Soral evolution, Telome concept
Soral evolution:-
Sorus:- It is brownish or yellowish cluster of spore-producing structures (sporangia) usually located on the lower surface of fern leaves. A sorus may be protected during development by a scale or flap of tissue called an indusium.
Types of sorus:-
i. Simple sorus:-
> The Simplices are primitive and extend to the Paleozoic.
> All sporangia within a sorus may be at the same stage of development (Simplices).
> The sporangia in such a sorus development simultaneously and all of them mature together.
> Eg.- Ophioglossum and Osmunda.
ii. Gradate sorus:-
> In such cases the internal space within the radial sorus gets filled by overlapping sporangia.
> Overcrowding results and there is no space for the proper dehiscence of sporangia and consequently some of them do not dehisce.
> The Gradate with an oblique annulus, occupy a middle position and have been discovered in the Mesozoic.
> Eg.- Dicksonia, Loxosoma, Trichomanes, Cyathea, Alsophila etc.
iii. Mixed sorus:-
> The spores are released in a staggered fashion and even if the environmental conditions are not favorable when the spores are initially released; this problem is solved by a constant crop of spores over a given length of time.
> Mixtae annulus is present but in a vertical fashion.
> Eg.- Polypodiaceae (Adiantum, Pteris, Pteridium, Davallia) Polypodiaceae
Indusial protection:- The indusium may be formed as an epidermal outgrowth covering receptacle and sporangia (true indusium) or it may merely be the in turning of the margin of the leaf to protect the sporangia (pseudo indusium or false indusium).
Position of sorus in ferns:- The Ophioglossales are group of interesting ferns in which the sporangia are marginal in position.
> In Maeattia, Angiopteris, Danaea etc. the sori are elongagated and are situated below the lateral veins.
> In christensia the sori are naked, circular and are irregular arranged between the lateral veins.
> In Adiantum the sporangia develop on the underside of special marginal flaps of lamina that become reflexed and protect the sorus
> The sori in Mattoniaceae are ventral in position and are usually arranged in two rows on either side of the midrib.
> In Glecheniaceae, the sporangia from distinct sori on the ventral surface of the leaves.
> The sporangia in Osmunda, Davalia, Trivhomanes and many other ferns are also marginal in position.
> In Marsileales and the Salviniales the sporangia develop in sori that are borne within distinct structure called the Sporocarp.
- In Salviniales the smaller sporocarp contain many microsporangia each and the larger ones contain one or more megasporangia per sporocarp.
- The sporocarp in Marsilea enclose sori that contain both micro and megasporongia.
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Telome concept:-
Telomes:-
> The name telome has been given to the simple ultimate terminal portions of dichotomously branched axis.
> These axes are undifferentiated and single nerved.
> Two telomes of a dichotomizing axis are united below the point of dichotomy to form a fused structure, called mesome.
> There are two types of telomes on the basis of their function:
i. Vegetative or sterile telomes:- These telomes are without sporangia and they are also called phylloids.
ii. Fertile telomes:- Those telomes which bore terminal sporangia are called fertile telomes.
> During the time of evolutionary development, telomes become grouped together and form a more complex structure, called syntelome or telome truss.
> When a syntelome consist of only sterile telomes is called phylloid truss and a fertile truss if it consisted of only fertile telomes and a mixed telome truss or mixed syntelome when it consisted of both sterile and fertile telomes.
Telome and the origin of higher land plants:-
> According to Zimmermann, the primitive vascular land plants have originated from unicellular green algae.
- Repeated divisions in unicellular forms resulted in the development of large undifferentiated forms with parenchymatous thalli.
- Further development and differentiation led to the establishment of meristematic tissue which gave rise to erect branches.
- It was followed by the appearance of heteromorphic alternation of generation.
- The branches of sporophyte were dichotomously branched and had a central mechanical strand.
> Zimmermann visualized such green algae as the immediate ancestors of the primitive vascular cryptogams (e.g. Rhynia, Horneophyton, Psilophyton, etc.) which evolved during the upper silurian lower and middle devonian.
> The sporophyte of these primitive vascular plants was relatively undifferentiated: the aerial telomes developed stomata and the basal telomes rhizoids. Internally the axis was undifferentiated with an axial typically protostelic vascular strand.
> Zimmermann suggested that the following five elementary process were responsible for the development of higher vascular plants from the early vascular cryptogams.
i. Overtopping
ii. Planation
iii. Syngenesis
iv. Reduction
v. Curvation
i. Overtopping:-
- In this process one of the two dichotomizing brnches of an axis become laarger, stronger and grew vertically upward as the main axis. The shorter dichotomy was displaced laterally and it served as precursor of megaphylls.
- Thus in this process the weaker branch was overtopped by the stronger branch. It resulted in the development of monopodial branches from equal dichotomies.
- Evolutionary studies have also shown that dichotomous branching is frequent in primitive pteridophytes and monopodial in advanced forms.
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ii. Planation:-
- The equal dichotomies of a system, which were in more than one plane (cruciate dichotomy) come to lie in a single plane (fan-shaped dichotomy).
- This process helps in the interpretation of the development of organs of bilateral symmetry from those of radial symmetry.
- Thus, planation must have led to the evolution of leaf.
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iii. Syngenesis:-
- In this process, telomes and mesomes came to lie within a common parenchymatous tissue.
- This process is also known as fusion or webbing. Syngenesis also involves the fusion of vascular strands of telomes.
- It is an important process which explains the formation of leaves with open dichotomous,pinnatified and reticulate venation polystelic condition found in Selaginella and some fossil members of the Devonian and Lower Carboniferous period (e.g. Cladoxylon, Steloxylon).
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iv. Reduction:-
- It involved transformation of a syntelome into a single needle like leaf.
- This process thus accounts for the evolution of simple microphyllous leaves of the lycopods.
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v. Curvation:-
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