Biology Today — December 2017

Affinities of Pteridophytes with Bryophytes

• Sporophyte is the
  dominant phase in the
  life cycle.


• Plant body is
  differentiated into root,
  stem and leaves.


• Vascular tissue is
  present.


• Sporophyte is
  autotrophic and
  independent.


• Gametophyte is the
  dominant phase in the
  life cycle.


• Plant body is thalloid
  or foliose.


• Vascular tissue is
  absent.


• Sporophyte is
  completely dependent
  upon the gametophyte.


• Jacketed sex organs.


• Flagellated male gametes.


• Fertilisation in film of water.


• Encapsulation of embryo in
  archegonium.


• Heteromorphic alternation of
  generation.


• Sexual reproduction oogamous;
  the male and female
  reproductive structures are
  known as antheridia and
  archegonia respectively.

Bryophytes Pteridophytes

Stelar System in Pteridophytes

Protostele

• Non-medullated stele, comprising of a central core of xylem,
  surrounded by phloem.


• Considered to be most primitive both phylogenetically and ontogenetically.

The central vascular cylinder of primary axis of pteridophytes is usually referred to as stele. It is delimited from cortex by the pericycle and

includes xylem, phloem and pith (if any). According to stelar theory proposed by Van Tieghem and Douliot (1886), stele is the fundamental

unit of vascular system. According to Schmidt (1982), two main types of stele are recognised in pteridophytes.

Siphonostele

• Medullated stele, comprises of a modified protostele with a central
  pith.


• More advanced than protostele.

Types of Protostele

Plectostele

• The central xylem core breaks into
  more or less parallel plates, each
  of which is surrounded by phloem.


• Found in L.clavatum and L. volubile.

Haplostele

• Simplest, primitive type of stele.


• Consists of solid xylem core
  surrounded by ring of phloem.


• Found in Psilotum,
  Selaginella, Lycopodium.
  Cortex
  Endodermis
  Pericycle
  Leaf trace
  Phloem

Actinostele

• Xylem is star shaped with many
  radiating arms.


• Phloem is present in small patches in
  between the radiating arms of xylem.


• Found in Psilotum, L. serratum.
  Cortex
  Endodermis
  Pericycle
  Xylem
  Phloem

Cortex

Endodermis

Pericycle

Leaf trace

Xylem

Phloem

Types of Siphonostele

Xylem

Ectophloic siphonostele

• Steles have a single phloem ring external to xylem.


• E.g., Equisetum, Osmunda, etc.
  Cortex
  Outer endodermis
  Outer pericycle
  Outer phloem
  Pith
  Xylem

Amphiphloic siphonostele

• Steles have a ring of phloem each external
  and internal to the xylem.


• Characteristically have two endodermal
  layers- outer endodermis - lies outside the
  outer phloem and inner endodermis - lies
  inner to inner phloem, outside the pith.

Outer endodermis

Inner endodermis

Outer pericycle

Outer phloem

Pith

Xylem

Inner phloem

Cortex

Inner pericycle

Leaf

trace

Mixed protostele

• The solid xylem core is broken into
  small groups of tracheids that remain
  embedded in phloem.


• Comprises of mixed pith i.e., groups of
  tracheids intermixed with parenchyma
  cells.
  Endodermis
  Pericycle

Xylem

Phloem

Cortex

Leaf

trace