Pteridophytes

Pteridophytes are the first terrestrial vascular plants. Carolus Linnaeus classified them under the group cryptogamae. Pteridophytes occupy a transitional position between bryophytes and spermatophytes. They do not produce flowers and seeds, hence they are also called Cryptogams. More than 12,000 species of Pteridophytes are found on Earth. The word ‘Pteridophytes’ comes from the Greek word Pteron meaning “feather” and python meaning “plants.” 

Characteristics of Pteridophytes

Some of the basic features of Pteridophytes are mentioned below. 

• They mainly thrive in moist and shady places. Some also grow in sandy soils.

• The main plant body has well-differentiated roots, stem and leaves. It is called a sporophyte.

• The stem is an underground rhizome.

• Some Pteridophytes have small leaves called microphylls (e.g lycopodium) and some have large leaves called megaphylls (e.g Pteris).

• Leaves may also have spores on the underside. Such leaves are called sporophylls. 

• Plants are flowerless and seedless. They reproduce by means of spores.

• Sometimes the sporophylls form compact structures known as cones or strobili.

• A well-developed vascular system with xylem (for conduction of water) and phloem (for conduction of food) is present.

Image: Parts of Pteridophytes

The Life Cycle of Pteridophytes

There is an alternation of generation in the lifespan of Pteridophytes. This is known as metagenesis. Just like in the seed-bearing plants and mosses there is a diploid generation alternating with a haploid generation. The diploid generation is the sporophyte that produces the spores. The haploid generation is the gametophyte that produces the gametes. Both the sporophyte and gametophyte are independent and free living. 

Sporophyte Generation

Some of the important features of the sporophyte generations are mentioned below.

• This is the dominant and longer phase than the gametophyte generation.

• The plant produces spores which are carried away by the wind to far off places.

• The spores are produced by spore mother cells through meiosis.

• Most plants produce similar spores. Such plants are called homosporous. E.g Lycopodium.

• Some plants are heterosporous as they produce two kinds of spores; microspores and macrospores. E.g Selaginella.

• Microspores produce male antheridia and megaspores produce female archegonia.

• Under favourable conditions, these spores germinate into small independent gametophytes called prothallus.

Gametophyte Generation

Important features of the gametophytic stage are mentioned below.

• The prothallus (gametophyte) can grow only in a moist and cool shady place. Their growth is restricted to only a few places.

• Gametophytes are multicellular and photosynthetic.

• They bear male and female sex organs.

• The male sex organ is called antheridia and the female sex organ is called archegonia. 

• The sperm in antheridia fuses with the egg in archegonia resulting in the formation of a zygote.

• The zygote later develops into a multicellular sporophyte.

• Thus the life cycle of Pteridophyte gets completed.

(Image will be uploaded soon)

Examples of Pteridophytes

There are many types of Pteridophytes. Common examples include:

• Club mosses- Lycopodium

• Spikemosses- Selaginella

• Quillworts- Isoetes 

• Horsetails- Equisetum

• Ferns- Pteris, Marsilea, Dryopteris

Important Features of Selaginella

Selaginella is an example of Pteridophyta, and some of its common features are mentioned below.

• Commonly known as spikemoss, Selaginella is found in moist and shady places.

• Morphologically the plant displays a lot of variation.

• The sporophyte is green and has adventitious roots.

• There are four rows of leaves on the stem; two rows of small leaves and two rows of large leaves.

• The plant reproduces by vegetative and sexual means.

• Vegetative reproduction is by means of buds and tubers.

• Sexual reproduction is through spores. It has two kinds of sporangia; microsporangia and megasporangia. The sporophyte is heterosporous.

• The sporophylls form strobila. 

• There is an alternation of generation between diploid sporophyte and haploid gametophyte.

(Image will be uploaded soon)

Classification of Pteridophytes

Plant kingdom was first divided into Cryptogams and Phanerogamae by Eichler. Cryptogamia was again divided into three phyla; Thallophyta, Bryophyta and Pteridophyta. Bryophyta and Pteridophyta were grouped under Embryophyta. A new term Tracheophyta was coined by Sinnott in 1935. Tracheophyta was divided into four groups by Eames in 1936. These groups were Psilopsida, Lycopsida, Sphenopsida and Pteropsida. Pteropsida was further classified into Filicinae, Gymnospermae and Angiospermae.

Another botanist Smith, classified Pteridophytes into Psilophytineae, Lycopodineae, Equisetineae and Filicineae. Pteridophytes were classified by Oswald and Tippo in 1942 into the following four classes:

• Psilopsida

• Lycopsida

• Sphenopsida

• Pteropsida  

Important Features of Psilopsida

• Most of the species are now fossils. This group includes some of the oldest vascular plants.

• The plant body does not have roots. The rhizome is subterranean, and it has an aerial shoot.

• The stem has dichotomous branches.

• Small rhizoids come out of the rhizome. These help in the absorption of water and salts.

• On the shoot, the leaves are either scale-like arranged in a spiral as in Psilotum or leaf-like appendages as in Tmesipteris. 

• There is no secondary growth in the stem.

• The sporophyte is homosporous as the spores are similar.

• The sporangia are present either at the tip or laterally directly on the stem; i.e they are cauline in position.

• The gametophyte is not photosynthetic as it is not green in colour. It grows as a saprophyte with a fungus.

• The antherozoids are flagellated and spirally coiled.

• An example of it is Psilotum. 

Important Features of Lycopsida

• The plant body is differentiated into well-defined roots, stem and leaves.

• The leaves are small, that is they are microphyllous.

• The branching is dichotomous.

• The sporangia are present in the axil of sporophylls. 

• The sporophylls are arranged as strobili.

• The sporophyte is either homosporous as in Lycopodium or heterosporous as in Selaginella.

• The gametophyte develops independently.

• Some of the examples include Selaginella and Lycopodium.

Important Features of Sphenopsida

• This class has only one living genus called Equisetum.

• The stems have distinct nodes and internodes.

• Branches are arranged in whorls.

• Very small leaves are arranged in whorls at the nodes of stems and branches.

• There is a special appendage called sporangiophore in which the sporangia are formed.

• Equisetum is homosporous.

• The gametophyte is photosynthetic.

• There are multiple flagella present on antherozoids.

• An example of it is Equisetum.

Important Features of Pteropsida

• The most prominent members of this class are ferns.

• There are more than 9000 species in this group, and it is the largest group of Pteridophytes.

• These plants are highly evolved than other Pteridophytes.

• Members of this class are found in all kinds of habitats; terrestrial, damp and shady places, even in water and some are epiphytes too.

• The plant body is differentiated into well-defined roots, stem and leaves. The leaves are arranged spirally.

• The rhizome is thick and short.

• The leaves are large in size (megaphylls). They are pinnately compound and are called a frond.

• Young fronds are coiled.

• Except for aquatic ferns, others are all homosporous.

• Sporangia are located at the margins and tip of leaves on the ventral side of leaves. These are present in clusters called sori.

• Some of the examples are Dryopteris and Pteris.

In conclusion, it can be said that we have learnt about the different classifications of Pteridophytes, we have also learnt about the life cycle of a Pteridophyte. We have also made the following conclusions-

• Many varieties of ferns are grown in gardens and have ornamental value due to their bright green foliage. 

• Some species like Selaginella are used for soil conservation.

• Marsilea is consumed by the tribals as a rich source of starch. 

• Many varieties of ferns are used as medicines. 

• Lycopodium clavatum is used in treating skin ailments. A variety of Equisetum is used as a diuretic. 

• Dryopteris is also used to make antihelminthic drugs; to treat tapeworm infections.