulate (i.e., netted) venation in
leaves
One ring or vascular bundles in stem
Have a true vascular cambium (lateral
meristem)
A radially symmetrical flower
Photo by Gita Ramsay
Flower symmetry
The sepals and petals are usually the most
conspicuous parts of a flower, and a variety
of flower types are described by the
characteristics of the perianth (combined
calyx and corolla). In regular
(actinomorphic) flowers such as tulips, the
members of the different whorls of the flower
consist of similarly shaped parts that radiate
from the center of the flower and are
equidistant from each other. The flowers are
radially symmetrical. In other flowers such
as orchids, one or more part of at least one
whorl are different from other parts of the
same whorl. These flowers are generally
bilaterally symmetrical and are said to be
irregular (zygomorphic).
A bilaterally symmetrical (irregular) flower
Photo by Gita Ramsay
Angiosperm life cycle
Eggs from within the embryo sac inside the ovules, which, in turn, are enclosed
in the carpels. The pollen grains, meanwhile, form within the sporangia of the
anthers and are shed. Fertilization is a double process. A sperm and egg come
together, producing a zygote; at the same time, another sperm fuses with the
polar nuclei to produce the endosperm. The endosperm is the tissue, unique to
angiosperms, that nourishes the embryo and young plant.