TEXT The name stele has been derived from Greek word meaning pillar or column. Sachs in 1875 proposed that the vascular system of the plant body is a continuous system. For the first time, the stelar organisation appeared in pteridophytes. Hence the pteridophytes are called as first vascular plants. The stelar theory was proposed by, Van Tiegham and Douliot in 1886. According to which the root and stem are fundamentally similar in gross anatomy, because in both the cortex encloses the central part of the axis, called the stele. According to them, stele is 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 vascular bundles (pericycle). The term stele is applied to the primary tissue only. On the basis of structural variations, in the primary vascular system, following types of steles have been recognized. 1. Protostele: This is the simplest type of stele. It consists of a solid central column of vascular tissue. Pith is absent. Xylem is located in the centre surrounded by phloem. On the basis of shape of xylem, the following types of protosteles have been recognized:
a. Haplostele: It has a smooth core of xylem surrounded by a uniform layer of phloem, e.g. Lycopodium cernum and Selaginella kraussiana (Fig. 1-A). b. Actinostele: It has a xylem core with radiating ribs (starshaped = stellate). In actinostele phloem is present in the form of separate patches alternating with projecting parts of xylem, e.g. Lycopodium serratum, Psilotum triquetrum (Fig. 1-B). c. Plectostele: In this type of protostele, xylem occurs in the form of separate plates which lie parallel to one another, with phloem situated between them. It is found in Lycopodium volubile, L. clavatum, etc. (Fig. 1-C). d. Mixed protostele with phloem: In this stele xylem groups are scattered in the form of irregular patches that are embedded in the ground mass of phloem, e.g., Lycopodium cernuum (Fig. 1-D). e. Mixed protostele with parenchyma: In this stele xylem groups are scattered in the form of irregular patches that are embedded in the ground mass of parenchyma, e.g. Hymenophyllum demissum. 2. Siphonostele or medullated protostele: A type of stele in which there is present a pith in the central region, is called a siphonostele or medullated protostele. In
siphonostele, vascular tissue is arranged in the form of a hollow cylinder, with distant pith in the centre. It is found in the stems of most members of Filicophyta. It is of the following two types: a. Ectophloic siphonostele; It is a type of siphonostele where xylem cylinder lies next to the pith and is surrounded by the phloem cylinder on the outer side. It is found in Osmunda, Equistem, etc. (Fig. 2-A). b. Amphiphloic siphonostele: In this type of siphonostele the pith is surrounded by inner endodermis, inner pericycle, inner phloem, xylem, outer phloem, outer pericycle and outer endodermis. So in this case phloem surrounds the xylem internally as well as externally. It is found in Adiantum, Marsilea, etc. (Fig. 2-B). In both ectophloic and amphiphloic siphonosteles, vascular tissue occurs as continuous cylinder. This is because the leaf traces do not break the vascular cylinder. Such plants in which the vascular supply to the leaf is without any break in vascular cylinder are called microphyllous. 3. Solenostele: This is similar to siphonostele in having central pith, but differs in producing leaf gaps, wherever leaf
traces originate. Thus due to production of leaf gapes, the cylinder becomes dissected at places. This type of solenostele may be ectophloic or amphiphloic, depending upon the type of siphonostele from which it is produced. Such plants where leaf gapes occur in vascular cylinder are called magaphyllous (Fig. 3). 4. Dictyostele: In many siphonostelic Filicophyta, leaves are inserted on the stem in close succession. In such cases, leaf gaps overlap in their longitudinal extent to such a degree that vascular cylinder of stem appears dissected into tubular network of interconnected longitudinal strands (meristeles), separated from one another by parenchymatous tissue (leaf gaps). These meristeles in a transverse section appear arranged in a ring. Such a stele is known as dissected siphonostele or dictyostele. The vascular parts of a dictyostele between two neighbouring leaf gaps, appearing in transverse section as separate strands, are termed as meristeles. In a Dictyostele, each meristele has the general structure of a protostele, e.g. Dryopteris filix-max (Fig. 4). 5. Eustele: This is a modification of siphonostele, in which vascular system consists of a ring of collateral or bicollateral vascular bundles. These are separated from one another by a wide medullary or interfasicular regions and leaf gaps are not
clearly distinguishable. It is found in the stems of gymnosperms and angiosperms (Fig. 5). 6. Atactostele: This is the most complex type of stele. In this type, the vascular bundles are irregularly dispersed in the ground tissue as in the stems of monocotyledonous (Fig. 6). 7. Polycyclic stele: When more than one steles are present in the axis of pteridophytes, e.g. 2 in Selaginella kraussiana, 16 in S. laevigata, the condition is called polycyclic stele (Fig. 7). B. Stelar evolution: According to Jeffrey (1898), protostele is the most primitive type of stele, from a phylogenetic stand point, from which other types of steles have evolved in the course of evolutionary specialization (Fig. 8). It is considered to be the fundamental stelar organisation that was present in the earliest vascular plants, and is now retained by some living vascular cryptogames such as Psilotum, Tmesipteris and Lycopodium, etc. The primitive vascular plants, like the extinct Psilophytales also possessed protostelic vascular organisation. Presence of exclusively protostelic stems in the earliest vascular plants, and their retention in some of the living vascular cryptogames, lends a strong support to
Jeffrey's view that protostele is phylogenetically a primitive type of vasculature. In its simplest form, protostele is haplostelic. During further elaboration, the central core of xylem became irregular and assumed almost a star-like shape. Such a modification of stele is termed as actinostele. As a result of further evolution, the xylem splits up into a number of parallel plates alternating with phloem. Such a modification is 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. It is regarded as Lycopsid line of evolution. The various types of protosteles may show relative variation in the position of protoxylem and metaxylem. It may be exarch with the protoxylem near the periphery of the xylem strands, or may be endarch with the protoxylem at the inner surface. In the mesarch condition, metaxylem is present both towards the outer and inner sides of protoxylem. The endarch condition is considered to be the most evolved and exarch condition as the primitive in the context of stelar 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 organisation and gave rise, on further elaboration, to complicated stelar types. Two theories have been proposed accounting the phylogenetic origin of the pith. These are: I) Intrastelar theory, II) Extrastelar theory I). Intrastelar theory: According to intrastelar origin of pith or expansion theory, the inner vascular tissue metamorphosed into the parenchymatous pith. The occurrence of mixed pith in some living forms supports this view. A mixed pith shows tracheids within the parenchymatous pith. This may be looked upon as a transition stage between a true protostele and true siphonostele. II) Extrastelar theory: According to the extrastelar origin of pith or invasion theory the pith is cortical in origin. The parenchymatous cortex is said to have intruded through the leaf gapes and branch gaps into the centre of vascular cylinder to give rise to the pith. But this cortical invasion could not have produced polycyclic siphonostele. Eames (1936) considered that in primitive forms, it may be intrastelar in origin, and in higher forms extrastelar in origin.
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, as follows: i). 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 named as ectophloic siphonostele. In its simplest form such a stele is uninterrupted by leaf gaps and is called cladosiphonic. In the magaphyllous vascular plants, the complete and uninterrupted cylinder of ectopholic siphonostele became interrupted by the appearance of leaf gaps, which is now called phyllosiphonic. In case the leaf gaps do not overlap, the stele is, interrupted only at considerable distances (nodes) by one leaf gap, so in between the two leaf gaps, the vascular cylinder remains complete. Such a stele is also called solenostele or siphoneustele. During the course of evolution, in certain pteridophytes, leaf gaps on the stem overlap and lead to formation of a much dissected stelar organisation, called eustele. It is made up of a number of separate and collateral vascular bundles. In certain cases, the vascular bundles are
scattered, as in monocotyledonous, this kind of stele is termed as atactostele. ii). During another line of evolution, the medullation of protostele was followed by the appearance of phloem on either side of the xylem; likewise internal pericycle and also appeared. As a result, the siphonostele consisted of central pith, surrounded by a internal endodermis, internal pericycle, internal phloem, external pericycle and external endodermis. Such a stele led to the formation of amphiphloic siphonostele. It may be cladosiphonic or phyllosiphonic. The phyllosiphonic amphiphloic siphonostele with only one leaf gap at the node is called amphiphloic solenostele. In case the leaf gaps overlap, the resultant stele is called the dictyostele. Dictyostele is very common in Filicophyta. In many eusporangiate and leptosporangiate ferns, the dictyostelic stems are protostelic at their bases. Recent experimental studies also reveal that dictyostelic condition can be changed to solenostelic, or even to protostelic condition by removing the young leaf primordial from shoot apices. All these observations prove that protostele is the basic or the fundamental stelar type, from which the complicated steles or vascular systems arose by elaboration.
Among the pteridophytes, polycyclic stele is the most advanced condition exhibited by some ferns like Marattia, Matonia, etc. It also originated from the protostelic condition by further elaboration. This is born out by the fact that in Matonia pectinata, there is a regular transition from protostelic condition to solenostele and then to polycyclic condition. This developmental phenomenon is termed as recapitulation. Occurrence of such a developmental phenomenon leads further support to Jeffrey's view that protostele is the primitive condition. Thus, the evolution of stellar organisation has taken place along several independent lines; even a single genus, like Gleichenia, shows some species with protostelic structure, and some with the siphonostelic organisation. So the stellar structure has not been of much significance in establishing phylogenetic relationships.