Reproduction in Pteridophytes

Although Pteridophytes commonly reproduces through vegetative means, reproduction through spores is the main mode of reproduction in Pteridophytes.

Table of Contents

Vegetative Reproduction in Pteridophytes

Reproduction in Pteridophytes though vegetative means usually occurs by formation of bulbils, by fragmentation, by formation of tubers and by formation of adventitious buds.

By the formation of gemmae or bulbils:

This vegetative reproduction in Pteridophytes is carried out by bulbils (bulblets) or gemmae. These are leafy side branches with wide base.
The gemmae fall on the ground and grow into a new young plant.
Species of Psilotum, Lycopodium phlegmaria, L. selago etc. Certain species of Selaginella also propagate by bulbils.

By fragmentation:

The death and decay of older sections of the stem result in the formation of stem or rhizome fragments.
Each fragment that has roots can grow into a new plant.
This method of vegetative reproduction in Pteridophytes is commonly found in species like Lycopodium, Selaginella, Dryopteris, Pteris, Adiantum, and others.

By formation of tubers:

The tubers originate from the paranchymetous regions of shoot and root at the onset of unfavourable conditions.
The tubers are formed at surface of the ground, called surface tubers and those developing underground are the underground tubers.
They consist of a group of cells with stored food materials and having the capacity to germinate into new plants during favorable season.
This method of reproduction in Pteridophytes, is common certain species of Marsilea, wherein irregular tuberous structures form on the stem. Similar tubers also develop in some species of Lycopodium, Selaginella, and Equisetum.

By formation of adventitious buds:

Adventitious buds can be induced on isolated bulbil leaves. When the stem is decapitated near its apex, it also triggers the formation of these buds.
Certain species of Lycopodium and Selaginella develop adventitious buds, as do some species of Asplenium, Diplazium, and Ophioglossum.
In Dryopteris, these buds arise in the leaf axils and can develop into new plants once detached from the parent plant.
Vegetative reproduction in Pteridophytes like in some ferns, such as Platycerium and Asplenium esculentum, the root apex directly develops into a leafy bud, which can then grow into a new plant.

Reproduction in Pteridophytes through spores

Reproduction through spores is the primary mode of reproduction in pteridophytes. Pteridophytes reproduce asexually via haploid spores, which are formed in structures called sporangia. These sporangia develop either on the underside or in the axils of leaves, which are known as sporophylls.

However, in Psilophytales, the sporangia are found on the stem. In Rhynia, the sporangia are located at the tips of fine aerial branches. In Equisetum and Selaginella, the sporophylls are organized into compact structures called strobili or cones. In genera like Azolla, Marsilea, and Salvinia, the sporangia are housed in specialized structures known as sporocarps.

In higher ferns, sporangia are arranged in well-organized clusters called sori (singular: sorus). In the case of Psilotum, the sporangia are trilobed structures, each lobe containing a sporogenous region, collectively known as a synangium.

On the basis of development of sporangia Goebel (1881) classified sporangial development into two types, i.e., Eusporangiate and Leptosporangiate. The sporangium developing from a group of initial cells is called eusporangiate while the development from a single initial cell is called leptospoangiate development.

Fig: Fern Life Cycle [Image source: Image by brgfx on Freepik]

Sporophytic phase

The spore-producing structure in pteridophytes is called the sporophyte. In the life cycle of pteridophytes, the sporophytic generation is dominant and more noticeable.
A typical pteridophyte’s life cycle features a regular alternation between the sporophytic (asexual) and gametophytic (sexual) generations.
In contrast, life cycle in bryophytes, where the gametophytic phase is dominant, and the sporophyte is dependent on the gametophyte.
In gymnosperms and angiosperms, the gametophytic generation is reduced and relies on the sporophyte.
Pteridophytes, positioned between these groups, are distinguished by both free-living gametophytic and sporophytic generations. However, the sporophyte is the dominant generation, eventually becoming independent of the gametophyte and growing significantly larger.

Pteridophytes exhibit two fundamental types of life cycles: homosporous and heterosporous.
Heterosporous pteridophytes produce two types of spores—larger megaspores and smaller microspores—which develop into female and male gametophytes, respectively.
Homosporous pteridophytes, on the other hand, produce only one type of spore, which typically develops into a hermaphroditic (monoecious) gametophyte.
Therefore, heterosporous pteridophytes are obligatorily heterothallic, while homosporous pteridophytes are usually homothallic.
Examples of heterosporous pteridophytes include Selaginella, Isoetes, Marsilea, Salvinia, Azolla, and Regnellidium.
The homosporous life cycle is found in Psilotum, Tmesipteris, Lycopodium, Equisetum, and homosporous filicopsids.

Gametophytic phase

The gametophyte represents the sexual phase in a plant’s life cycle.
It begins with the haploid spore, which is the first cell of the gametophyte. These spores are haploid and result from reduction division in the sporogenous cells of the sporangium.
Upon germination, the spore develops into a prothallus.
Typically, the prothalli are green, simple, somewhat branched, and aerial structures. However, in certain genera like Lycopodium, the prothalli are subterranean, branched, colorless, and saprophytic.

The prothallus gives rise to two sex organs: antheridia and archegonia.
In most homosporous pteridophytes, the prothalli are monoecious, meaning they produce both types of sex organs.
In contrast, the prothalli of heterosporous pteridophytes are usually dioecious, producing either male or female sex organs.

In most cases, the antheridia and archegonia are embedded within the prothallus.
The antheridium is always surrounded by a jacket layer and produces antherozoids.
During reproduction of Pteridophytes like Lycopodium, Selaginella, and similar genera, the antherozoids are unicellular, uninucleate, and biciliate, while in Psilotum, Tmesipteris, Isoetes, Equisetum, and ferns, they are multiciliate.
The archegonium is made up of a protruding neck and a lower, embedded part known as the venter. The neck contains neck canal cells, while the venter houses a ventral canal cell and an egg cell.

Fertilization and formation of zygote

Fertilization in pteridophytes requires water. When a film of water flows between the prothallus and the substrate, it causes the cap cells of the antheridia to open, releasing the antherozoids.
In the archegonium, the neck canal cell and ventral canal cell break down, their protoplasm becomes mucilaginous, absorbing water from the surrounding jacket cells and swelling.
This swelling exerts pressure on the four cover cells at the top of the archegonium, causing them to separate.
Mucilaginous substances containing malic acid accumulate at the opening of the archegonium.
The antherozoids are attracted to the archegonium by this malic acid (chemotactic response), and they swim toward the archegonium.
Many antherozoids enter the neck of the archegonium, but only one fuses with the egg to form a diploid zygote.

Typically, cross-fertilization occurs because the antheridia mature before the archegonia due to the protandrous nature of the prothalli.
This fertilization results in the formation of a diploid zygote, which is the first cell of the sporophytic generation and eventually develops into a fully formed sporophyte.

Development of embryo

The zygote develops into an embryo, starting with a typically transverse division.
Following this initial transverse division, the zygote forms a two-celled structure.
This is followed by a vertical division, resulting in a quadrant of cells.
Subsequent divisions continue to produce and develop the young sporophyte.

Alternation of generation in Pteridophytes

Reproduction in Pteridophytes exhibit a true alternation of generations.
In this process, the dominant sporophyte stage produces spores through meiosis.
The gametophytic generation, during reproduction in Pteridophytes, on the other hand, forms gametes through mitosis.
Pteridophytes have distinct haploid and diploid stages: a multicellular haploid gametophyte with n chromosomes alternates with a multicellular diploid sporophyte with 2n chromosomes.
The mature sporophyte produces spores via meiosis, reducing the chromosome number from 2n to n. These haploid spores germinate and develop into haploid gametophytes.
Once mature, the gametophytes produce gametes through mitosis.
When two gametes—either from different individuals of the same species or from the same individual—fuse, they form a zygote, which then develops into a diploid sporophyte.
The life cycle of pteridophytes involves an alternating sequence between the sporophyte and gametophyte stages, a phenomenon known as alternation of generations.
This cycle represents a survival strategy, allowing the plant to alternate between different reproductive methods.

Abnormalities in the life cycle of Pteridophytes

In the typical life cycle of vascular plants, two generations alternate: one haploid and one diploid. In pteridophytes, this regular alternation of chromosome numbers can sometimes be disrupted by two phenomena known as apospory and apogamy.

Apogamy:

During reproduction in Pteridophytes, when a sporophyte develops directly from the gametophyte without the formation of gametes or sexual fusion (syngamy) is called apogamy.
In this case, the sporophyte retains the same haploid chromosome number as the gametophyte.
Apogamy during reproduction in Pteridophytes was first observed by Farlow in Pteris cretica in 1874 and is a common occurrence in ferns.
It has been documented in various ferns, including Pteris, Pteridium, Dryopteris, Adiantum, Osmunda, Todea, Athyrium, and Asplenium.

Apospory:

During reproduction in Pteridophytes, when it involves the development of gametophytes from the vegetative parts or cells of the sporophyte, bypassing meiotic division and spore formation is called as apospory.
These gametophytes are diploid.
The phenomenon of apospory during reproduction in Pteridophytes, was first discovered by Druery in Athyrium filix-femina in 1884.
It has been observed in several pteridophytes, including Pteridium aquilinum, Asplenium dimorphum, Osmunda regalis, and Todea.