M.Sc. Pteridophytes notes pdf

PTERIDOPHYTES 

INTRODUCTION: Pteridophyta (Gr, Pteron = feather, phyton = plant), the name was originally given to those groups of plants which have well developed pinnate or frond like leaves. 

• Pteridophytes are cryptogams (Gr. kruptos = hidden, and Gamos = wedded) which have well developed vascular tissue. Therefore, these plants are also known as vascular cryptogams or snakes of plant kingdom. 
• They are represented by about 400 living and fossil genera and some 10,500 species. 
• Palaeobotanical studies reveal that these plants were dominant on the earth during the Devonian period and they were originated about 400 million years ago in the Silurian period of the Palaeozoic era.

Fern Ally: It is a general term covering a somewhat diverse group of vascular plants that are not flowering plants and not fern. Like ferns, these plants disperse by shedding spores to initiate an alternation of generations. 

Example: Lycopodium, Selaginella.

True Ferns: They are vascular plants differing from lycophytes by having true leaves (megaphylls). They differ from seed plants (gymnosperms and angiosperms) in their mode of reproduction - lacking flowers and seeds. Similar of flowering plants, ferns have root, stem and leaves.

Example : Ophioglossaceae, Hymenophyllaceae, Marattiaceae.

Adaptive features of vascular land plants: The following adaptive features are mainly prevail to land vascular plants - 

• Development of an anchorage and water absorbing system, such as underground stems and root.
• Distinct photosynthetic organ i.e. true leaves.
• Conducting tissue, xylem and phloem.
• Prevention of desiccation by the formation of a waxy layer (cuticle on aerial branch system).
• Development of structure for gaseous exchange (stomata).
• Production of spores with the cell wall impregnated with sporopollenin (a substance that prevents desiccation and is virtually indestructible by microorganism).
• Independent sporophytic phase (asexual) to be dominating one and distinct free living prothalli (sexual) i.e. the alternation of generation present.

Alternation of Generations :

Hofmeister (1851) used this term in plants. He observed that in Mosses and Ferns there are two types of morphologically distinct individuals in the life cycle. Both alternate in a life cycle i.e.,there are some events which lead one generation to produce the other and thus cause alternation. The actual phenomenon responsible for bringing about alternation was exposed by the significant discovery of “the periodic Reduction of Chromosomes” by Strasburger (1894). Strasburger discovered the process of meiosis in plants. This discovery revealed that the reduction in the number of chromosomes leads to the formation of a new individual in the life cycle. This individual has haploid number of Chromosomes in its nuclei. It bears sex organs and is concerned with sexual reproduction It was given the name ‘gametophyte’ and represented gametophytic generation. The haploid gametes unite (fertilization or syngamy) and establish a diploid nucleus or a Synkaryon. Fertilisation establishes a diploid cell or the zygote, which is a pioneer structure of the diploid individual or the sporophyte. It germinates to form the embryo, which in turn develops into the sporophyte individual. This generation is termed the sporophyte generation. The sporophyte individual bears sporangia which produce spores as a result of meiosis. These spores are haploid and are also known as meiospores. The spores are the pioneer structures of the gametophyte generation. They germinate to give rise to the gametophyte individual known as the prothallus in vascular cryptogams.

Both the generations can reproduce vegetatively and effect an increase in the number of their individuals.

The above events, as displayed by the normal life cycle of a vascular plant. lead us to the following conclusions: 

1. There are two distinct individuals in the life cycle of vascular cry ptogams.
2. These individuals are produced by the germination of the haploid spores and the diploid zygote. 
3. The one produced by the spores is the gametophyte plant. To it is attributed the function of sexual reproduction and is haploid. 
4. The zygote produces the embryo which gives rise to the diploid individual called the sporophyte. It bears haploid spores (or meiospores) after a process called the meiosis. 
5. The two generations alternate with each other in the life cycle.  
6. Meiosis and syngamy (fertilization) are the two significant stages that switch on the life cycle from one generation to the other. 
7. The number of chromosomes is halved during meiosis and is doubled during syngamy or fertilisation

In the vascular plants the sporophyte individual is a complicated, independent and a dominant generation, whereas the gametophyte is comparatively much reduced. The gametophyte in the homosporous forms, though inconspicuous and comparatively shortlived, is independent and may be surface living and green (autophytic) or subterranean and non-green (Lycopodium). They are always exosporic and thus are not enclosed by the spore wall. They are also infected by an endophytic or amy corrhizic fungus in some genera (Lycopodium, Psilotum and Tmesipteris). The heterosporous land plants, surprisingly enough, display a considerable reduction in their
gametophytes. The gametophytes are reduced and endosporic.

They have, as a rule, separate male and female prothalli. The male prothallus is extremely reduced and represented only by a single prothallial cell. The female prothallus, on the contrary, is well developed because it has to nourish the developing embryo. Such a dioecism is unknown in homosporous land plants, but is a regular feature in the heterosporous ferns. The life cycles of homosporous and heterosporous vascular plants are represented diagrammatically in the below Figure.

The sporophyte in the vascular cryptogams is no doubt independent but it has to depend upon the gametophyte during its earlier stages of development. It achieves its complexity after establishing its independence. The sporophyte, however complex it may be, has never achieved complete independence. The huge and giant sporophytes of the gymnosperms, during their earlier stages of development, are completely at the mercy of the food stored in the gametophyte. This is true even in the even in the case of apogamously formed sporophytes. This food in the homosporous plants is truly gametophytic in origin, but in the heteroporous plants the food is mainly drawn from the sporophyte plant and is only stored in the gametophytic tissue. The angiosperms. on the other extreme, tell a different tale. The food stored in the endosperm is derived from the sporophyte and is stored in a

triploid tissue. The morphological complexities of the sporophyte of vascular plants coupled with their elaborate and well organised anatomical set-up equipped with efficient means of dispersal confer upon them the potentialities to live under varied environmental conditions. These capabilities are responsible for their being stamped as efficient land dwellers.

Both the homosporous and heterosporous vascular plants exhibit heteromorphic type of alternation of generations because the sporophyte and the gametophyte individuals present marked morphological and anatomical differences.

While studying the life cycle of a heterosporous individual a common feature that attracts our attention is the difference in the size of spores. It poses a serious question before us. Why the microspores (smaller in size) give rise to the male gametophyte and the megaspores to the female gametophyte? This question has not been satisfactorily answered. Two poss doubt been suggested.

The first suggestion tends to explain the cause of difference in size. The megaspores are larger in size and produce female prothalli. The latter have to lodge the developing embryo sporophyte that requires lot of nourishment. The larger spores have greater nutritional store and can produce a well developed female prothallus that is capable of hoarding food sufficient for the development of embryo sporophyte. The microspores are smaller in size and thus have little food stored in their protoplasts. They cannot afford to produce massive prothallial tissues. They germinate to develop one or two-

celled prothallus that bears a single antheridium. The function of the antheridium is to produce spermatozoids. After their liberation it has nothing to do and, therefore, perishes. This can be the cause of their smaller size. The microspores are produced in large numbers as they are likely to be wasted during dissemination

The cause of their being unisexual appears to be a contrivance towards cross pollination. The monoecious or bisexual gametophytes of homosporous vascular plants have many chances of self- fertilisation, unless they are submerged under water. The dioecious prothalli have extremely rare chances of self-fertilisation and suffer from a great disadvantage if they grow under terrestrial conditions. Under such circumstances they have little chance of fertilisation because the spermatozoids cannot reach the archegonia. This difficulty is solved to a greater extent by the phenomenon of heterospory. The microspores that need not carry much food are light in weight and smaller in size. They can be

easily carried by wind and other agents to the female prothalli. The chances of their wastage are eliminated to a great extent, on account of their large number per microsporangium. Those which fall on the female prothallus have absolutely no difficulty in effecting fertilisation as the spermatozoids have not to travel a long distance and require only a little moisture which is provided by dew drops. Chances of self-fertilisation under these circumstances are rare, and cross-fertilisation is usually effected. This leads to outbreeding and ultimately to variations and rapid evolution. Heterospory has, as a matter of fact, freed the sporophyte from aquatic environments. Such an emancipation from aquatic habitats has conferred on the sporophyte the freedom to grow under varied environmental conditions. Heterospory is, therefore, regarded as a necessary step towards the evolution of seed habit and must be significant development during the struggle to conquer land.

Origin of Alternation of Generations :

Celakovsky (1874) listed three distinct types of alternation of generations in plants. These are | —

1. The Antithetic Alternation of Generations by which he meant the interpolation of a new phase between the successive gametophyte generations. Such an alternation, according to him, makes up the life cycles of the bryophytes and the pteridophytes.

1.  The Homologous Alternation of Generations that was believed by him to be prevalent among algae and fungi.

1.  Strophogenesis or the alternation of shoots.

Bower (1935) dropped the terms antithetic and homologous and replaced them with Interpolation theory for antithetic theory and Transformation theory for homologous theory. These two theories have been advanced to explain the origin of alternation of generations. Both of them are aimed at explaining the origin of sporophytic individual in the life cycle.

What are the similarities between bryophytes and pteridophytes ?

• Mainly the taxa of both the group belongs to terrestrial habit.

• After the fertilization zygote is formed and developed into embryo, so the plants are commonly known as embryophytes.

• Female sex organ or archegonia are similar in both groups.

• Antheridia are male reproductive organ.

• Antherozoids are ciliated.

• For fertilization water is required.

• Sporangia are multicellular and spores are developed in similar way in both the groups.

What are the dissimilarities between bryophytes and pteridophytes ?

• Dominating phases of the life cycle is sporophytes (diploid) in case of pteridophyte and in case of bryophyte i.e. the gametophytes (haploid).

• In case of pteridophyte two distinct independent phases (sporophyte and gametophyte) are present in their life cycle whereas the bryophytes are having sporophytic plant body, dependent entirely on the gametophytic plant body.

• Plant body of pteridophytes are differentiated into leaf, stem and roots whereas in case of bryophytes the plant body is thalloid or foliose in nature, but no true leaves, stems or roots are present.

• Pteridophytes bear the vascular tissue with well developed conducting system but that are absent in bryophytes.

What are the similarities between gymnosperm and pteridophytes ?

• The sporophytic generation is the dominating in the life cycle phase.

• Vascular tissue and well developed conducting systems are present.

• Both groups are known as embryophytes as the zygote develops to form embryo.

• Both groups are known as archegoniate.

What are the disimilarities between gymnosperm and pteridophytes ?

• Pteridophytes are non-flowering plants whereas gymnosperms are flowering plants bear naked seed.

• In pteridophytes the gametophytic phase is free living and independent but the gametophytic phases of gymnosperm is comparatively reduced and completely dependent on sporophyte.

• In pteridophytes the mode of reproduction is zooidogamy i.e. the sperm reaches the egg through neck canal cell but in gymnosperm mainly siphonogamy is seen, exception present.

• Root system is comparatively less developed in pteridophytes but in gymnosperm it is more developed.