Biology Today — December 2017

• The diploid spore mother cells or sporocytes within the sporangia undergo
  meiosis or reduction division to form spores. These spores may be similar
  as in majority of pteridophytes called homosporous e.g., Lycopodium
  and Dryopteris or different with two types of spores (microspores and
  megaspores) called heterosporous e.g., Selaginella and Salvinia.


• Spores upon germination give rise to haploid gametophytes or prothallus
  bearing multicellular, jacketed sex organs.


• Gametophytes formed from homospores are monoecious (both antheridia
  and archegonia are borne on same prothallus) while those formed from
  heterospores are dioecious (antheridia and archegonia develop on separate
  male and female prothallus).


• Antheridia are small and sessile comprising of androcytes, each of which
  produces a motile antherozoid. Archegonia are partially embedded and consist of 4-rowed neck.


• Water is essential for fertilisation, as it assists in carrying bi-or multiflagellate sperms to archegonia.


• Pteridophytes exhibit alternate succession of sporophytic and gametophytic generation.

Evolution of Seed Habit

• The ability of plant to form seed is called seed habit. It is considered to have originated in pteridosperms during Devonian-
  carboniferous periods. The development of zygote into young embryo within female gametophyte in heterosporous plants e.g.,
  Selaginella is a precursor to the evolution of seed habit.


• The differentiation of spores into microspores and megaspores and their dependence on the parent sporophyte for the nutrition
  are certain features considered as pre-requisites for the formation of seeds.

Classification of Pteridophytes

Pteridophyta

Psilophytopsida

• These are oldest and
  simplest vascular plants.


• All members are extinct.


• Plant body was sporophytic,
  stem dichotomously branched.


• Rootless, rhizoids were present.


• Homosporous


• Their gametophytes
  are not known.


• E.g., Rhynia, Homeophyton, etc.

Sphenopsida

• At nodes, whorls of small
  leaves are present.


• Sporangia develop stalked
  sporangiophores bearing
  compact strobilus at tips.


• Ridges and grooves
  are found in stem.


• Always homosporous,
  e.g., Equisetum.

Lycopsida

• Commonly called
  club mosses or
  spike mosses.


• Roots, stem and
  leaves are present.


• Members are
  microphyllous.


• Homosporous
  (Lycopodium) or
  heterosporous
  (Selaginella).

Pteropsida

• Largest and most highly
  evolved group.


• Sporangia develop in
  groups called sori on
  ventral surface of leaves.


• Homosporous
  (Dryopteris, Pteris) or
  heterosporous (Marsilea).

Economic Importance of Pteridophytes

Medicines

Rhizomes of male shield fern are used to

obtain anti-helminthic drug. Equisetum

is used in preparation of diuretic,

haemostatic and haemopoietic drugs.

Ornamentals

Ferns are known for their attractive

foliage, hence are grown as ornamental

plants and used in bouquets and floral

decorations.

Nitrogen fixation

Some ferns e.g., water fern Azolla form

symbiotic association with nitrogen fixing

cyanobacterium Anabaena azollae. These are

used as biofertilisers in paddy fields.

Scouring

Stems of some plants are used in

scouring or cleaning of utensils and

polishing of metals, e.g., Equisetum.

Binding of soil

Pteridophytes bind the soil even

along hill slopes, thus, preventing

soil erosion.

Food

Pteridophytes constitute a good source of food

to animals. Sporocarps of Marsilea are cooked

and eaten by certain tribals.

Economic Importance