Evolutionary Concepts in Pteridophytes
Discipline: Botany Paper: Pteridophytes Lesson: Evolutionary Concepts in Pteridophytes Lesson Developer: Dr.P.L.Uniyal, Ms. Priti Department/College: Department of Botany, University of Delhi Lesson Reviewer: Dr Satish Agarwal Department/College:Deshbandhu College Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL
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Evolutionary Concepts in Pteridophytes
Table of Contents
History Theories of algal origin of pteridophytes
Theories of biphylitic origin
Telome theory
Stelar System in Pteridophytes
Heterospory and seed habit
Apospory and Apogamy
Exercise
Glossary
References
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Evolutionary Concepts in Pteridophytes Origin of pteridophytes Pteridophytes show close affinities with bryophytes is having the similarity in the gametangial structures and the sexual reproduction. However, the remarkable event of evolution from bryophytes to pteridophytes was the evolution of dominant and independent sporophyte phase which symbolize the true root, stem, leaves and vascular tissue. Church and Arnold, considered the brown algae (member of phaeophyaceae) as immediate ancestor of pteridophytes. On the other hand Bohlin, Fritsch and Lotsy put forward the concept that the vascular plants were possibly from chaetophoraceae green algae. Andrews, Merker proposed that pteridophyts have polyphyletic origin. Telome Theory Present day plants with leaves and branches are the result of continuous evolution in the axial sporophytes of the primitive land plants. earliest vascular lands plants (Rhynia, Horneophyton, Psilophyton, Asterroxylon), had a simple dichotomously branched axes and bore sporangia terminally or laterally. Zimmermann (1930) proposed that dichotomous branching system and leaves in the primitive plants evolved by the process of elaboration which is the telome theory. Telome Telome represent the simple ultimate terminal portions of dichotomously branched axis. These axes were undifferentiated and single nerved. Two telomes were united below the point of dichotomy to form a fused structure, called mesome. The telome, which bears a sporangium at its tip is called a fertile telome, while the sterile telome, phylloids or vegetative telome are those, which does not bear a sporangium During the course of evolutionary development, telomes did fuse together to form a more complex structure, called syntelome or telome truss. If a syntelome is represented by only sterile telome, it is called phylloid truss. if telomes had only sporangia, it was known as fertile truss. When the plant consisted of both sterile & fertile telomes it was named as mixed telome truss or mixed syntelome.
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Evolutionary Concepts in Pteridophytes
Fig.1 Telome: Diagrammatic representation of hypothetical primitive
vascular plant.
Telome and the origin of Higher Lands Plants Zimmermann stated that the primitive vascular land plants (Rhynia, Horneophyton, Psilophyton, etc) have originated from unicellular green algae during the upper Silurian and Lower and Middle Devonian periods. He proposed that the shoot axes and leaves of higher vascular plants have evolved from earliest land plants as a result of a few organogenetic processes of progressive differentiation which are called “elementary processes”. The early vascular plants werer transformed to higher plants through following elementary processes: (1)
Overtopping
(2)
Planation
(3)
Syngenesis
(4)
Reduction
(5)
Curvation
Overtopping: In this process one of the two dichotomizing branches of an axis became sturdy and grew vertically longer similar to main axis. The shorter branch was shifted laterally and it appears to be a megaphylls .So the weaker and shorter branch was overtopped by the stronger and longer branch. It initiated the development of monopodial branches from equal dichotomies. Dichotomous branching is considered to be primitive character and is found in primitive pteridophytes and monopodial branching is a advance character that is represented in the advanced forms.
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Evolutionary Concepts in Pteridophytes
Fig.2 Overtopping of telome
Planation: The equal dichotomies of telome, which were in more than one plane (cruciate dichotomy), come to lie in a single plane and appear to be fan shaped dichotomy). This process is called planation. It is interpretive of the development of organs of bilateral symmetry from those of radial symmetry. Planation process has given the way to the evolution of leaf.
Fig.3 Planation of telome Syngenesis: In this course, telomes and mesomes came to lie within a common tissue by fusion or webbing. The vascular strands of telome branches were also fused together. By the result of this process leaves with open dichotomous, pinnate and reticulate venation are
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Evolutionary Concepts in Pteridophytes created and Polystelic condition was also arose as found in Selaginella and some fossils (Cladoxylon, Steloxylon)
Fig.4 Webbing of telome Reduction It involved transformation of a syntelome into a single narrow leaf, which led to the evolution of simple microphllous leaves of the lycopods.
Fig.5 Reduction of telome Curvation It is brought about by unequal growth of tissues on two opposite flanks of the telome. The following two types of curvation processes have been recognized. 1. Recurvation. Downwards bending of telomes leads to the recurvation.
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Evolutionary Concepts in Pteridophytes
Fig.6 Recurvation of telome 2.
Incurvation. This process accounts for the shifting of sporangia to the ventral surface of the leaf in fern.
Origin of Sporophylls Zimmermann also attempted to explain the origin of sporophylls in three main groups of vascular cryptograms. According to him, a sporophyll is composed of one or more telomes of both, telomes and mesomes. 1. Lycophyta 2. Spenophyta 3. Pterophyta
Origin of sporophyll in Lycophyta The members of lycophyta are characterized by the presence of axillary sporangia and Zimmermann the viewed that axillary sporangia are formed through. (1) the aggregation of sterile and fertile telomes, (2) the decrease in the number of telomes and mesomes and (3) the decrease in the number of sporangia
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Evolutionary Concepts in Pteridophytes
A
B
C
D
Fig.7 Development of sporophyll in Lycophyta: A-B Aggregation of sterile and fertile telomes C-D Reduction in the number of fertile and sterile telome (also note the shift of sporangia to axillary position). Origin of sporophylls in Spenophyta In the Spenophyta sporangia occur on the underside of stalked peltate disc, called sporangiophore. Such structure was formed by by the: (1)
downward bending (recurvation) of fertile telomes.
(2)
and recurved telomes then come to lie within a common parenchymatous flap (webbing), which converted into the peltate disc.
A
B
C
D
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Evolutionary Concepts in Pteridophytes Fig.8 Development of sporophyll in Sphenophyta A-C.Downward bending of telomes, D.Formation of peltate disc by the fusion of telomes and mesomes.
Origin of sporophylls in Pterophyta Most of the Pterophyta have large pinnately divided sporophylls. Zimmermann stated that the elementary processes, such as overtopping, reduction, syngenesis, and incurvation have resulted in development of such sporophylls. Following steps were seems to be involved in the development of sporophylls of the Pteropsida. (1)
Overtopping of sterile and fertile telomes has led to the development of pinnate sporophylls.
(2)
Lateral fusion (webbing) of mesomes has resulted in the development of pinnate sporophylls with marginal sporangia.
(3)
Incurvation resulted in the shifting of sporangia from marginal to lower superficial position.
A
B
Fig.9 Development of sporophylls in Pterophyta (Red line showing webbing)
Origin of Leaves
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Evolutionary Concepts in Pteridophytes Zimmermann proposed that the microphyllus leaf originated as a result of the process of reduction whereas megaphyllous as a result of syngenesis.
Merits of Telome Theory (i)
The theory illustrates the origin and evolution of sporophytes of land plants to a great extent.
(ii)
It provides the details of the structure of the sporophytes of earliest known land plants.
(iii)
It clarifies that the entire sporophyte is an axis containing root and shoot.
(iv)
It explains the exact position and relationship between root, stem and leaves.
(v)
It highlights the phylogenetic relationship between fossil and living taxa.
(vi)
This theory elucidates the evolution of vegetative and reproductive structure of Cordaitales.
Demerits of Telome Theory (i)
The theory does not describe the formation of whorled and spiral arrangements of sporangia, which is found in some ancient and primitive plants.
(ii)
The telome theory does not fit into the pattern of origin of Lycopsida.
(iii)
Telome theory does not explain how a telome-like characteristic body has been developed.
(iv)
Telome theory was criticized because it fails to explain the derivation of the dictyostelic condition.
Stelar System in Pteridophytes The central vascular cylinder of the primary axis of pteridophytes is usually referred to as stele (stele = column). It includes xylem and phloem and pith (if present) and is delimited from the cortex by the pericycle. The concept vascular system as a fundamental unit was proposed by Van Tieghem and Douliot (1886). They proposed stelar theory, stating that the root and stem have the same basic structure consisting of cortex and central cylinder and endodermis (innermost layer of the cortex) as anatomical boundary between these two. A distinct endodermis is present between the cortex and stele in pteridophytes, however, higher plants usually lack a distinct endodermis and pericycle.
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Evolutionary Concepts in Pteridophytes In Selaginella and Lycopodium, the stele of the axis and the leaf traces developed independently, the mainly cauline and leaf traces are attached only to its surface. On the other hand, in higher pteridophytes, such as Pteridium, Pteris the stele is a composite structure consisting of vascular cylinder of the stem and leaf traces. Steles in Pteridophytes Mainly two types of steles found in pteridophytes : (1)
Protostele
(2)
Siphnostele
I.Protostele: Such stele consists of central solid core of xylem, surrounded by a band of phloem. Pith is totally absent. There is a single or multiple layer of pericycle outside the phloem which is surrounded externally by a continuous sheath of endodermis. It is the most simple and primitive type of stele, both phylogenetically as well as ontogenetically. Primitive taxa of pteridophtes like Psilotum,Timesipteris as well as fossils Psilophytes Rhynia, Horneophyton show this kind of stele. The protostele gives off leaf traces without any break in the continuity of the enveloping endodermis. With the departures of leaf trace there is no marked change in the solid xylem core. Each leaf trace is surrounded by its own epidermis. On the basis of the position and shape of xylem the protostele may be distinguished into following types. 1. Haplostele. It consists of a solid xylem core with smooth circular outline, which is surrounded by a ring of phloem. It is found in the fossil (Rhynia, Horneophyton, Cooksonia) as well many living pteridophytes (Psilotum, Seleginella, Lycopodium).
Cortex
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Evolutionary Concepts in Pteridophytes
Endodermis Pericycle Xylem Phloem
Fig.10 Haplostele 2.
Actinostele. The xylem forms star shaped structure with many radiating arms. The phloem is present in small patches in between the radiating arms of the xylem. Such type of protostele is found in many living (Psilotum, Lycopodium serratum) and fossil forms (Asteroxylon, Sphenophyllum).
Cortex Endodermis Pericycle Xylem Phloem
Fig.11 Actinostele 3. Plectostele. The central xylem core breaks into more or less parallel plates and each xylem plate is surrounded by phloem. Aerial shoots and cone axis of Lycopodium clavtum and L. volubile have this type of stele.
Cortex
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Evolutionary Concepts in Pteridophytes
Endodermis Pericycle Xylem Phloem
Fig.12 Plectostele 4.
Mixed protostele. Sometimes the solid xylem core of the protostele is broken into small groups of tracheids which remain embedded in the phloem. Such type of protostele is found in the stem of Lycopodium ceranum.
Cortex Endodermis Pericycle Xylem Phloem
Fig.13 Mixed protostele II. Siphonostele (medullated stele) In this stele a pith is present in central region which is surrounded by the xylem and then phloem. This type of stele is an advancement over protostele. Such stele is found in the stem of most of the ferns (Filicophyta). Origin of siphnostele: Siphonostele has originated by the development of pith in the centre of protostele. Two views have been given regarding the origin of pith in stele. (a)Inter-stelar origin of pith. Boodle (1901), Gwynne-vaughani (1908), Bower (1911) and Fahn (1960) supported the view that the is pith originated as a result of transformation
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Evolutionary Concepts in Pteridophytes of tracheary elements. Thus the pith is completely surrounded by xylem. Botrychium ternatum, B.virginianum, Osmunda regalis etc show such stele where tracheids are scattered throughout the pith. (b)Extra-stelar origin of pith. Jeffery (1897, 1902, 1917) stated that protostele has transformed into a siphonostele by the movement of cortical cells into the stellar zone. In this event some gaps such as leaf and branch gaps probably provided way for the transfer of parenchyma into stele. Types of siphonostele: Depending on the presence of vascular tissues, the siphonostele has been recognized into following two types (i)Ectophloic siphnostele. In such stele xylem encloses the and phloem occurs only outside the xylem. It is present in Equisetum and some fern like Osmunda and Schizaea.
Cortex Endodermis Pericycle Phloem Pith Xylem
Fig.14 Ectophloic siphonostele (ii) Amphiphloic siphonostele. The phloem occurrs both on the inner side and outer side the xylem and a central pith is present. This type of stele is found in Adiantum and Marsilea.
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Evolutionary Concepts in Pteridophytes
Cortex Outer endodermis Outer pericycle Pith Xylem Outer phloem Inner phloem Inner endodermis endendodermis Inner pericycle Fig.15 Amphiphloic siphonostele Modification of siphonostele. Depending on the presence of non-overlapping and overlapping leaf gaps, siphnostele are distinguished into the following types. (a)
Solenostele
(b)
Dictyostele
(c)
Polycyclic stele
(i)Solenostele. A siphonostele with non-overlapping leaf gaps is known as solenostele. It may be ectophioic or amphiphloic solenostele.
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Evolutionary Concepts in Pteridophytes
Cortex Endodermis Phloem Leaf trace Leaf gap Xylem
Fig.16 Ectophloic solenostele
Cortex Outer endodermis Outer pericycle Xylem Leaf trace Leaf gap Inner phloem Outer phloem
Inner pericycle Outer pericycle Fig.17 Amphiphloic solenostele.
(ii)Dictyostele. A siphonostele with overlapping leaf gaps with each other is known as dictyostele. Such stele has many scattered vascular strands and each strand is called a meristele. It is represented by some fern like Dryopteris, Pteris, Ophioglossum, etc.
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Evolutionary Concepts in Pteridophytes
Leaf trace Meristele
Leaf gap
Fig.18 Dictyostele (iii)Polycyclic stele. Sometime two or more concentric rings of vascular tissue are present in the stem. This type of stele is called polycyclic stele. It is found in some higher ferns such as two concentric rings of vascular strands are found in Pteridium aquilinum, three in Montania pectinia and four in Pteris podophylla and Platycerium aethiopicum. It may be polycyclic solenostele or polycyclic dictyostele.
Meristeles of outer ring
Meristeles of inner ring Scleranchyma of ground tissue
Fig.19 Polycyclic stele.
(a)
Polycyclic solenostele. When the outer cylinder is solenostelic, it is called polycyclic solenostele.
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Evolutionary Concepts in Pteridophytes
Solenostele Siphonostele
Leaf trace Leaf gap Fig.20 Polycyclic solenostele (b)
Pith
Polycyclic dictyostele. In a polycyclic stele the inner cylinder is dictyostelic.
Leaf traces Leaf gap Meristele Fig.21Polycyclic dictyostele. HETEROSPORY In majority of pteridophytes, a plant produces only one kind of spores (similar in shape and size), which is called homosporous. However, in some pteridophytes a species produces two distinct types of spores, which differ in size. The larger spores are called megaspores, produced in megasporangia comparatively less in number. Smaller spores are called microspores which are formed in microsporangia in comparatively large number. They are called heterosporus. The difference in size of the spores is related to sex of the gametophyte. The megaspore on germination gives rise to female gmetophyte which produce only archegonia. The microspore on germination gives rise to male gametophytes which bear only antheridia. The phenomenon where two kinds of spores differing in size, structure and function are formed on the same plants is known as heterospory.
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Evolutionary Concepts in Pteridophytes
Smaller spores
Larger spores Fig.22 Development of two types of spores on same plant. Heterospory is common in nine genera of pteridophytes: Selaginella, Isoets,Stylites,Marsilea,Pilularia,Regnellidium,Salvinia,Azolla and Platyzoma. Some of these genera also approach to seed habit. Significance of Heterospory (i)
Heterospory expresses sex determining capability of the plant.
(ii)
In homosporous species, differentiation of sex take place at the gametophytic stages, whereas in heterosporous species. differences in size of the spores is related to the sex of the gametophyte;a microspore always gives rise to male gametophyte and megaspore gives rise to female gametophytes. Thus in heterosporous forms the sex of the gametophyte can be predicted at the spore stages.
(iii)
Development of gametophyte in heterosporous forms is endosporic and the nutrition for the developing gametophyte is derived from sporophytes. Hence the development of gametophytes is not affected by ecological factors as in case of independently growing gametophytes.
(iv)
The megaspore is retained by the parent even after fertilization. This ensures nutrition for the developing embryo. This is an important step towards the seed habit.
Advantage of Heterospory (i)
In homosporous pteridophyte the spore germinate on soil to produce gametophytes that are independent of sporophytic plant and the gametophytes of heterosporous pteridophytes with much vegetative tissue,
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Evolutionary Concepts in Pteridophytes has to manufacture not only its own food but also for the developing embryo which is dependent upon it. (ii)
In a heterosporous taxa spores germinate within sporangium and gives rise to prothallus. The gametophytes which is greatly reduced, is retained wholly or largely within the spores wall depending entirely upon the food deposited in spores.
(iii)
As the gametophytes of heterosporous taxa obtain their nourishment from the parent sporophytes, they are independent of external conditions in comparison to those of the free living gametophytes of homosporous pteridophytes.
Seed Habit The requirement for the formation of seed are as follows. (i)
Formation of two types of spores –microspores and megaspores (heterospory).
(ii)
Reduction in the number of functional megaspores to one per megasporangium.
(iii)
Retention of megaspores in the megasporangium until embryo development.
(iv)
Elaboration of the apical part of megasporangium to receive microspores or pollen grains.
(v)
Availability of sufficient nutrition for the development of embryo.
Most of the species of Selaginella are heterosporous and they have only one functional megaspore mother cell which gives rise to four megaspores after meiosis. Only a single functional megaspore in a sporangium is present in Selaginlla rupestris, S. monospora,and S, erythopus like higher plants. The development of female gametophytes, fertilizations and embryo development take place within the megasporangium. Thus, evolution of seed habit took place in such species of Selaginella. But they are not called true seeds because (i)
The megasporangium is not covered with integuments, and
(ii)
There is no resting stage after embryo development; the development of embryo is accompanied with the development of shhot and rhizospore.
Apospory and Apogamy
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Evolutionary Concepts in Pteridophytes Life cycle of ferns is complete with two alternating phases, one is the sporophytic generation, which is represented by the complete plant with root, shoot, leaves and sporangia and the other is gametophytic generation which is symbolized by the prothallus with antheridia and archegonia.
. The sporophytic generation is large, leafy and diploid
nad prolonged, whereas the gametophytic generation is relatively small and haploid and short. The gametophyte gives rise to the sporophytic forms by the fusion of gametes and the sporophyte produces haploid spores which on germination give rise to gametophyte. However, sometimes certain stages may become interpolated in the normal cycle or some essential phases are entirely omitted. Two important processes which involve the elimination of essential events in the life-cycle are apospory and apogamy .
Apospory Development of gametophyte directly from the vegetative cells of the sporophyte without the involving spores. Thus apospory does not involve the event of spore production (meiosis) and spore germination in the life cycle. Therefore, aposporous gametophytes have the same number of chromosomes as sporophyte,i.e., they are diploid. The event of apospory leads to the development of polypioid gametophytes and sporophytes. Triploid, tetraploid and even higher polyploids have been produced by inducing apospory in Osmunda regalis, Aspidium marginale and Woodwardia virgnica. An aporosporous prothallus develops from the cells of an abortive sporangium or directly from the cells of the leaf or stem.
Sporophyte Gametophyte Fig.23 Event of Apospory
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Evolutionary Concepts in Pteridophytes Druery(1884) reported apospory for the was first time as natural phenomenon in Athyrium filix-femina var.clarissima. In this taxon prothalli develop on pinnules in place of sporangia. This phenomenon is also called as soral apospory. Many families of ferns show apospory as a pathway of natural polyploidization. It is also found in cultivated plants of Trichomanes kaulfusi and Polystichum angulare var.pulcherrium where leaves bore no sori and instead gametophytes developed on the margins. In Polystrichum angulare prothalli develope at the tip of the pinnules (apical apospory). Aposporous gametophytes have also been observed on the young leaves of Osmunda regalis and O. javanica. Apospory can also be induced in many ferns. Experimental studies on apospory have reported by Bristwo (1962), Steil (1994), Munore and Bell (1970) Tactaria trifoliata), Pteris cretica, Pteridium aquilinum). Different parts of sporophytes when grown in a medium with little or no sugar, develop into aposporous gametophytes. Factors Regulating Apospory Aposporous development of gametophytes is influenced by a numbers of factors. (i)
Nutrition
(ii)
Light intensity
(iii)
Age of sporophytic cells
[I]Nutrition Sucrose play an important role in triggering different gene blocks into action that ultimately determine the nature of the regenerating tissue. Complete absence of carbohydrates in growth medium (starvation medium) induces development of aporosporous gametophytes. Growth medium with 1% carbohydrate induces callus formation, which is reported to be partly gametophytes and partly sporophytes (i.e., gametosporophytes). The culture medium with 2%carbohydrates induce only sporophytes on the callus. [II]Light intensity Low light intensity promote development of aposporous gametophytes. Studies have shown that under high light intensity leaves of Drynaria rididula, Polypodium aurenum, Platycerium bifurcatum and P. grande develop sporophytic buds, but they develop into prothalli under low light intensity. [III]Age of cells
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Evolutionary Concepts in Pteridophytes The nature of tissue regenerated by a sporophytic cell is greatly influenced by its age. Younge cells shoot apex or leaf tip, forms gametophytes.Mature cells of the basal part of the shoot or leaf regenerate into sporophytic tissue. Apogamy Development of sporophyte from the gametophytic tissue without involving fusion of male and female gametes is known as apogamy. Such sporophytes grow by budding from the gametophytes, hence they have the same number of chromosomes as gametophytes. Apogamy was first observed by Farlow (1874) in Pteris cretica. In ferns apogamy is more widespread than apospory. Apogamy is reported in more than 50 species of ferns in which apogamous sporophytes are known to occur naturally due to the absence of or non functional nature of one or both sex organs. Such apogamy is known as obligate apogamy which reported in Dryopteris, Adiantum, Diplazium, Pteris, Polystichum, Osmunda and Asplenium. The prothalli of these ferns have functional antheridia but archegonia are either non –functional or absent.
Sporophyte Gametophyte Fig.24 Development of Apogamy Generally dry conditions promote apogamy. Ferns of dry habitats usually do not get sufficient water for fertilizations hence fail to reproduce sexually. In such forms obligate apogamy is an adaptation to dry conditions. Gametophytes having functional sex organ can be induced to bear apogamous sporophytes .This type of apogamy is known as induced facultative apogamy.
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Evolutionary Concepts in Pteridophytes Factors regulating Apogamy Development of apogamous sporophytes in ferns is determined by a some factors such as: (i)
Nutrition
(ii)
Light quality
(iii)
Hormones
(iv)
Genetics
[I]Nutrition When the gametophytes are placed in 4% sucrose solution at the initiative phase, they produce apogamous sporophytes. Further development can, however, proceed normally even if we modify the concentrations of sucrose and all mineral nutrients. [II] Light quality Gametophytes generally show good growth in white light. Far-red light (705 nm) has been reported to be most effective in inducing apogamous sporophytes, followed by blue (445nm) and white light. [III] Hormones Some hormones like 2,4-D (2,4,dichlorophen aceti acid), gibberellic acid(GA3), indole 3acetic acid (IAA) and tryptophan stimulates gametophytes to produce apogamous sporophytes even at very low concentration (0.5%)of carbohydrates.Besides,formation of apogamous plants is increased in prothalli exposed to ethylene. [IV]Chromosomes number Apogamy is more common in polyploidy gametophytes than in normal haploid prothalli.Thus, there seems to be a correlation between ploidy of the gametophyte and apogamy Summary: Telome refers to a simple ultimate terminal portion of dichotomously branched axis. These axes are single nerved. Telomes show dichotomous branching at the apical portion. Fused telome structure is called mesome. The telome, that does not bear a sporangium at its tip, is called a sterile telome, phylloid or vegetative telome. The telome with sporangium at its tips is called a fertile telome. According to Zimmermann, the primitive vascular lands plants have originated from unicellular green algae and proposed
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Evolutionary Concepts in Pteridophytes that green algae are immediate ancestors of the primitive vascular cryptogams (Rhynia, Horneophyton, Psilophyton, etc) which evolved during the upper Silurian and Lower and Middle Devonian periods. He stated that shoot axes and leaves of higher vascular plants have evolved from earliest land plants as a result of few organogenetic processes of progressive differentiation. These process are called “elementary processes” such as Overtopping, Planation, Syngenesis,Reduction, Curvation which were responsible for the development of higher vascular plants from the early vascular cryptograms. Regarding the origin of sporophyll, Zimmermann explained that a sporophyll is composed of one or more telomes of both, telomes and mesomes. He also proposed that the microphyllus leaf originated as a result of the process of reduction whereas megaphyllous as a result of syngenesis. Roots were differentiated from the creeping underground rhizomes of early plants and perhaps before the differentiation of leaves. Root and stem have the similar basic structure mainly composed cortex and central cylinder with an endodermis as an anatomical boundary between these two units. In lower vascular cryptogams like Selaginella and Lycopodium, the stele of the axis and the leaf traces developed independently, they are mainly cauline and leaf traces are attached only to its surface. On the other hand, the stele of higher ferns like Pteridium, Pteris is a composite structure consisting of vascular cylinder of the stem and leaf traces. There are mainly two types of steles in pteridophytes, Protostele (without pith) and Siphnostele (with pith). Protostele may be modified into Haplostele, Actinostele, Plectostele and Mixed protostele. On the basis of location of phloem, siphonostele is further devided into two types, Ectophloic siphnostele, Amphipholic siphnostele. Most of pteridophytes, a plant produces only one kind of spores that are called homosporous. However in pteridophytes a plant produces two kinds of spores, differing in size. The larger spores are called megaspores which are borne in megasporangium and the smaller spores are called microspores Tat are produced in microsporangium. The megaspore on germination gives rise to female gmetophyte which produces only anrchegonia. Microspore on germination gives rise to male gametophytes that produce only antheridia. The phenomenon where two kinds of spores differing in size, structure and function are formed on the same plants is known as heterospory. Pteridophytes comprise two alternating phases in their life cycle, one is the sporophytic and other is gametophytic generations. Former one is diploid has shows stem roots and leaves and prolonged life, whereas the latter one is relatively small, thalloid and haploid and short lived. Gametophyte gives rise to the sporophytic forms by the fusion of gametes and the sporophyte produce haploid spores which germinate to give rise to gametophyte. Two important processes which involve the elimination of essential events in
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Evolutionary Concepts in Pteridophytes the life-cycle are apospory and apogamy. These phenonena are frequently observed in pteridophytes. Exercise Important Questions 1. What is telome? Explain the steps involved in the origin of megaphyllous leaves from telome truss. 2. Describes the merits and demeritis of the telome theory 3. Write notes on: (i)
Overtopping,
(ii)
Webbing,
(iii)
Origin of sporophylls of Lycopsides.
(iv)
Origin of sporophylls in Sphenopsida.
4. What is apospory? Explain it giving suitable examples. 5. What is apogamy? Give its special feature and describe various factors that affect this phenomenon in pteridophytes. 6. What is heterospory? Describe briefly the significance of heterospory. 7. What are the characteristics of seed habit? Comment on the seed habit in pteridophytes with reference to Seleginella. 8. What is stele? Give an illustrative description of the various types of steles found in pteridophytes. 9. Write notes on: (i)
Protostele
(ii)
Siphonostele
(iii)
Dictyostele
(iv)
Origin of siphonostele.
10. Fill in the blanks (i)
A siphonostele with leaf and branch gaps is called ………..
(ii)
A stele with a central solids mass of xylem surrounded by phloem and endodermis is called………………
(iii)
Amphiphloic siphonostele is found in ……………..
(iv)
According to Van Tiegham and Douliot, the ……..is the unit of vascular organization.
Institute of Lifelong Learning, University of Delhi
Evolutionary Concepts in Pteridophytes (v)
A protostele in which the central xylem core breaks into more or less parallel plates is known as …………..
(vi)
Development of gametophyte directly from the vegetative cells of sporophyte without the formation of spores is known as ……………
(vii)
Development of sporophyte from the gametophytic tissue without involving fusion of gametes is known as.............
(viii)
A siphonostele that has phloem only on the outside of the xylem is known as…………..
(ix)
The phenomenon where two kinds of spores differing in size, structure and function are formed on the same plants is known as…………..
(x)
This process accounts for the shifting of sporangia to the ventral surface of the leaf in fern is known as………………
Glossary
Apogamy: The production of sporophyte from a gametophyte directly, without the fusion between the male and female gametophytes. Apospory: The production of a gametophyte from a sporophytic directly, without the formation of spores. Dictyostele: A solenostele that is broken up by crowed leaf –gaps into a network of distinct meristeles, each surrounded by an endodermis. Ectophloic: A siphonostele that has phloem only on the outside of the xylem. Fertile telome: The telome that bears a sporangium at its tip. Heterospory: The condition when a plant produces two kinds of spores differing in size, i.e., the smaller microspores and the larger are megaspores. Leaf gap: A break in the continuity of a siphonostele associated with the departure of a leaf trace. Leaf trace: The vascular strands extending from the stem to the leaf. Mesome: Two telomes are united below the point of dichotomy to form a fused structure. Phylloid: A syntelome is called phylloid truss
Institute of Lifelong Learning, University of Delhi
Evolutionary Concepts in Pteridophytes Polystelic: Stem having more than one stele. Sterile telome: It does not bear a sporangium at its tip. It is also called phylloids or vegetative telome. Syntelome: hen telomes are grouped together to form a more complex structure. It is also called telome truss. References 1.S.N.Pandey;S.P. Misra &P.S.Trivedi.1972;A text book of botany;Vikas publishing house;Delhi(India).
Institute of Lifelong Learning, University of Delhi
