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Pteridophytes: Alternation of Generations, Reproduction, Life Cycle, Classification and Economic Importance

Did you know that pteridophytes exist as well-differentiated plants and non-differentiated thalli? In fact, the chromosome numbers of the two body forms of a pteridophyte are also different. Isn’t that amazing? Why do you think it happens? 

It is because their life cycle alternates between two generations, one depicted by the developed plant body and the other by the thallus. 

Also, do you think ferns are exactly similar to horsetails in all aspects? In fact, are all pteridophytes across the world exactly similar to each other?? Doesn’t sound right, does it? 

Had all pteridophytes been exactly similar to each other, they would not have been such a diverse group of plants, wouldn't it? This brings us to the conclusion that the division pteridophyta must be further classified into classes based on basic similarities or differences.

Plants belonging to different species of pteridophytes also contribute significantly to the environment and to the needs of humans. Let us now learn more about how pteridophytes are classified and how important they are to us and the ecosystem they live in.

Alternation of Generations in Pteridophytes

The life cycle of pteridophytes alternates between two generations:

  • A diploid sporophyte
  • A haploid gametophyte

Alternation of generation in pteridophytes

Sporophytic Generation 

Sporophytic plant body

Sporophyte is the free-living, multicellular, diploid dominant phase. It has a well-differentiated plant body and produces haploid spores by meiosis, which germinate to form gametophytes. The sporophyte can be homosporous or heterosporous.

Difference between Homosporous and Heterosporous Pteridophytes

Homosporous pteridophytes

Heterosporous pteridophytes

Produce spores of a single type.

Produce spores which are of two different types.

The spores are small.

These produce small microspores and large megaspores.

The spores germinate into bisexual gametophytes.

Microspores germinate into male gametophytes and megaspore germinate into female gametophytes.

Majority of pteridophytes are homosporous.

Selaginella and Salvinia are heterosporous pteridophytes.

 

Heterospores of Selaginella

Gametophytic Generation

The gametophyte is a haploid, free-living, mostly photosynthetic, multicellular thalloid structure known as a prothallus.

 Gametophyte

Gametophytes require cool, shady and damp places to grow and the presence of water for fertilisation. They bear the sex organs that give rise to haploid gametes by mitosis. 

Sex

Name of organ

Gamete produced

Male 

Antheridium

Antherozoids

Female

Archegonium

Egg cell

The gametophyte can be:

  • Unisexual: Male and female sex organs are borne on different gametophytic thalli.
  • Bisexual: Male and female sex organs are borne on the same thalli. This is mostly seen in homosporous pteridophytes.

Bisexual gametophyte showing presence of male and female sex organs

Reproduction in Pteridophytes

The male antherozoids produced by the antheridium of the gametophyte swim in water to reach the egg cell in the archegonium. One of the antherozoids fuses with the egg cell to form a diploid zygote that develops into a diploid sporophyte.

The sporophytes develop haploid spores within the sporangia by meiosis which germinate into haploid gametophytes. 

Life Cycle of Pteridophytes

The life cycle of pteridophytes is said to be haplo-diplontic as it exhibits equal prevalence of the alternating haploid gametophyte and diploid sporophyte generations.

Haplodiplontic life cycle of pteridophytes

Precursor to Seed Habit

In heterosporous pteridophytes, the female gametophytes are retained on the sporophytic plant body. Fertilisation and development of the zygote occurs within the female gametophyte while it is still attached to the sporophytic plant body. This phenomenon is considered as an important step towards the development of seed habit in gymnosperms and angiosperms.

Classification of Pteridophyta

The division Pteridophyta is further classified into 4 classes:

Class

Unique features

Examples

Images

Psilopsida

  • Rootless sporophyte.
  • Plant body differentiated into subterranean rhizome and aerial shoot.
  • Dichotomous branching.
  • Sporangia develop on aerial shoot, terminally or axillary.

Psilotum

 Psilotum

Lycopsida

  • Primitive vascular plants.
  • Plant body is well-differentiated.
  • Leaves are small microphylls.
  • Sporangia develops from axil of sporophylls (spore bearing leaves).
  • Sporophylls form compact cones or strobili.

Selaginella, Lycopodium

 Selaginella

Sphenopsida

  • Stem is differentiated into nodes and internodes.
  • Leaves are borne at nodes.
  • Sporangia develop on sporangiophores (stalks bearing sporangia) that form compact strobili.

Equisetum (horsetails)

Equisetum (horsetails)

Pteropsida

  • Plant body is well-differentiated.
  • Large, pinnately compound leaves.
  • Sporangia develop on the ventral surface of leaves.

Dryopteris (fern), Pteris, Adiantum

Pteris

Economic Importance of Pteridophytes

  • Pteridophytes are used as soil binders as they hold the soil with their roots and prevent it from getting eroded.
  • Ferns are grown as ornamental plants because of their beautiful foliage.
  • Equisetum stems are used for scouring of utensils and polishing of metals.
  • Azolla exists in symbiotic association with the nitrogen fixing cyanobacterium Anabaena. This symbiotic combination is used as a biofertiliser in paddy fields.
  • Pteridophytes help to prevent pollution by assisting in absorption of heavy metals from soil.
  • They have multiple medicinal uses, such as, Lycopodium is used in the treatment of rheumatism, roots of Adiantum help to cure throat infections etc.

Practice Problems of Pteridophytes

Question1. Consider a fern plant having 26 chromosomes. What will be the chromosome number of the prothallus?

A. 13
B. 12
C. 6
D. 52

Answer: Prothallus represent the haploid gametophyte generation of ferns. It is formed after the germination of the haploid spores produced by the diploid sporophytic plant body. As the diploid fern plant has 26 chromosomes, the chromosome number of the prothallus will be 13.

Hence, the correct option is a.

Question2, Which of the following statements is correct about the sporophyte of a fern plant?

A. Sporophyte of a fern is diploid
B. Sporophyte of a fern is haploid
C. Sporophyte of a fern is formed after germination of spores
D. All of the above are true

Answer: Sporophyte develops from the zygote that is formed due to fusion of haploid male and female gametes produced by the gametophytes. Hence, the sporophyte of a fern plant is diploid in nature.

Hence, the correct option is a.

Question3. Based on the life cycle of pteridophytes, which of the following sequences are correct? 

A. Sporophyte, meiosis, spores, gametophyte 
B. Sporophyte, mitosis, gametes, zygote 
C. Gametophyte, meiosis, gametes, zygote 
D. Gametophyte, mitosis, spores, sporophyte 

Answer: Life cycle of pteridophytes (haplo-diplontic life cycle): 

Sporophyte (2n)

Spore mother cells (2n)

↓ Meiosis

Spores (n)

Gametophyte (n)

↓ Mitosis

Gametes (n)

↓ Fertilisation

Zygote (2n)

↓ Mitosis

Embryo (2n)

Sporophyte (2n)

Hence, the correct option is a.

Question4. Match the columns I and II, and choose the correct combination from the options given.?

Column I

Column II

(A)

Pteropsida

(i)

Equisetum

(B)

Lycopsida

(ii)

Adiantum

(C) 

Psilopsida

(iii)

Selaginella

(D

Sphenopsida

(iv)

Psilotum

A. (A)---(ii), (B)---(iii), (C)---(iv), (D)---(i)
B. (A)---(iv), (B)---(i), (C)---(ii), (D)---(iii)
C. (A)---(ii), (B)---(iii), (C)---(i), (D)---(iv)
D. (A)---(iv), (B)---(i), (C)---(iii), (D)---(ii)

Answer: The division Pteridophyta is further classified into 4 classes:

  1. Psilopsida
  2. Lycopsida
  3. Sphenopsida
  4. Pteropsida

The examples of each class of division Pteridophyta are -

Column I

Column II

(A)

Pteropsida

(ii)

Adiantum

(B)

Lycopsida

(iii)

Selaginella

(C) 

Psilopsida

(iv)

Psilotum

(D

Sphenopsida

(i)

Equisetum

Thus, the correct option is a.

FAQs of Pteridophytes

Question1. Differentiate between homosporous and heterosporous sporophytes.?

Answer: The main plant body of the pteridophytes is a diploid sporophyte which produces haploid spores by meiosis of spore mother cells present in its sporangia. Based on whether a sporophyte produces one or more than one type of spores, sporophytes can be homosporous or heterosporous, respectively.

Homosporous pteridophytes

Heterosporous pteridophytes

Produce spores of a single type.

Produce spores which are of two different types.

The spores are small.

These produce small microspores and large megaspores.

The spores germinate into bisexual gametophytes.

Microspores germinate into male gametophytes and megaspore germinate into female gametophytes.

Majority of pteridophytes are homosporous.

Selaginella and Salvinia are heterosporous pteridophytes.

Question2. Which phenomenon in pteridophytes is considered as an important step towards the development of seed habit in gymnosperms and angiosperms?

Answer: Heterosporous pteridophytes produce two kinds of spores: microspores and megaspores. The megaspore germinates into a female gametophyte on the parent sporophyte plant, which produces an egg that gets fertilised, forming zygote. Zygote, while remaining attached to the sporophytic plant body, develops into an embryo. This phenomenon is considered as a precursor to seed habit in higher plants like gymnosperms and angiosperms.

Question3. What is a prothallus?

Answer: Prothallus is the gametophyte of pteridophytes which is free-living, bears sex organs and is green in colour due to the presence of chlorophyll. Pteridophytes are distributed over a narrow range of geographical regions because they need damp places to survive and need water for transfer of gametes.

Hence, the correct option is b.

Question4. State four economic importances of pteridophytes.?

Answer: The five economic importances of pteridophytes are -

  • They are used as soil binders as they hold the soil with their roots and prevent soil erosion.
  • Ferns are grown as ornamental plants because of their beautiful leaves.
  • Equisetum stems are used for scouring of utensils and polishing of metals.

A symbiotic combination of Azolla and cyanobacteria Anabaena is used as a biofertiliser in paddy fields.

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Cell Cycle and Division Transport in Plants Mineral Nutrition
Photosynthesis in Higher Plants Respiration in Plants Plant Growth and Development
Digestion and Absorption Breathing and Exchange of Gases Body Fluids and Circulation
Excretory Products and their Elimination Locomotion and Movement Neural Control and Coordination
Chemical Coordination and Integration

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