The below mentioned article provides an overview on the vascular tissue system of plants.
The vascular tissue system consists of the complex tissues, xylem and phloem, which constitute discrete conducting strands called vascular bundles. These are usually primary in nature.
The elements of xylem and phloem have already been discussed in the preceding chapters. In spite of the occurrence of supporting and other cells the function of the vascular bundles is primarily conduction, xylem for the conduction of water with dissolved mineral matters, and phloem for the conduction of elaborated food matters in solution.
The term fibrovascular bundle had been in use in the past mainly because of association of sheaths of fibrous tissues with the bundles. But in view of the fact that the fibrous sheaths do not always form a part of the bundle, the term fibrovascular bundle has been discarded and replaced by simply vascular bundle.
The vascular bundles originate from the procambium of the apical meristem. In the promeristem stage normally all the cells are isodiametric. During the longitudinal divisions they undergo, some cells are set apart as patches or strands.
These cells are smaller and elongate ones and possess dense cytoplasm. This meristematic tissue (Fig. 569) is called procambium, which is destined to produce the elements of vascular bundles.
In recent years the terms ‘provascular tissue’ or ‘provascular meristem’ have also been used to designate this tissue. Procambium appears early near the apex of the axis and gradually differentiates out backwards, so that the course of development is acropetal.
The procambium shows early differentiation into two parts. The part destined to give rise to phloem takes dense stain and shows different planes of division than the other part which would eventually produce xylem.
So the terms phloic procambium and xyloic procambium have been used for the two parts. This fact really justifies the use of the term provascular meristem.
The phloem elements mature earlier than the Xylem elements; the order of maturation is always acropetal. But that of xylem elements may be both acropetal or basipetal.
Thus the progressive development of the vascular elements from the procambium strands may be both centripetal and centrifugal. The order of differentiation in phloem is probably always centripetal, i.e., towards the centre of the axis.
But in case of xylem three different conditions are possible as regards the order of differentiation of elements. In the first type the initial xylem elements are located furthest from the axis, where the course of development is obviously centripetal or towards the axis.
This type of xylem is said to be exarch, what is characteristic of the roots (Fig. 570B). In the second type, the condition is just the reverse, the initial ones occurring nearest the axis and the latest furthest from it. Here the course of differentiation is centrifugal and the Xylem is called endarch (Fig. 570A). This type of xylem is common in the stems of spermatophytes.
There is a third type where the course of differentiation proceeds in two directions, i.e., both centripetally and centrifugally. The xylem is that case is known as mesarch, what is found in some ferns (Fig. 570C).
The first-formed xylem is called protoxylem and the lately-formed one is known as metaxylem. Protoxylem differentiates from the procambium when the organ continues elongation and is often subjected to considerable stretching.
It consists of tracheids, vessels and parenchyma, the fibres being usually absent. The cells and elements are elongate and slender bodies with cellulose cell walls, reinforced by lignified secondary walls. The secondary wall layers are deposited in form of rings and spirals (Figs. 583B & 585B) which keeps the thin and plastic walls of empty water-conducting elements in proper position and prevent collapse during the elongation of the organs.
Annular and spiral thickenings are thus characteristic of the protoxylem elements. Scalariform or ladder-like thickening may also occur. Of all the above-mentioned types the spiral elements are much more abundant.
The water-conducting elements of protoxylem are tracheids in pteridophytes, gymnosperm and some angiosperms and vessels in many angiosperms. The annular cells or vessels are often subjected to so much stress during elongation that the primary wall is destroyed and secondary wall distorted, so that even a canal-like body, called protoxylem lacuna, may be formed (Fig. 518C).
The lately-formed xylem or metaxylem elements are not subjected to stretching as they mature only when organs have completed growth in length. They have more extensive secondary walls in form of network (reticulate) or pits.
Scalariform thickening may also be present. The two parts of the Xylem often intergrade. Metaxylem is the main water-conducting portion in plants which have no secondary increase in thickness. It is more complex than protoxylem and possesses more tracheary elements. The type of primary Xylem, whether exarch, endarch or mesarch, is determined by the position of protoxylem in relation to metaxylem (Fig. 570).
In exarch condition protoxylem occurs towards the circumference and- metaxylem towards centre; in endarch the position is just the reverse, i.e., protoxylem towards centre and metaxylem towards circumference; in mesarch protoxylem is flanked on two sides or remains surrounded by metaxylem.
The first-formed elements of phloem are called protophloem. The cells of protophloem differ considerably from the lately-formed ones, known as metaphloem elements, in size and shape. The protophloem elements are slender and elongate bodies with cellulose cell wall. They are subjected to considerable stretching during the rapid growth in length of the organs.
The metaphloem elements mature after the completion of growth in the surrounding tissues and so they are not subjected, to stretching. Primary phloem consists of protophloem and metaphloem. In secondary phloem protophloem is absent, because secondary tissues are formed when growth in length has ceased.
The protophloem constitutes the first vascular elements to mature from the procambium. It is composed of sieve elements—sieve tubes in angiosperms and sieve cells in gymnosperms and pteridophytes. These are small cells more or less similar to the procambium ones.
In keeping with the rapid elongation of the organs the cells are elongate and slender. Companion cells are scarce or lacking. Parenchyma and fibres are formed later from the procambium. Metaphloem is rather complex, consisting of all the elements —sieve tubes or cells, companion cells, parenchyma and fibres.
The primary phloem persists throughout the life of the organs and carries on its physiological functions, where secondary phloem is not formed. This condition prevails in the monocotyledons and in some dicotyledons like Cucurbita.
But in plants having secondary growth primary phloem is of short duration, as the considerable amount of secondary phloem formed later takes over the physiological function, and the primary phloem often gets crushed.
However, in some plants with secondary growth, like Solanum of family Solanaceae, Aster of family Compositae the amount of secondary phloem is small and primary phloem persists all through.
Normally phloem occurs on the outerside of the xylem in the vascular bundles of stems and on the abaxial side in the leaves and leaf-like organs. So phloem is said to be external with reference to Xylem. But in a number of dicotyledonous families like Solanaceae, Cucurbitaceae, Compositae, Apocynaceae, Asclepiadaceae, Convolvulaceae and Compositae a part of phloem may be present on the internal side as well.
This is known as internal phloem, as opposed to normal external one. It usually occurs as large or small strands, in close association with primary xylem, as in Cucurbita (Fig. 585), or it may often form independent strands in the outer part of the pith, as found in potato (Fig. 571), Calotropis (Fig. 588) and others.
Early workers called it intraxylary phloem, but that term has been abandoned now. The external and internal phloem is practically similar in composition, structure and arrangement of cells, but internal phloem develops later than the external one.
It should not, at any rate, be confused with another type called ‘interxylary’ or included phloem found in some dicotyledonous families like Combretaceae, Loganiaceae, Acanthaceae.
Interxylary phloem is really secondary phloem formed due to peculiar behaviour of the cambium cells and it ultimately gets embedded in secondary Xylem.
As already stated the procambium cells differentiate and mature into Xylem and phloem elements. In plants having no secondary growth in thickness, as in lower vascular plants and monocotyledons, all the procambium cells ultimately mature into vascular tissues.
But in the stems of dicotyledons and gymnosperms, which grow in thickness, a part of the procambium remains meristematic. It is called cambium (Fig. 583). It is a lateral meristem occurring parallel to the axis.
Cambium produces secondary tissues and is thus responsible for growth in thickness of the organs. The cambium cells have vacuolate protoplast and thin cell wall composed of cellulose, often with primary pit-fields. The cells divide periclinally and produce secondary tissues.
The vascular cambium is composed of two types of cells, viz., elongated cells with tapering ends, called fusiform initials, and small, more or less isodiametric cells known as ray initials.
The fusiform initials by cell division give rise to the secondary tissues—secondary xylem and secondary phloem which remain arranged along the long axis of the organ. The ray initials produce the ray cells of Xylem and phloem, which occur in transverse or horizontal series.
The leaves of gymnosperms like pine possess a peculiar type of conducting tissue in addition to normal vascular tissues. This tissue, called transfusion tissue (Fig. 572), consists of rather short tracheid-like cells.
They have thin cellulose walls with characteristic thickenings of the tracheids, viz., bordered-pitted, scalariform or reticulate ones. The cells are devoid of protoplasts.
Barring those two characters-localised thickenings of the walls and absence of protoplast, they resemble elongate parenchyma cells. They occur adjacent to Xylem in the bundle, partly or wholly surrounding the latter.
Some workers are of opinion that transfusion tissue derives its origin from centripetal xylem, while others consider it to be transformed parenchyma cells outside the vascular tissues. Their function is uncertain.
But as they connect the veins with the mesophyll of the leaves, taking the position of the vein-lets, they may be regarded as modified vascular tissues. According to some workers it is a water- storage tissue.
Types of Vascular Bundles:
The complex tissues, xylem and phloem, are usually associated in the formation of the vascular bundle.
They show three common types of arrangements, viz., (i) the two complex tissues occur side by side, (ii) one tissue remains surrounded by the other, (iii) the two tissues are separated from each other.
According to mode of occurrence of the elements, the vascular bundles are of the following types:
1. Collateral (Fig. 573 A & B):
This is the most common type of vascular bundle in the stems and leaves of angiosperms and gymnosperms. Here xylem and phloem remain side by side arranged on the same radius, phloem on the outer side, i.e., external, and xylem towards the pith, i.e., internal.
In the stems of most dicotyledons and gymnosperms, a strip of lateral meristem, the cambium, occurs between xylem and phloem (Fig. 573A).T he bundles in those cases are called open, whereas those without cambium, e.g., monocotyledons, are said to be closed (Fig. 573B).
2. Bicollateral (Fig. 573 C):
These are collateral bundles, where, in addition to the external phloem, another patch of phloem occurs on the inner side, what may be called internal phloem. Though rather uncommon, this type occurs in the family Cucurbitaceae.
Two strips of cambium and two patches of phloem are present on the outer and inner sides of Xylem. Naturally the sequence is outer phloem, outer cambium, Xylem, inner cambium and inner phloem. Obviously the bundles are always open.
3. Concentric (Fig. 573 D & E):
Here one kind of vascular tissue completely surrounds the other. Thus concentric bundles are of two types—xylem surrounding phloem, called amphivasal or leptocentric bundles (Fig. 573 E) or, phloem surrounding xylem called amphicribral or hadrocentric bundles (Fig. 573D).
Though less common, the amphivasal bundles occur in some monocotyledons, particularly in the nodal regions and in some rhizomes. In dicotyledons the medullary bundles are amphivasal.
Amphicribral ones are frequently found in the ferns. The small bundles of flowers; fruits and some leaves of dicotyledons are of this type. The concentric vascular bundles are always closed.
4. Radial (Fig. 573 F):
These bundles are fundamentally different from the types mentioned above, in view of the fact that here Xylem and phloem occur in separate patches on alternate radii on the axis intervened by non-conducting tissues.
Early workers used the term ‘conjoint’ for all the types discussed above with Xylem and phloem occurring on the same radius, as opposed to the radial arrangement. Radial vascular bundles are characteristic of the roots.
These are the common types of vascular bundles. But variations in structure may occur and intergradations of the types are also possible, even with transitional conditions. In some grasses the bundles are collateral, where xylem occurs in form of letter V, the two metaxylem occupying the flanks and phloem located between them.
The same bundle may differ in the arrangement of the elements along its course, so much so that it may be collateral at one level, amphivasal at another and even transitional somewhere between the two levels.
The internal phloem in the stems of some plants, e.g., family Solanaceae, may be detached from other parts of the bundle and occur as independent patches in the pith. In some families like Piperaceae, Amarantaceae, etc., even all the vascular bundles may be scattered in the pith. These are referred to as medullary bundles (Fig. 649), which are usually amphivasal.
Similarly bundles may occur in the cortex in members of families Melastomaceae, Cactaceae, Oleaceae, etc. They are called cortical bundles (Fig. 658).
The Vascular Skeleton:
The vascular system is continuous in the two parts of the axis, the stem and the root, and is also connected with the lateral expansions, the leaves. The vascular bundles, in fact, form a skeleton comparable to the skeleton of the animal bodies.
The skeleton (Fig. 574) differs in the various plant organs, and this difference is constant and characteristic. Every species has its own plan and arrangement what is different from other species. This vascular skeleton becomes increasingly more complex in the plant kingdom from pteridophytes to the spermatophytes.
The two parts of the axis, stem and root, possess different types of vascular bundles— collateral bundles in the stem with endarch Xylem and radial bundles in the root with exarch Xylem.
Nonetheless, the continuity of the vascular system in the axis is maintained. Some orientations take place in the region between the root and the stem, usually the hypocotyl. That region is referred to as transition region.
There are diversities as regards the methods of changes taking place in transition regions but the xylem usually splits and swings laterally by 180 degrees and ultimately joins up with phloem.
Thus the radial bundles of the root with exarch xylem become continuous with the collateral bundles of the stem having endarch xylem.
At each node of the stem the vascular bundle runs into the leaf; it is called a leaf trace or foliar trace (Fig. 575). A trace is nothing but an extension of the vascular tissues of the stem into the leaf.
The terminal part of the trace bundle is made of xylem alone, and the basal part of Xylem and phloem. The traces supplying the leaves, forming what is known as leaf supply, vary from one to many; but the number is constant for a particular species, and even for a family. The traces have characteristic forms.
The bundles may remain separate in the stem; but if a trace is followed along its descent towards stem it is found to join ultimately with another bundle which has entered from a lower leaf.
Questions actually arose on the point as to how much of the vascular supply belongs to astern and how much to the leaf. In fact, bundles had been said to be of three types, viz., leaf trace bundles, cauline bundles and common bundles.
Leaf trace bundles, as already reported, are connected with the leaves. Cauline bundles (caulis—stem) are those which form the vascular skeleton of the stem and do not enter the leaves. Thus they belong to the stem proper.
Common bundles run through the stem in unbranched condition for some distance and finally terminate as leaf traces. Thus they are common both to the stem and the leaves. In lower vascular plants like Lycopodium and Selaginella the leaves are very small and simple.
The traces are naturally small and superficially connected with the strongly developed vascular skeleton. But higher vascular plants—ferns, gymnosperms and angiosperms. have larger leaf traces. So here the vascular system of the stem is closely associated with the leaves, in fact, forming a vascular skeleton with interconnected leaf traces.
Just above the trace parenchymatous cells, instead of vascular tissues, differentiate up to a limited distance. These parenchymatous regions are called leaf gaps (Fig. 575).
It should be noted that gaps are not breaks in the vascular system, but they are the areas or openings where cortex and pith become continuous. Lateral connections are established below and above the gaps, so that the continuity of the system remains undisturbed.
Leaf gaps are absent in lower vascular plants like Lycopodium, Equisetum, etc., but they are constant in the ferns, gymnosperms and angiosperms.
The number of traces and gaps is variable (Fig. 575A). It is usually one in pteridophytes; one or two in gymnosperms; and one, three, five or many in angiosperms.
Accordingly the common types of nodes in the dicotyledons are said to be unilacunar with one gap and one trace; trilacunar with three gaps and three traces to a leaf; and multilacunar, with many gaps and traces gaps being also known as lacuna.
Anatomists believe that trilacunar condition is more primitive in the dicotyledons and the other two types have evolved either by reduction or amplification in the number of traces (Sinnot, 1934).
This assumption has been refuted (Bailey and others) by many workers. In fact volume of recent researches on nodal anatomy (Gunkel & Wetmore, 1946; Marsden & Bailey, ’56 & others) lead to the reputation of Sinnot’s assumption regarding the primitiveness of trilacunar node.
Four main types of dicotyledonous nodes are now recognised. They are:
(1) unilacunar two-trace, in which the two traces are connected to opposite halves of the eustele;
(2) unilacunar with a single trace,
(3) trilacunar with traces from three gaps; and
(4) multilacunar, in which more than three traces and three gaps per node are present.
The trilacunar type remains a form from which the unilacunar one trace and multilacunar types have been derived. The discovery of the fourth type of nodal anatomy (unilacunar—-two traces) by Marsden & Bailey led to revision of concepts of nodal evolution.
It now appears to be basic in angiosperms. Bailey, Canright (’55) & others have put forward the following reasons in support of their contention:—(a) this condition seems to be wide-spread and basic in vascular plants other than angiosperms, what is expected in case of ‘pteropoid’ origin of angiosperms; (b) a large percentage of cotyledonary nodes appear to have two traces and retain that condition; (c) dicotyledons with many other primitive features exhibit unilacular two-traces or some derivative form, e.g. Austrobaileya showing this condition throughout the entire plant.
Apart from many other families, occurrence of this condition has been curiously noted in
advanced family like Verbenace. So the sequence followed in the evolution of nodal anatomy would be (1) two-traces unilacunar, trilacunar, and multilacunar; or (2) two-trace unilacunar, one-trace unilacunar, trilacunar, and multilacunar. The one- trace unilacunar could have been derived from the trilacunar as well.
In the monocotyledons the vascular bundles follow a different course. The common type of vascular skeleton here is known as ‘palm type’ which occurs in the palms and other monocotyledons (Fig. 574B). The traces supplying a leaf here are numerous.
They are usually of two kinds—small and large. The small ones which pass into a stem from the leaf base are located peripherally in the stem practically encircling it.
The larger traces penetrate up to the centre of the stem in the upper parts and move towards the periphery in the lower, where they fuse with others. Since the penetration of the traces in the stem is not uniform, the bundles appear scattered in cross- section.
The continuations of the vascular system into the lateral branches constitute the branch traces or ramular traces (Figs. 576 & 577). Like leaf traces they also prolong into the axis and ultimately merge with the vascular system.
In dicotyledons and gymnosperms there are usually two branch traces, in some plants there may be one, and in others they may be more than two. Gaps, known as branch gaps, are also present here accompanying branch traces. They occur in all vascular plants having pith. These are larger and more extensive than leaf gaps.
Thus the longitudinal course of the vascular bundles forming a discrete skeleton, is evident from the continuity of the root-stem axis, and occurrence of leaf traces and branch traces which tie up all the parts of the axis and the appendages.
The anatomy of the node is being studied intensively now, particularly in view of its importance in taxonomy and comparative morphology of the organs concerned.
The central core of the axis is called stele. It includes the vascular tissues and the ground tissues like pericycle and pith, when present. These are referred to as intrastelar ground tissues.
The stele remains surrounded by the cortex, what constitutes extrastelar ground tissue, the endodermis being the innermost layer. Both stem and root possess stele enveloped by the cortex. In view of this fundamental similarity in the two organs of the axis in anatomical nature, the stelar theory was proposed in the later part of the nineteenth century.
The theory was readily accepted and it profoundly influenced investigations on comparative anatomy and proved to be immensely helpful in the interpretation of stem anatomy, particularly of the lower vascular plants. The proponents of stelar theory considered endodermis as the innermost layer of cortex and pericycle as the outermost portion of stele.
In recent years some anatomists have suggested that the boundary between the stele and cortex is still doubtful, and so the stelar theory needs a thorough re-examination.
But that at any rate does not minimise the classical importance of the stelar theory, which has been ‘of unmistakable value in emphasising the unity of the structure of vascular system’—as stated by Prof. Esau. Different types of steles (Fig. 578) were recognised, a brief review of which is given here.
The simplest type of stele consists of a solid column of vascular tissues having no pith. This is known as protostele (Fig. 578A). This is the most primitive one from phylogenetic point of view, from which other types have evolved.
In the simplest condition in a protostele xylem forms the core and remains completely surrounded by phloem. In cross- section the protostele may appear as a column—circular, angular, stellate or even irregularly lobed in outline.
Accordingly a few terms have been in use. Protostele with smooth core of xylem is called haplostele, which is considered most primitive (Fig. 579A). That with xylem in form of radiating ribs is known as actinostele (Fig. 579B).
The most advanced type of protostele is one where xylem and phloem intermingle and xylem occurs as separate plates usually lying parallel to one another. This is known as plectostele (Fig. 579C).
The stele of the root of dicotyledons with pithless central column is also regarded as protostele. Some authors called it radial stele (Fig. 578B), in view of radial arrangement of vascular tissues where xylem radiates like arms towards pericycle, phloem patches lying alternatingly.
Protostele is common in the lower vascular plants. It also occurs in the earliest parts of shoot of ferns and in some aquatic plants of the angiosperms.
Haplostele is found in Lycopodium cernuum, Selaginella kraussiana; actinostele in Isoetes coromandeliana, Psilotum triquetrum; and plectostele in Lycopodium clavatum.
The next type is known as siphonostele or tubular stele (Fig. 578 C to F), which is characterised by the differentiation of a pith in the central region.
It is considered to be derived phylogenetically from the protostele and thus represents an advance from the point of view of evolution. This is undoubtedly the most prevailing type in ferns, gymnosperms and angiosperms.
In cross-section siphonostele also shows various outlines. It is of two types, according to distribution of the vascular tissues, viz., ectophloic siphonostele (Figs. 578C & 580A), when phloem occurs on the outer side of xylem, and amphiphloic siphonostele (Figs. 578D & 580B), when the phloem is present both on the outer and the inner side (internal) of xylem.
In simplest cases, as in lower vascular plants, siphonostele has no leaf gaps; in some others the gaps are very small and thus not overlapping, so that a section through the internode shows a continuous ring of vascular bundles.
Such a siphonostele without overlapping gaps is also called solenestele. In ferns leaf gaps are fairly large and overlapping. As a result the whole stelar system is dissected into a netlike structure.
This type is known as dictyostele or dissected siphonostele (Figs. 578E & 580C), and the intervening strands of the vascular tissues, each resembling a miniature protostele and occurring laterally to two overlapping gaps are called meristeles.
As the term dictyostele was used by some early authors in a different sense, modern workers have preferred to use eustele, meaning true stele, for dissected siphonostele, what is very common in dicotyledons and gymnosperms.
In majority of the monocotyledons the vascular bundles remain scattered in the ground tissues, so that the semblance of a stele is lost. This complex type of stele (Figs. 578F & 580D) consisting of dispersed strands has been called atactostele, (atactos, meaning, without any order).
A very complex type of stelar construction is noticed in some pteridophytes. These are called polycyclic steles—having two or more concentric rings of vascular tissues.
Those in the inner cylinder usually form a siphonostele and the outer one is either a solenestele (Fig. 581 A), as in Matonia pectinala; or a dictyostele (Fig. 581 B), as found in Pteridium latiusculum.
The terms morwstele and polystele had been used by early workers. Monostele used to mean one stele with vascular tissues forming a unit structure.
Polystele was applied to the strands of dissected siphonostele, where each strand, particularly one with amphicribral bundle, appeared like a protostele in cross-section.
Thus protostele and unbroken siphonostele had been called monostele, and some dissected siphonosteles were termed polystele. These terms have been discarded now. True polysteles do not occur in living plants.
It is clear from the above discussion that two principal types of steles are protostele and siphonostele. It is generally accepted that siphonostele has evolved from protostele. The methods of evolutionary changes have been controversial.
In fact, two theories had been proposed in this connection. The first theory holds that the central part of the stele remains unspecialised during the process of evolution and ultimately becomes pith.
Thus according to proponents of this theory the origin of pith is intraxylic-—it morphologically belongs to vascular tissues. They have suggested that by metamorphosis of vascular tissues parenchymatous pith has been formed.
This is known as expansion theory—rather an unfortunate expression, because expansion of cortex to pith is not established here. In some primitive plants tracheary elements have been found scattered in parenchymatous pith.
The stele in the rhizome of Ophioglossum lusitanicum (Gewirtz & Fann, 1960) is peculiarly protostelie at the base and slphonostelic at the upper portion (Fig. 581 A) with parenchymatous pith mixed with tracheidal elements.
These are strong points in support of above theory. The second one, known as invasion theory, demands that cortex has invaded the central cylinder during phylogenetic advance in the vascular plants, the leaf gaps and branch gaps being the channels of invasion.
According to this theory pith is cortical in origin and thus does not belong to stele. So pith is extrastelar in nature from morphological point of view.