In this article we will discuss about the evolution of stele in a plant.
Stele (= Greek word meaning a column) can be defined as the unit of vascular system that is made up of xylem, phloem, interfascicular tissues, medullary rays, pericycle and pith (if present).
The term stele refers to the central core of plant axis and is restricted only to primary tissues. The term is used in case of pteridophytes and is seldom applied in case of angiosperms and gymnosperms. Endodermis delimits a stele on the peripheral side.
There are two basic types of stele:
1. Protostele, and
2. Siphonostele.
1. Protostele:
A protostele is composed of a solid core of xylem mass surrounded by phloem, which in turn remains encircled by pericycle. Endodermis delimits protostele on the peripheral side. In protostele pith is absent and the protoxylem is exarch. There is no leaf gap. Some dicotyledonous roots have radial stele where pith is completely absent. Such radial stele is also referred to as protostele.
There are three types of protostele (Fig. 15.1) which are as follows:
(A) Haplostele:
Haplostele has a cylinder of phloem that surrounds a smooth core of xylem. The xylem mass appears circular or oval in outline as seen in cross-section. Protoxylem is exarch. Ex. Selaginella, Lygodium, the extinct psilophytes Rhynia and Homeophyton etc.
(B) Actinostele (Figs. 15.1 & 15.3A):
Actinostele has a cylinder of phloem that surrounds a star-like mass of xylem. As seen in cross-section, the xylem mass has radiating ribs of varying number. Protoxylem occurs at the tip of radiating ribs. Phloem also occupies the position between the xylem lobes and furrows. Ex. Psilotum and the extinct psilophyte Asteroxylon.
(C) Plectostele:
Plectostele has masses of xylem that are in the form of plate-like lobes. As seen in cross-section the plates are of different sizes and some of the plates are united at one end. Cylinder of phloem surrounds xylem masses and phloem also occurs between xylem plates. Ex. Lycopodium clavatum.
Lycopodium cernuum has mixed protostele (Figs. 15.1 & 15.3B). In this type of stele as seen in cross-section the xylem is mesh-like mass that is uniformly distributed and appears to be embedded in the ground mass of phloem.
2. Siphonostele:
Siphonostele, where xylem is in the form of a hollow cylinder, has parenchymatous pith at the central region of xylem. The xylem is surrounded by phloem that in turn remains encircled by pericycle. The whole stele is limited outside by a continuous endodermis. In siphonostele xylem and phloem are in the form of a continuous or split vascular cylinder.
The following two types are recognized (Fig. 15.1) on the basis of position(s) of phloem in relation to xylem in siphonostele:
(a) Ectophloic siphonostele:
Ectophloic siphonostele has a continuous cylinder of phloem surrounding the peripheral side of xylem. Parenchymatous pith occurs at the central region of xylem. The whole stele is delimited outside by a continuous endodermis. Sporne (1976) defines ectophloic siphonostele as ‘medullated protostele’. Leaf gap is absent in ectophloic siphonostele. Ex. ferns like Osmunda, Schizaea etc. and dicotyledonous angiosperm like Phlox, Lindenbergia etc.
(b) Amphiphloic siphonostele (Figs. 15.1 & 15.3C):
Amphiphloic siphonostele has cylinders of phloem on the peripheral and inner side of xylem. The peripheral phloem is termed as outer phloem and the other as inner phloem. Pericycle and endodermis appear both outside and inside of vascular tissues. To distinguish them the terms outer and inner pericycle, and outer and inner endodermis are used.
The outer pericycle occurs surrounding the peripheral side of outer phloem whereas the inner pericycle is situated on the inner side of inner phloem. Outer endodermis delimits the whole stele and occurs between cortex and outer pericycle. Inner endodermis occurs between inner pericycle and pith. Ex. Marsilea and Adiantum etc.
Ectophloic and amphiphloic siphonostele may be cladosiphonic or phyllosiphonic. In cladosiphonic siphonostele the vascular tissues, in cross section, appear as continuous cylinder as the leaf traces are without gaps (e.g. Selaginella). The traces of phyllosiphonic siphonostele are with gaps and so the vascular tissues appear as isolated bundles in transverse section (e.g. Polypodium).
i. Solenostele:
Solenostele can be defined as a type of amphiphloic siphonostele with non-overlapping leaf gap. The leaf gaps are distantly spaced. Solenostele consists of two vascular strands-the small leaf trace and the large principal vascular strand as seen in a cross-section of stem at node. The principal vascular strand appears horse-shoe-shaped due to the presence of parenchymatous leaf gap.
In the vascular strands phloem appears both outside and inside of xylem. The two vascular strands have individual continuous endodermis. The vascular strand is in the form of a continuous cylinder between leaf gaps. The vascular cylinder is interrupted at the places corresponding to the origin of leaf traces. Ex. Anemia, Adiantum pedatum, Davallia etc. It is to note that ectophloic siphonostele with non-overlapping leaf gap is also referred to as solenostele.
ii. Dictyostele:
Dictyostele can be defined as a type of amphiphloic siphonostele with overlapping leaf gaps. The upper part of a leaf gap overlaps the lower part of the upper adjacent leaf gap. The gaps are not distantly spaced from each other and occur in parallel manner. As a result a longitudinal cylindrical network of interconnected vascular strands (Fig. 15.1C) is formed when viewed as three- dimensional object.
The vascular strand is perforated as seen in cross-section. The vascular strands are arranged in a ring-like manner and parenchyma occurs in between the vascular strands. Each vascular strand is composed of xylem surrounded by phloem.
This amphicribral vascular strand is surrounded by a pericycle and the whole being bounded on the outside by a continuous endodermis. Ex. Mohria, Polypodium falcatum, Ophioglossum, Dryopteris etc. Each vascular strand of dictyostele (Fig. 15.2) is referred to as meristele.
iii. Eustele:
Eustele can be defined as a type of ectophloic siphonostele with overlapping leaf gaps. The leaf gaps occur parallel to each other and are not distantly spaced. The upper part of a gap overlaps the basal part of the upper adjacent gap. When viewed as three-dimensional object the vascular strands form an interconnected network. The vascular strands are separate as seen in cross- section. Each vascular strand, also called vascular bundle, is conjoint and collateral.
Parenchyma occurs at interfascicular region. All vascular bundles are arranged in a ring like manner. Pericycle surrounds the vascular bundles on the peripheral side, the whole being bounded by a continuous endodermis. Eustele is the characteristic of gymnosperm and dicotyledonous stem. Ex. Helianthus, Xanthium etc. Eustele with bicollateral vascular bundle is observed in the families Cucurbitaceae, Solanaceae etc.
iv. Atactostele:
Atactostele can be defined as a type of eustele where collateral vascular bundles are arranged in an irregular manner (Fig. 15.2). It is the characteristic of monocotyledonous stem where there is no distinction between pith and cortex.
Parenchyma bounded by epidermis is designated as ground tissue in monocotyledons. The vascular bundles are scattered on the ground tissue as seen in cross-section of stem. The vascular strands form an interconnected network when viewed as three-dimensional object. Ex. stem of Zea mays, Asparagus (Fig. 15.3E) etc.
v. Polystele (Fig. 15.3D):
Polystele can be defined as having more than one protostele as observed in the cross-section of a stem. In Selaginella willdenowii (Fig. 15.1) three protosteles occur. Each protostele has xylem surrounded by phloem, the whole being bounded by endodermis.
In angiosperm polysteles occur in the families like Acanthaceae, Nymphaeaceae, Palmae etc. In cross-section polysteles appear to be scattered or organized into a ring. In longitudinal section it is revealed that individual steles, by anastomosis among themselves, form a network.
vi. Polycyclic stele:
Polycyclic stele can be defined as having two or more coaxial cylinders of vascular strands as observed in the cross-section of a stem. The individual cylinders are interconnected at the base of inner stele. Polycyclic steles are also referred to as polycyclic siphonostele where the innermost vascular cylinder is amphiphloic siphonostele (Fig. 15.2). The other cylinders remain separated by parenchyma. Ex. Matonia pectinata.