In this article we will discuss about the evolution of stelar forms in vascular plants.
The well known genus Rhynia represents the simplest kind of vascular plant. It is rootless, leafless and the axis is dichotomously branched. A stele runs through the centre of stem. The cells in the very centre of stele consist of tracheids only and this xylem is among the simplest kind of wood cells known. A zone of phloem tissue surrounds the xylem core.
Phloem is conspicuous among living plants, but not especially evident in extinct plants. This very simple kind of cylindrical stele where phloem surrounds a solid core of xylem is termed as protostele. Protostele is found not only in ancient plants like Rhynia and Psilophyton, but also in stems of Lycopodium, some ferns and in many kinds of dicotyledonous roots.
With the increase in complexity of sporophyte of vascular plants the vascular system elaborates. Both phyolgenetically and ontogenetically protostele is regarded as most primitive (Fig. 15.4). During evolution protostela undergoes certain modifications that result in the formation of different forms of protostele and siphonostele.
Haplostele is regarded as most primitive among protostele. Actinostele is somewhat more advanced kind of stele, e.g. extinct Asteroxylon, extant Lycopodium serratum etc. Plectostele is most advanced kind of stele in protostele, e.g. Lycopodium clavatum. It is thought that the line of evolution among protostele proceeds from haplostele to actinostele and then to plectostele.
In the course of evolutionary specialization the circular vascular strand of haplostele, as seen in cross-section, becomes stellate thus forming actinostele. This happens when the smooth core of vascular cylinder folds up at different places. Plectostele results when the folding becomes complete followed by separation of xylem masses.
It is always observed that non-living conducting cells like tracheids etc. are in close association with living cells of some kind. There is no known physiological basis of this association. Anatomists believe that ‘because of the very close association between non-living conducting cells and living cells, there must exist some unexplained relationship between them.
In haplostele, when the stele is small, each tracheid is not very far from a living cell, that is, phloem that surrounds the xylem. With the growth of sporophyte the stele also increases by the formation of living cells. Many tracheids may be of considerable distance from living cells.
So to have a close association between non-living and living cells the xylem mass of haplostele becomes ridged and furrowed to form actinostele. During the course of evolutionary specialization plectostele arises where the living cells surround the xylem plates and intersperse between xylem masses.
The living-nonliving cell relationship is also maintained by the formation of pith. Pith is composed of living cells. Pith occupies the centre of stele and is composed of parenchyma cells. Most of the higher vascular plants exhibit steles with pith in the centre.
Pith remains surrounded by xylem that in turn is surrounded by phloem on the peripheral side (ectophloic siphonostele) only or on both outside and inside (amphiphloic siphonostele). Such steles are referred to as siphonostele.
Ectophloic siphonostele, in its primitive form, has a continuous cylinder of vascular tissues. The advanced form consists of a network of bundles. In cross- section the bundles appear as separate vascular strands that are arranged more or less in a ring. Parenchyma exists at interfascicular regions.
These regions are usually the position from where leaf traces originate from the vascular cylinder of stem. This interfascicular region is referred to as leaf gap. The leaf gaps may be considerably distant from each other. In the other case the upper part of one gap is at the level of the lower part of another gap.
The gaps are said to be non- overlapping when they are distant from each other. In overlapping gaps the base or apex of one gap is at the level of apex or base of another gap. In the course of evolutionary specialization non-overlapping leaf gap arises and this results in the formation of siphonoeustele (also termed as ectophloic solenostele).
In this stele the vascular tissue is interrupted at the place where leaf trace appears. In other places the vascular tissue is in the form of a continuous cylinder. Overlapping leaf gaps dissect the stele and thus eustele originates. In course of evolution atactostele arises where there is no definite arrangement of vascular bundles.
Siphono-eustele, eustele and atactostele have collateral vascular bundles. The evolutionary specialization proceeds from ectophloic siphonostele to siphonoeustele and then to eustele. This line of evolutionary sequence is exhibited in seed plants.
Amphiphloic siphonostele, in its primitive form, is in the form of a continuous cylinder. Non-overlapping leaf gap appears in course of evolution and dissects the vascular cylinder to form solenostele (also termed amphiphloic solenostele).
Ultimately overlapping leaf gaps appear and a dictyostele results. This line of evolutionary specialization proceeds from amphiphloic siphonostele to amphiphloic solenostele and then to dictyostele. This line of evolutionary sequence is exhibited in cryptogams.
The polycyclic stelar structure as seen in cross-section of Matonia etc. is regarded to have arisen from protostele by elaboration. In favour of this view the stelar structure of Matonia pectinata has been cited. The basal portion of this plant is protostele. In the higher region the stele is solenostele and ultimately the stele is polycyclic.
From phylogenetic point of view protostele is regarded as primitive than siphonostele. In the course of evolutionary specialization siphonostele originated from protostele. Protostele became large with the addition of living cells.
The living cells at the centre never became modified into nonliving tracheids. They remained living and are referred to as pith.
Thus siphonostele arises and the followings are the two schools of thought regarding the origin of pith namely- expansion theory and invasion theory:
i. Expansion Theory:
Proponents of this theory believe that pith has originated from stelar tissues. During differentiation of xylem certain living cells at the centre never modified into non-living xylem. In protostele the solid central core of xylem mainly consists of tracheids. The siphonostele (of pteridophyta) possesses parenchyma cells at the centre that is pith, surrounded by tracheids.
In the transitional forms some amount of parenchyma remains mingled with tracheids- termed mixed pith. So the evolution occurred in the following way: stele without pith-stele with mixed pith-stele with pith (siphonostele). In favour of this view Gewirtz and Fahn (1960) cited the stelar structure of Ophioglossum lusitankum.
The basal portion of this species is protostele, the upper portion shows siphonostele dictyostele and in the transition zone, below the level of the first leaf gap, some parenchyma cells are found at the centre in association with tracheids. The number of parenchyma cells gradually increases in the more upper level.
It appears that during transition from protostele to siphonostele, some of the initials of tracheids are transformed to the initials of parenchyma that formed pith. This view also finds support due the presence of isolated tracheids in the pith of Botrychinm lunaria and Osmunda regalis.
ii. Invasion Theory:
According to this view pith is regarded as extrastelar in origin. Jeffrey (1917), who proposed the invasion theory, is of opinion that the cortical parenchyma cells invaded the central core of xylem of protostele where they established as pith. According to Jeffrey the invasion occurred through the leaf-and branch gaps.
As evidence Jeffrey cited the occurrence of inner endodermis between pith and vascular cylinder as endodermis is regarded as the integral part of cortex. Jeffrey opined that endodermis together with cortical parenchyma penetrated through the leaf-and branch gaps towards the centre of protostele. This view has been a subject of much debate because in Selaginella Ptendium etc. the endodermis is stelar in origin. This view gained little support.
In this context it is worthy to mention the researches of Namboodiri and Beck (1968) who establish that leaf gaps do not exist in seed plants like ferns. The origin of eustele in seed plant is different from that of dictyostele. According to Namboodiri and Beck haplostele is the original protostele. During evolution the xylem protrudes in the areas from where leaf trace originates.
The rest of the areas of xylem strand become indented. Thus the margin of xylem mass becomes undulated and this results in the formation of actinostele. This stage is exampled by Aneurophyton (Fig.15.5). During evolutionary specialization the protruding areas of actinostele become isolated. As a result a stele is formed that consists of interconnected vascular strands.
Parenchyma surrounds the vascular strands. This parenchyma is cortical parenchyma and it extends between the regions of vascular strands. This type of stele is represented by the extinct plant Stenomyelon (Fig. 15.5). In Archaeopteris (Fig. 15.5) the number of individual vascular strands increases and the vascular strands are arranged more or less in a ring surrounding a well-defined region of parenchyma.
This parenchymatous region is termed as pith and it is also present in conifers (Fig. 15.5) and in other seed plants. The above mentioned evolutionary sequence regarding the appearance of pith reveals that the seed-plant-eustele is not a dissected siphonostele.
Previously the node of seed plants was interpreted on the basis of nodal anatomy of ferns and to describe it the terms leaf gap, leaf trace, cauline bundle and common bundles were used. But the information and ideas presented by Namboodiri and Beck reveal that leaf gaps do not exist in seed plants as they do in ferns.
Diagram illustrating the stelar organization of plants presented to hypothesize about the origin of pith in seed plants.
Whatever might be the reason, pith originated in protostele and that resulted in the formation of siphonostele. Siphonostele may be cladosiphonic or phyllosiphonic. In cladosiphonic siphonostele the vascular cylinder is interrupted at the region of branch gap. Branch gap is associated with branch trace.
Branch trace originates from central vascular cylinder and enters into lateral branches. Cladosiphonic siphonostele is observed in the microphyllous plants present in Psilopsida, Lycopsida and Sphenopsida. In phyllosiphonic siphonostele the vascular cylinder is interrupted due to the presence of leaf gaps. It is observed in plants present in Pteropsida.