Pteridophytes including their life cycle and classification

Introduction- Pteridophytes are commonly known as ferns and horsetails. They are a group of vascular land plants (cryptogams) which were preceeded by bryophytes and are succeeded by gymnosperms. Pteridophytes have a number of amazing characteristics which helps us to understand the evolutionary hierarchy of the kingdom plantae.
Special characteristics- Their special features can be categorized as :

1. First trachaeophytes- For the first time in the plant kingdom,vascular bundles appeared in pteridophytes, though they were very different from those present in angiosperms or flowering plants. For example, they lack sieve tube ,trachea, companion cells which are present in angiosperms
2. Closer to seed plants- The more advanced dioecious pteridophyta shows heterospory (formation of two type of spores) which is considered to be precursor of seed habit found in gymnosperms and angiosperms.
3. Well differentiated body- For the first time in the plant kingdom, plant body got differentiated into roots,stems and leaves in pteridophytes.
4. First plants with sporophytic life cycle- The pteridophytes were the first land plants to have a diploid (2n) sporophytic life cycle,although free existance of both gametophytic and sporophytic generations could be seen in them.Their life cycle is haplodiplontic type.
5. Appearance of stelar system- Stelar system (central axis containing vascular bundles) appeared for the first time in rhynia like pteridophytes.

Life Cycle- Life cycle of pteridophytes consists of the following:

• Homosporous species like Dryopteris forms similar type of spores (homospores) whereas heterosporous species forms two different type of spores-megaspores and microspores.
• For homosporous species, the spores germinate to form photosynthetic monoecious gametophyte called prothallus.The gametophyte bears both male and female reproductive units.
• For heterosporous species, both the spores germinate to form female gametophyte (archegonia) and male gametophyte (antheridia) respectively. They can be called as dioecious as male and female reproductive structures are borne on different individual.
• Due to the specific requirement of cool,damp,shady places to grow, these gametophytes are restricted to narrow geographical locations.
• The gametophyte produces male and female gametes.
• Sperms (male gametes) are flagellated and they require water to swim upto the mouth of archegonium.(Female gamete bearing sac)
• Ultimately,the sperms fuse with female gametes to form zygote,which developes into embryo.

                                       

Flowchart showing life cycle of pteridophyta.

Classification- 
They can be classified into following classes-

1. Psilotopsida (wisk ferns)- 12 species.
2. Lycopsida- It consists of three orders- Selaginellales (450 species), Isoetales (130 species), lycopodiales or club moss (400 species)
3. Equisetopseda (horsetails)- 15 species.
4. Polydiopsida- 10560 species.

Some members of this phyla- 

1. Adiantum caudatum (Walking fern) - They propagate by using their leaf tips,creating a walking effect.
2. Equisetum myriochaetum (giant horsetail) - It is the largest horsetail species,reaching upto 7 metres.
3. Ophioglossum (Adder's tongue fern) - One of the species of ophioglossum contains 1260 chromosomes which is highest among all living organisms.
4. Azolla is an aquatic water fern which is used as biofertilizer.