Biology Today — December 2017

ANGIOSPERMS

• Angiosperms are seed plants in which sporophylls are organised into flowers and seeds are produced inside fruits. They are
  the most highly evolved plants and form the dominant vegetation of present day earth.

General Characteristics

• Angiosperms or flowering plants occur in most environments on the earth.


• Plant body is sporophytic and represented by herbs, shrubs, trees, twiners, trailers, climbers, etc.


• Sporophyte is differentiated into roots, stem and leaves.


• Xylem possesses tracheids and vessels while phloem possesses sieve tubes and companion cells. Vascular bundles are conjoint
  and collateral and open in dicots, hence show secondary growth.


• Sporophylls are aggregated to form flowers. Both microsporophylls and megasporophylls are specialised to form stamen and
  carpel, producing male and female gametes respectively.


• Archegonia is absent and female gametophyte is represented by embryo sac.


• Pollination takes place by several agencies viz., wind, water, animals, etc.


• Double fertilisation is characteristic feature of angiosperms, wherein of two male gametes, one fuses with egg to form zygote
  while other fuses with central cell to form PEN (Primary endosperm nucleus).


• Endosperm is triploid, formed after fertilisation and represents a new structure.


• The ovary develops into fruit and ovules ripen into seeds, after fertilisation. This is another peculiar feature of angiosperms.

Classification of Angiosperms

• Angiosperms are divided into two sub-groups – dicots (two cotyledons) and monocots (single cotyledon).

Table: Comparison between dicots and monocots

Features Dicots Monocots

Cotyledons Usually two One cotyledon

Flowers Penta or tetramerous Trimerous

Pollen grains Three germ pores Single germinal furrow

Venation Reticulate Parallel, except a few

Primary root Often long lived forming tap root system. Adventitious roots

occur in some.

Short-lived, tap roots absent, instead adventitious roots are

found.

Stem tissue Concentric arrangement of tissue system viz. epidermis, cortex,

endodermis, pericycle, pith, etc.

Tissue system undifferentiated, a ground tissue occurs.

Vascular bundles

of stem

Arranged in a ring and possess cambium (open), thus,

secondary growth present.

Scattered, cambium is absent (closed).

Root Pith absent or small. Vascular bundles 8 or less. Pith present. Vascular bundles more than 8.

Xylem vessels Polygonal in outline. Rounded in outline.

Affinities of Angiosperms with Gymnosperms

• Shows a variety of habits-
  trees, shrubs or herbs.


• Universal occurrence of xylem
  vessels and companion cells
  (except in Family Winteraceae).


• Flowers are generally bisexual.


• Ovules enclosed within ovary.


• Embryo sac is present instead of
  archegonia.


• Triploid endosperm, as formed
  after fertilisation i.e., triploid
  fusion.


• Double fertilisation present.


• Mostly woody trees.


• Absence of xylem vessels
  and companion cells (except
  in Gnetales)


• Strobili are usually unisexual.


• Naked ovules.


• Female gametophyte consists
  of archegonia.


• Haploid endosperm, as
  formed before fertilisation.


• Double fertilisation absent.


• Sporophytic plant body
  differentiated into root, stem and
  leaves.


• Vascular system of stem shows
  conjoint, collateral and open
  vascular bundles.


• Stem shows increase in girth due
  to secondary growth.


• Fertilisation is siphonogamous.


• Development of megaspore into
  female gametophyte takes place
  inside megasporangium (ovule).

Gymnosperms Angiosperms