In this article we will discuss about the comparative morphology of pteridophyta:- 1. Stele 2. Root 3. Leaf 4. Sporocarp.
Stele:
The vascular elements of the primary axes of practically all pteridophytes consist of a solid core of xylem surrounded by phloem. Archiac Psilophytales such as Rhynia and Horneophyton and present-day Psilotales, Psilotum has retained this organization even in adult forms in subterranean stems.
This simplest possible organization, a prototype, is termed as protostele. Therefore, phylogenetically as well as ontogenetically protostele is considered to be most primitive type of stele. A protostele of circular outline is termed as haplostele (Fig. 11.1 A) and of star-shaped configuration, actinostele (Fig. 11. 1B).
The latter is characteristic of Asteroxylon, aerial axes of Psilotum and certain species of Lycopodium. However, in certain other species of Lycopodium central xylem breaks up into plates and each is surrounded by phloem. This specialized form is termed as plectostele (Fig. 11.1C).
To keep pace with overall increasing diameter of stem axis, the stelar tissue in the central region undergoes parenchymatous development. This is best exemplified by Psilotum, the basal region of aerial axis has, a central medulla or pith and the ultimate branches are without it. Medullated protostele is termed as siphonostele (Fig. 11.1D) and represents an advancement.
The origin of pith, however, has been controversial. The pith has been considered to result from degradation of tracheary elements and, therefore, stelar in origin. Forms with mixed pith serve as support examples of this view point. In contrast, is the view, suggesting extrastelar origin of pith i.e., resulting from migration of cortical cells into the stelar axis.
Support for this view point is derived from forms having solenostele (Fig. 11. 1E), that has two endodermal layers; one delimits the stele from cortex and other delimits it from the pith.
The stele also has two zones of phloem that border the xylem internally as well as externally and is, therefore, termed as amphiphloic siphonostele or solenostele. During the departure of leaf traces in megaphyllous forms the solenostele undergoes parenchymatous development and forms leaf gaps. The axial view of solenostele with nonoverlapping leaf gap is shown in Fig. 11.11.
Due to overlapping of leaf gaps (Fig. 11.1 J) a solenostele gets dissected and results in a dictyostele. A dictyostele in transection appears as a scattered series of bundles; each bundle is a meristele (Fig. 11. 1F) consisting of a central strip of xylem surrounded by phloem followed by a pericyclic and endodermal layer.
In the axial form these strands are inter-connected and form a tubular network (Fig. 11.1K). A dictyostele is, therefore, an amphiphloic tube with overlapping leaf gaps. Under natural conditions, the origin of dictyostele can be envisaged when incipient vascular tissue on being subjected to tensile stress undergoes parenchymatous development. Dictyostele is the most advanced type of stelar organization in higher pteridophytes (ferns). It can also be polycyclic (Fig. 11.1G, H).
For a more detailed discussion of stelar organization a reference is made to a review wherein are recognized:
(a) Protostele (divided into: ectophloic, endophloic and amphiploic)
(b) Siphonostele (divided into: ectophloic and amphiploic; it is further divided into solenostele and dictyostele)
(c) Eustele (divided into eustele sensu stricto and subtypes pseudosiphonostele, reduced eustele and polycylic eustele).
Root:
Embryonic root in Pteridophyta is short-lived and the roots on adult plants are adventitious structures. The lateral roots originate from endodermis. Similar to shoot apex, growth centres can be visualized to exist, but nothing is known about these centres. Moreover, due to geotropic nature of root physical factors should be taken into consideration.
Leaf:
On the basis of their leaf structure, Pteridophyta can be classified as microphyllous and megaphyllous. The microphylls, as the name suggests, are small leaves but may attain considerable dimensions as in Isoetes. However, their distinctive character is simple form, and an unbranched vein which is not accompanied with the formation of leaf gap in the stem stele.
Exception to this generalization with two veins in leaf blade is species of Selaginella with a branched vein. The microphyllous forms have been classified into ligulatae and eligulatae based on the presence or absence of ligule.
In contrast to univeined simple microphylls, the megaphylls are large and usually pinnatified with a complex series of veins, and the origin of leaf trace is accompanied with the formation of prominent leaf gap in the vascular cylinder.
Phylogenetic Origin:
About the phylogenetic origin of a leaf there are two contrasting viewpoints. A widely accepted viewpoint is that microphyll represents an upgrade progression from leaflessness. Contrary to this is the theory of downgrade reduction.
The microphylls are described to have originated as superficial lateral outgrowths or enations on aerial axes of Psilophytales. The progressive evolution of a microphyll can be traced in Psilophyton,. Initially enation was merely a scale-like appendage without any leaf trace as in Psilophyton, then the appendage acquired a rudimentary leaf trace which terminated at the base, as seen in Asteroxylon, and finally evolved the univeined microphyll, of living forms such as Psilotum.
Quite early the megaphyll was suggested to be of stem nature and this concept formed the basis of Telome Theory. Accordingly, an early vascular plant such as Rhynia consisted of dichotomous axes the ultimate branches of which terminated either into fertile telomes (sporangia) or remained as sterile telomes (phylloids). A telelome (Fig. 11.2A) in a broad sense is one of the distal branches of a dichotomized axis.
A megaphyll can be conceived to have evolved by a process involving overtopping, planation, and webbing (Fig. 11.2D). To begin with, from an equal dichotomously branching axis, characteristic of archiac forms, originated the sympodial form i.e. unequal dichotomy.
This was accompanied by overtopping of weaker branch by stronger branch, the former on being laterally displaced served as precursor of megaphyll and the latter continued growth as the main axis. In the next step termed as planation, the branching in this precursor became restricted to a single plane and finally through fusion or webbing arose the lamina of open dichotomous venetion.
At the time of formulation of Telome concept Rhynia was selected as a prototype and new discoveries of pteridophytes of the past, such as Cooksonia and Renalia are additional supports to this theory. Cooksonia from mid-silurian is a dichotomous axis with terminal sporangia.
Renalia, another rhiniophyte, supports as to how overtopping and reduction served to produce lateral branches with terminal sporangia. From a megaphyll, a result of planation and webbing, a group of telomes became progressively reduced resulting in formation of single veined ‘needle leaf’ or microphyll.
In latest exposition of telome theory one can find a new dimension of this concept. According to the author evolution is a result of progressive modification/s spread over a span of millions of years and new types of organs (fertile and sterile) are the result of changes in genotype that in turn are expressed ontogenetically. This concept of phylogeny is described as ‘Hologenetic’.
Leaf Segment Theory:
The vascular supply to the peduncle, and the vascular supply within the sporocarp indicate it to be a modification of a leaf segment rather than an entire leaf.
The sporocarp has been interpreted to result from an apposition of two pinnae, enfolding of several pairs of pinnae or two rows of pinnules. A parallel has been drawn between Marsileaceae and Schizaeaceae; especially Schizaea rupestris in which sterile leaf is flattened and simple but the sporophyll has six to ten serratted pinnules (Fig. 11.3A) on either side.
These pinnules are reflexed to the abaxial side (Fig. 11.3B) so that the lower surface to which marginal sporangia are attached face inwards (Fig. 11.4C). If two similar series of pinnules were flexed adaxially instead of abaxially, with the same alternation and marginal position of sporangia, by fusing them a structure similar to Marsilea sporocarp could be derived.
The sporocarp of Marsilea is comparable to entire leaf tip with four leaflets (Fig. 11.3D). The bumps that occur at the proximal end of the sporocarp and are occasionally vascularized represent the vestigeal pair of pinnae. The body of the sporocarp is made up of two distal leaflets and two proximal ones greatly reduced. The coming together of leaflets has been proposed by a recurvature of leaflet margins towards the rachis.
On this basis, the sporocarp can be derived as shown in Fig. 11.3E-G. Figure E is a cross section showing margins curled downward and inward, their sori partly enclosed by indusia facing inward. Fig. 11.3F shows the development of wing-like shoulders which are curled downwards and inwards until they meet enclosing the sori.
Sporocarp of Marsileaceae is considered to be specialized fertile segment of a leaf, as in schizaeaceae.
An equation of the sporocarp to a single leaflet has been made (Fig. 11.3H) with as many pinnules as the number of lateral bundles in the sporocarp. The same view has been expressed in different words; the sporocarp is a leaflet with as many lobes as the number of lateral bundles.
A derivation of the sporocarp of Marsilea is possible from a fern with gradate sorus surrounded by involucroid indusium, rather than from schizaeaceous ancestor. The sporocarp is conceived to have evolved from a cup-like indusium through an unequal growth on opposite sides forming hood-shaped indusium. The next step involved an apposition of the two lateral margins of the abaxial side of the pinna Fig. 11.3I.
The occurrence of 10-15 sporocarps on one side of petiole in M. polycarpa is considered to be a strong evidence for the equivalence of the sporocarp to the pinna of once compound frond.
Whole Leaf Theory:
The origin of sporocarp in M. quadrifolia by a single initial sharply contrasts with leaf pinna which arises from several marginal cells. The sporocarp is more than a pinna or a group of fused pinnae is evidenced by the development of secondary and even tertiary sporocarps on the peduncle of primary sporocarp.
The ontogeny of sporocarp in M. polycarpa, also, contradicts the pinna interpretation.
Sporocarp:
In the sporocarp of Marsileaceae, the sori are marginal structures. The presence of protecting body has greatly distorted the normal structure. The morphological nature of this specialized structure -sporocarp- has been controversial. It is held to be a modified fertile segment of a leaf or equivalent to an entire leaf. These two viewpoints have come to be known as leaf segment theory and whole leaf theory. A new interpretation has also been given to the morphology of sporocarp.
New Theory of Sporocarp Morphology:
Marsileaceae has synthesized a new organ the sporocarp, which changes its morphological nature in midontogeny. The sporocarp in Marsilea arises from an initial with three cutting faces, a condition unknown for any fern pinna.
In its early stages of development, it is comparable to the sporangial stalk initial of a fern. Therefore, the morphological nature of sporocarp can be interpreted in terms of differential expression of genome i.e. sporangial ontogeny in early stage and pinna ontogeny in later stage.
None of these theories explains the evolution of heterospory in marsileales and there is possibly a correlation between evolution of heterospory and sporocarp, as all the homosporous ferns lack a sporocarp.