Stele in Pteridophytes

Stele in Pteridophytes

What is Stele?

Stele (greek stele=column) represents the core of the axis which includes the vascular system, interfascicular portion, the pith (if present) and some surrounding portion of the fundamental tissue in the vicinity of the vascular bundles (pericycle). This concept of stele was proposed by Van Tiegham and Duoliot (1886) supported by the plant morphologists and plant anatomists. The term ‘stele’it used only with reference to primary vascular tissue.

Sachs (1875) first developed the idea that the vascular system of the plant body is a continuous system. This idea was favoured with a strong emphasis by Van Tiegham and Duoliot (1886) and they proposed and developed the the ‘Stelar Theory’. This theory explained that the root and the stem are basically similar in gross internal organization of tissue (anatomy) because in both the cortex encloses the central part of the axis. They called this central part or core the axis as ‘stele’.

Types of Steles

Two principal types of steles are recognized among the vascular plants. These are; (A) Protostele and (B) Siphonostele.

(A) Protostele

According to Jeffrey (1903) and the most of present day Pteridologists, the Protostele is the most primitive and simplest type of stele. It consists of solid mass of xylem completely surrounded by phloem, pericycle and endodermis in regular order. Protostele is permanently retained in the adult stems of many living pteridophytes e.g. Lygodium, Lycopodium, Hymenophyllum, Selaginella etc.

The Protostele is fundamental type of stele for the vascular plant and all others steles have been derived from it during the course of evolution.

The Protostele may exist in any of the following forms:

  1. Haplostele-

    A Protostele with central solid and smooth core of xylem surrounded by phloem is known as a haplostele. It occurred in primitive Psilophytales like Rhynia and Horneophyton and is found in a number of living generas, e.g. , Lycopodium cernum, Selaginella kraussiana and Lygodium.

  2. Actinostele-

    Actinostele is that in which the central xylem cone has radiating ribs and assumes a star shaped appearance e.g. Psilotum triquetrum and Lycopodium serratum.

  3. Plectostele-

    Plectostele is that in which the xylem is divided into a number of separate plates, which lie parallel to radiate to one another e.g. Lycopodium volubile, L. clavatum.

  4. Mixed Protostele-

    Mixed Protostele is that in which the xylem and phloem are uniformly distributed. Xylem elements remain embedded in phloem tissue. e.g. Lycopodium cernnum. The stele is without pith.

(B) Siphonostele (Medullated Protostele)

Siphonostele is that stele in which the pith is present in the centre of hollow vascular cylinder. So Siphonostele is actually a modified Protostele with pith and therefore botanists believe that siphonostele has arising from protostele either by the intra stellar origin of pith or by the invasion of cortical tissues (extrastelar origin of pith). The first view, which has been proposed by Bordle has been accepted by most of the workers. It is found in the stems of most members of the Fillicophyta.

Siphonosteles may be of following three types:

  1. Ectophloic Siphonosteles-

    In the ectophloic siphonostele, the xylem is in the form of a hollow cylinder surrounding a pith with the phloem occurring only outside the xylem. It is found in Equisetum and some ferns like Osmunda and Schizaea.

  2. Equistelic Siphonostele-

    In this type of stele the central is occupied by pith. In this case, the xylem is surrounded on both external and inner sides by the rings of phloem, pericycle and endodermis. e.g. Marsilea, Adiantum and Dryopteris.

  3. Equistelic Siphonostele-

    In this type of stele the xylem and phloem cylinders breaks into a large number of collateral vascular bundles which are arranged in a ring e.g. Equisetum.

(C) Solenostele

Solenostele is that stele in which the stele is perforated by single leaf. It may be ectophloic or amphiphloic solenostele in the same way as in Siphonostele e.g. Ferns.

(D) Dictyostele

Dictyostele is that stele in which there occur more than one leaf gap. The portion of vascular tissue which lies in between two gaps is termed as Meristeles. Each meristeles protosteleic in nature, e.g. Ferns (Drypteris, Pteris, Ophioglossum Lasitanicum).

(E) Polycyclic Stele

In certain pteridophytes complex type of stellar structure is seen in which two or more concentric rings of vascular tissues are present. This type of stele is known as Polycyclic stele.

In Pteridium aquilinum (fern), there are two concentric rings of vascular tissues in which the inner rings forms a siphonostele and the outer ring forms a dictyostele consisting of numerous meristeles.

In Matonia Pectinate there are three rings of vascular tissues.

Evolution of Stelar System in Pteridophytes

Jeffrey’s opinion (1898) that protostele is the primitive type of stele in pteridophytes is favoured by most of the pteridologists. It is considered to be a fundamental stelar organization that was present in the earliest vascular plants and is now retained by some living vascular cryptogams like Psilotum, Tmesipteris, Selaginella, Lycopodium, Gleichenia, etc. The primitive vascular plants like the extinct psiophytales also possessed protostellic vascular organization e.g. Rhynia, Horneophyton, Asteroxylon. Presence of exclusively protostellic stems in the earliest vascular plants and their retention in some of living vascular cryptogams gives a strong support to Jeffrey’s view that protostele is phylogenetically a primitive type of stele. In its simplest form the protostele is haplostelic. During further elaboration the central core of xylem became irregular and assumed an almost star-like shape (Asteroxylon, some species of Selaginella and Lycopodium). Such modification of protostele was termed as achinostele by Brebner. As a result of further elaboration, the xylem splits up into a number of parallel plates alternating with phloem. Such a modification was called plectostele. It is found in some species of Lycopodium. Haplostele to actinostele and then to plectostele is considered to be one line of evolution of the protostele and can be regarded as a Lycopsid line of evolution.

Another very important evolutionary change that occurred in the protostele was the appearance of the central pith. This step led to many important changes in the protostelic organization and gave rise, on further elaboration, to complicated stellar types. The origin of pith has been explained variously by various authors. Two theories have been put forth to accord for its origin. These are the intrastelar theory and the extrastelar or invasion theory. Appearance of pith led to the conversion of the protostele into a new type of stele called the siphonostele. Elaboration of siphonostele also followed two courses of evolution :

  1. The appearance of pith resulted in the formation of a stele consisting of a central pith surrounded by a complete ring of xylem, which in turn was surrounded by a complete ring of outer phloem, pericycle and endodermis. Such a stele was designated as ectophloic siphonostele (medullated protostele). In its simplest form such a siphonostele is uninterrupted by leaf gaps and is also called Cladosiphonic. In the megaphyllous vascular plants the complete and uninterrupted cylinder of ectophloic siphonostele becomes interrupted by the appearance of leaf gaps. It is now called phyllosiphonic. In case the leaves do not overlap the stele is interrupted only at considerable distances (nodes) by one leaf gap. In between the two gaps the vascular cylinder remains complete. Such as stele is also called solenostele or siphonoeustele. In case the leaves on the stem overlap the leaf gaps also overlap and lead to the formation of a much dissected stellar organization. It is made up of a number of separate and collateral vascular bundles. This condition is called Eustele and is met with in the seed plants. In case the vascular bundles are scattered, as in monocotyledons, the stele is termed as atactostele.
  2. During another line of evolution the medullation of the protostele was followed by the appearance of phloem on either side of the xylem and likewise internal pericycle and endodermis also appeared. As a result the siphonostele consisted of a central pith surrounded by internal endodermis internal pericycle, internal phloem, xylem, external phloem, external pericycle and external endodermis. Such a siphonostele is called amphiphloic siphonostele (Marsilea, Adiantum). It may be cladosiphonic or phyllosiphonic. The phyllosiphonic amphiphloic siphonostele with only one leaf gap at the node is called amphiphloic siphonostele. In case, the leaf gaps overlap the resultant stele is called the dictyostele. Dictyostele is very common in the filicophyta. In many eusporangiate and leptosporangiate ferns the dictyostelic stems are protostelic at their bases. One such example is afforded by Ophioglossum lusitanicum. Recent experimental studies also reveal that dictyostelic condition can be changed to solonostelic or even protostelic condition by removing the young leaf primordial from shoot apices (Dryopteris dilatata). All these observations prove that protostele is the basic or the fundamental stellar type from which the complicated steles or vascular systems arose by elaboration.

Polycyclic condition exhibited by some ferns like Marattia, Matonia, Pteridium, Cyathea, etc. also originated from the protostelic condition by further elaboration. This is borne out by the fact that in Matonia pectinata there is a regular transition from protostelic condition to solenostele and then to a protostelic condition. The stem is protostelic at the base, then becomes entric circles of solenostelic and ultimately polycyclic. (Three concentric circle of siphonosteles). This developmental phenomenon is termed as recapitulation. Occurrence of such a developmental phenomenon lends further support to Jeffrey’s view that protostele is a primitive condition.

A number of recent workers have presented a different interpretation of the evolution of stellar system in the seed plants. They believe that the leaf gaps of the seed plants (Gymnosperms and Angiosperms) are not morphologically equivalent to those of ferns. Nambeodri and Beck (1968) and Beck (1970) have put forth an explanation that the eustele in gymnosperms is not derived from filicean type of siphonostele nor are there any filicean type of leaf gaps. These authors studied the stellar organization in primitive gymnosperms like Calamopitys foerstei and Lyginopteris oldhamia and found that the eustele in these originated direct from the protostele through longitudinal dissection, whithout any intervening siphonostelic stage. Such a mode of organisation of the primary vasculature in the woody angisoperms that are now living has been studied. According to their view extended by pteridologists, filicean leaf gaps are absent in the eustele of primitive gymnosperms like Lyginopteris. It has now been held that angisoperms originated from the Pteridosperms and if this view is corret then three are no fern type leaf gaps in the angiosperms also. So according to this interpretation the eustele originated directly from the protostele without the intervention of Siphonostele.

A new concept has been put forth that parenchymatous areas may arise in the stele without having any connection with the leaf gaps. Such a conclusion is based on detailed study of the primary vasculature in gymnosperms and angiosperms and is against Jeffrey’s leaf gap concept for ferns.

The above view supports another new interpretation that the primary vasculature of the stem is Cauline and not foliar. The protostele is clearly axial, not a foliar structure. “That this is true becomes very apparent when one considers that the ancestral psilophytes from which the protosteli, progymnosperms must have evolved were leafless”. Usually primary vasculature of stem is considered to be foliar in nature but the above view of its cauline nature is at variance with this.

Leaf Traces and Leaf Gaps-

A leaf trace is that part of the vascular cylinder that extends between the leaf base and the point where with the vascular system of the stem. The number of the leaf traces associated with one leaf may be more than one. So leaf traces connect the vascular system of the leaf with that of the stem.

In ferns and the seed plants the portion, lying immediately above the point of divergence of the leaf trace from the vascular cylinder of the stem, becomes parenchymatous. In this parenchymatous region, the vascular elements are absent, but this region is limited to only a short distance up, beyond which the vascular tissue is present. These parenchymatous region in the vascular cylinder are described as Leaf gaps or lacunae. The vascular plants in which the leaf traces leave leaf gaps are called megaphyllous. There are no leaf gaps in the lower vascular plants such as Selaginella and Lycopodium. In these cases, the leaves are small and their comparatively thin leaf traces do not lave leaf gaps. Such plants are called microphyllous.

Branch Traces and branch gaps-

The branches develop from buds and have vascular connections with the main axis. These vascular connections that connect the vascular system of a branch with that of stem are called branch traces. Similarly there are branch gaps which are the parenchymatous areas in stem vascular system just above the origin of branch trace. The leaf gaps and the branch gaps are not actually breaks in the continuity of the vascular tissue maintains lateral connection just above the below the gap.

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