ANGIOSPERMS
Angiosperms are seed bearing plants or flowering
plants. Unlike gvmnosperms where the ovules are
naked, in angiosperms the sporophylls are organised
into flowers and the seeds are produced inside fruits.
1 here arc about 230000 species of angiosperms in the
nature. They evolved about 130-160 million years
ago.
Habitat
Angiosperms are found in wide range of habitats,
from the land to upto 6000 m in Himalayas or
Antarctica and Tundra, dry hot deserts, cold deserts,
tropics, freshwater up to 600C, underground, over
other plants as parasites, saprophyte, etc. Zostera is a
marine angiosperm.
Plant Body
These plants are sporophytic, in the form of herbs,
shrubs, trees, climber, creepers, etc. The smallest
angiosperm is water plant Wolffia and tallest is
Eucalyptus regnans (100 m and above). Primary root
develops from radicle of the germinated seed.
It forms tap root system. In many angiosperms roots develop
from places other than radicle, these are adventitious roots. Stem
develops from plumule.
Leaves
These can be simple or compound. The leaves bear axillary buds
which can grow into stem branches.
Vascular Tissues
Angiosperms have vessels in xylem. Phloem contains sieve tubes
and companion cells in regard to gymnosperms which do not
have companion cells.
Classification of Angiospenns
George Bentham and Joseph Dalton Hooker presented the
system of classification of angiosperms published in Genera
Plantarum (1862-1883) which appeared in three volumes.
This system of classification is used by most of the well-known
Herbaria of the world. The details of this system are not
described here. Conveniently, on the basis of the number of
cotyledons angiosperms are classified in two broad groups. i.e.,
Monocotyledonae and Dicotyledonae.

Flowers
Flowers are the reproductive structures formed by the union of
one or both types of sporophylls (microsporophylls or stamens
and megasporophylls or carpels).
Microsporophylls or Male Sex Organs
Stamens are considered as the male sex organs of a flower. Each stamen
has two main parts, i.e., a slender filament with an anther (at the
tip). An anther contains four microsporangia where microspore
mother cells become differentiated to form four microspores, each
developing into a pollen grain.
Megasporophylls or Female Sex Organs
Carpel or pistil is called the female sex organ of the flower. Each
carpel has three parts (i.e., an ovary, style and stigma). A
megaspore mother cell is differentiated in the nucellus and
undergoes meiosis, ultimately one functional
megaspore gets enlarged and forms the female
gametophyte known as embryo sac.
Knowledge Plus
Embryo Sac Each embryo sac contains a three-celled eg
apparatus consisting of one egg cell and two synergids
three antipcxial cells (at the opposite end) and two polar
nuclei (in the central cell). The polar nuclei eventually
fuses and forms diploid secondary nucleus.
Pollination
Pollen grains after dispersal from the anthers are
carried by various ways such as wind, water or by
various other agencies to the stigma of the pistil.
Fertilisation and Development of a Seed
Each pollen grain germinates on the stigma forminga
pollen tube that carries two male gametes to the
embryo sac, growing through the tissues of stigma
and style. One of the male gametes fuses with the eg
cell to form a zygote (syngamy).
The other male gamete fuses with the diploid
secondary nucleus to produce the triploid Primary
Endosperm Nucleus (PEN). Because of the
involvement of above mentioned two events, it IS
known as double fertilisation. After fertilisation
synergids and antipodal cells degenerate.
Endosperm
The zygote develops into embryo and the primary
endosperm nucleus develops into an endosperm•
Fruit
The ovule gradually transforms into a seed and the ovary becomes
the fruit. A fruit is actually a ripened ovary. They not only protect
the seed but also help in their dispersal.
Economic Importance
of Angiosperms
(i) The angiosperms are major source
of food, fibres, spices and beverages.
(ii) They also provide valuable timber
and medicines.
(iii) These also add beauty to our environment as well.