The following points highlight the five major reasons of anomalous secondary growth in plants. The reasons are: 1. The Activity of Normal Cambium is Abnormal 2. Abnormally Situated Cambium Forms Normal Secondary Vascular Tissues 3. Formation of Secondary Tissues by Accessory Cambium 4. Formation of Interxylary Phloem 5. Formation of Intra-Xylary Phloem.
Reason # 1. The Activity of Normal Cambium is Abnormal:
Cambia of this category function mostly in two ways. Certain segments of cambia cease producing secondary xylem; instead these segments donate secondary phloem only towards exterior. The other segments of cambium produce secondary phloem and secondary xylem normally.
As a result a ridged and furrowed stele is formed (ex. Bignonia). In other cases the interfascicular cambium forms non-vascular tissues. Vascular- tissue formation is restricted to fascicular cambium only (ex. Aristolochia, Tinospora, Clematis etc.).
Mature branches of the different species of Bignonia (Fig. 22.1) exhibit (in t.s.) wedges of bast in xylem and separate xylem-masses owing to fission. These anomalies are not present in the very young branches that show a ring of secondary xylem where the vessels are with small lumina.
Mature branches of Bignonia have four wedges of phloem that form a definite pattern. The wedges of bast form an orthogonal cross if it is imagined that wedges of phloem continue as far as pith and intersect with one another at right angles at the pith.
The primary vascular bundles of Bignonia capreolata are conjoint, collateral and open. The stems have normal ring of primary vascular bundles and the bundles are of different sizes. Fascicular cambium exists in the primary vascular bundles. During intrastelar secondary growth the interfascicular parenchyma, by dedifferentiation, forms interfascicular cambium.
Fascicular- and interfascicular cambium together form a normal cambium ring. Initially the stem shows normal cambial ring and normal ring of vascular bundles. All the segments of cambium ring produce secondary phloem outside and secondary xylem inside, the latter being in excess of secondary phloem.
The xylem has vessels with small lumen. As soon as the cambial ring starts producing vessels with large lumen the formation of wedges of phloem commences. Four opposite small segments of cambial ring cease to form secondary xylem. Instead, these segments donate secondary phloem outside only.
The four alternate segments of cambial ring continue to produce normal secondary xylem and phloem. As a result four wedges of phloem are formed in the cylinder of secondary xylem. The wedges of phloem are symmetrically arranged and corresponding in position to the larger primary vascular bundles.
As the stem grows in thickness the wedges of secondary phloem become deeper in xylem. At this stage the vascular cambium is no longer in the form of a complete ring. It is split into eight strips. Four of them occur at the bottom of the wedges of phloem. The rest are present at the peripheral margin of the projected woods that lie alternate to wedges of bast.
Most of the species of Bignonia have four wedges of phloem. In other species more than four wedges of secondary phloem appear (e.g. Bignonia aequinoctialis) between the original four. This happens after the projecting portion of secondary xylem has grown in thickness for some time by the entire cambial strips occurring external to secondary xylem.
The development of additional four wedges of phloem occurs when small cambial segments from the cambial strips that occur at the margins of projected wood suddenly begin to produce increased amount of bast and reduced amount of wood. The newly formed wedges of phloem do not penetrate so deeply into the secondary xylem as the original wedges.
The recently formed wedges form angles of 45° with original wedges (Fig. 29.16A). Thus four more wedges may be developed bisecting the original four interspaces. As a result a total of eight wedges of phloem are developed. If the same phenomenon is repeated at frequent intervals more wedges of bast may be observed in the transverse section of stem at successive stages.
Schematic illustrations of the anatomy of stele of lianas in cross-sectional view.
In transverse section the appearance of the wedges of bast may be of two kinds. In simpler case the wedges of bast are of equal thickness throughout. The boundary between such wedges of phloem and the adjacent wood is a straight line (e.g. Bignonia capreolata). In others the wedges of bast widen from base upwards and the line of separation of wood and bast is like a staircase, e.g. Bignonia aequinoctialis.
This is due to successive broadening of the bast-wedges from base to periphery. The bast-wedge is in the form of an inverted arrow. The sequential change in the activity of small cambial region of normal cambium results in the development of such wedge-bast.
In another anomaly the mass of wood exhibits fission, e.g. Bignonia catharinensis stem of sufficient age. The xylem-mass has wedges of bast from the beginning. At later stages fission occurs by cell-division and dilatation in the parenchyma of the wood and pith.
Anomaly in the stele of the different species of Bignonia stem is due to presence of followings that form the characteristics of the genus:
i. Four or more wedges of phloem that are traversed by bast fibres.
ii. The peripheral boundary of wood is ridged and furrowed.
iii. The vascular cambium is not in the form of a continuous ring.
iv. The cambia are in the form of strips and the number of strips corresponds to the sum total of projecting woods and wedges of bast.
v. The cambium ring does not function uniformly around the stem.
vi. Occurrence of wedge-bast at regular intervals.
vii. Cambium is confined to the inner surface of wedge-bast and to the peripheral surface of the projecting portions of wood.
viii. Cambium is absent from the radial surfaces, i.e. the lateral boundary between wood and wedge-bast.
ix. As cambium is absent from the lateral boundary between wood and wedge-bast the wedge of phloem appears to be sliding along the lateral surfaces of the furrow during growth in thickness.
x. In fresh materials of Bignonia stem it is observed that fissures appear on the lateral surfaces of wedge-bast that subsequently close again. This is due to ease of sliding of wedge-bast in the furrow.
The stems of Bignonia, Doxantha etc. of sufficient age, in addition to original wedges of bast, exhibit split or fissured xylem. The cells of pith and xylem parenchyma divide and expand to split the xylem mass. The wood of Bignonia is dichotomously split.
In some other members of Bignoniaceae such as Paragonia (Fig. 22.2), Pyrostegia, Petastoma, Clytostoma, Amphilophium etc. a normal cambium ring is formed and it functions normally until a thin ring of secondary phloem and secondary xylem is formed at the peripheral and center side respectively.
After a brief period of activity four small segments of cambium, located opposite to each other, become unidirectional, i.e. produce secondary phloem on the peripheral side only. These segments of cambia do not produce secondary xylem at all and as a result remain stationary within the stem and push out narrow wedges of secondary phloem on the peripheral side.
The other four cambium segments, those situated alternate to unidirectional cambia, continue as bi-directional meristem, i.e. produce secondary xylem and phloem normally.
These four bi-directional cambia gradually move outward as more and more secondary xylem are produced. After a period of activity, it is noticed that the four unidirectional cambia are deep within the secondary xylem masses, which are produced by the bi-directional cambia.
The narrow wedges of secondary phloem, formed by the unidirectional cambia, slide past the secondary xylem on their sides. In this way, the shoot grows and secondary growth continues. After the shoot grows for a period, small segments within the bi-directional cambia are converted to unidirectional one, which also produces narrow wings of secondary phloem only on the peripheral side.
This process is repeated several times. In mature and large stems, where this process has occurred many times, it is noticed that there are several strips of unidirectional cambia situated at different sites of varying depths in the secondary xylem. These cambia have produced and pushed out narrow wings of secondary phloem.
Usually, in Bignonia and Doxantha, the numbers of wedges of phloem correspond to the numbers of orthostichies present in the stem and the furrows alternate with the orthostichies. An experiment was performed on Doxantha unguis-cati (Fig. 22.3) with four orthostichies to test the relation between the orthostichies and the wedges of phloem with very interesting results by Dobbins, 1969.
Removal of leaves from two or four orthostichies was used as experimental material to study the activity of the vascular cambium. In the control plant, where no leaves were removed, the four wedges of secondary phloem were produced. In the experimental plant from where leaves from all four orthostichies were removed, the wedges of phloem were not formed and little amount of secondary tissues was developed.
In the other plant, from which leaves were removed from two successive orthostichies only, the wedges of bast were not produced at the two large primary vascular bundles corresponding to the treated orthostichies. These observations clearly indicate the effect of leaves on the activity of vascular cambium.
In Aristolochia stem anomaly in the stele is due to the production of secondary ray parenchyma at the interfascicular region by interfascicular cambium.
The stele of Aristolochia is siphonostele. Endodermis delimits stele on the peripheral side. Just below the endodermis there occurs a cylinder of sclerenchyma that is composed of fibrous cells. As the fibres are located on the periphery of the vascular cylinder it is referred to as perivascular fibre.
Perivascular fibres are not related ontogenetically to phloem. Parenchyma occurs in between perivascular fibre and vascular strands. According to stelar terminology perivascular fibre and the subjacent parenchyma are to be referred as pericycle.
The primary vascular system of stem consists of conjoint, collateral and open strands. In transverse section the vascular bundles are arranged in an oval ring surrounding parenchymatous pith. The individual bundles are of different sizes, wedge-shaped and remain separated by wide interfascicular regions. The smallest bundles are the trace bundles.
Each vascular bundle has fascicular cambium on the periphery of which there occurs the primary phloem. Primary xylem is situated inside the fascicular cambium. At the onset of secondary growth the parenchyma cells present at the primary medullary ray become meristematic.
The meristematic cells originate at the level of fascicular cambium by dedifferentiation. This secondary meristem is the interfascicular cambium. Fascicular- and interfascicular cambium join with each other thus forming a continuous cambium ring.
Production of secondary vascular tissue is restricted to fascicular cambium only. The fascicular cambium divides tangentially and the inner derivative cells form the secondary xylem mother cell. The peripheral derivative cells are the secondary phloem mother cell. Secondary xylem mother cell, secondary phloem mother cell and fascicular cambium form a cambial zone.
The secondary xylem mother cell and secondary phloem mother cell later differentiate into secondary xylem and secondary phloem respectively. As a result of formation of secondary vascular tissues, the primary phloem is pushed towards the peripheral side and the primary xylem is pushed inside.
The primary phloem and secondary phloem consist of sieve tubes, companion cells and phloem parenchyma. Phloem parenchyma is abundant and is arranged in radial system. Sieve tube and companion cell compose the axial system. The most conspicuous feature is that phloem contains no fibres. In secondary phloem sieve tubes and phloem parenchyma occur as bands.
Bands containing sieve tubes and associated parenchyma alternate with tangential bands of parenchyma. At later stages sieve tubes cease to function and get crushed by the formation of more secondary phloem. As a result a conspicuous banding appears in the phloem where uncompressed parenchyma alternates with compressed cells.
In the xylem all four elements are present where vessel, tracheids and xylem fibre are arranged in axial system. The radial system consists of abundant xylem parenchyma. Vessels consist of protoxylem and conspicuous metaxylem. The interfascicular part of cambium forms rays that are made up of parenchyma cells only.
The parenchyma is similar to that present in the primary interfascicular region. Interfascicular cambium produces ray parenchyma both inside and outside. As a result the vascular strands remain discrete. Ray parenchyma cells occur in radial rows and thus they compose the radial system of stems.
The markings of growth increment are conspicuous in secondary xylem. The elements of xylem formed in the early and late part of growth season exhibit size difference (Fig. 29.15A). Rays associated with xylem also exhibit such size difference. The tissues formed at the late part of growth season are relatively smaller than those formed in the early part of growth season.
Schematic illustrations of anatomy of stele of lianas in cross sectional view.
As a result of secondary growth the stem increases in circumference. The individual vascular strand widens towards the periphery. It is observed that new rays are interpolated into widening of vascular wedges. Continuous secondary growth causes squeezing of pith. At later stage pith cells and the associated ray cells are partly crushed.
The continuous cylinder of perivascular fibres offers resistance to the expanding vascular system. As pith and medullary rays form a continuous structure, the pith cells and associated ray cells are crushed probably owing to resistance offered by the perivascular fibre. Secondary vascular tissue formation continues and the cylinder of perivascular fibre has not enough strength to resist the expanding vascular strands.
Eventually the cylinder ruptures (Fig. 29.15A). In happens mostly in front of rays and the adjacent parenchyma cells fill up the breaks. These parenchyma cells in some species differentiate into sclereids that function like perivascular sclerenchyma.
In some species of Aristolochia, in old stems, xylem groups become fissured owing to the development of medullary rays. The most conspicuous feature of cambium ring is that certain cambial segments form ray-like parenchyma only. As the stem increases in circumference due to production of secondary vascular and non-vascular tissues new cambial segments develop and these cambial segments also donate rays of parenchyma.
Thus a fluted vascular cylinder originates. In Aristolochia triangularis (Fig. 22.4A) the vascular bundles have forked structure towards periphery owing to development of rays of parenchyma by certain cambial segments. So the vascular cylinder has fan-shaped appearance.
In Tinospora stem the anomaly in the stele is due to formation of secondary ray parenchyma at the interfascicular region by interfascicular cambium.
The stele of Tinospora stem is siphonostele (Fig. 22.4B). Endodermis delimits stele on the peripheral side. Just below the endodermis there occurs a cylinder of sclerenchyma. This is pericycle and is composed of many layers of cells. In some regions the sclerenchymatous pericycle arches over the vascular bundles.
In the stele the primary vascular bundles are arranged more or less in a ring and remain surrounding central parenchymatous pith. Each vascular bundle is conjoint, collateral and open. The vascular bundles remain separated by wide interfascicular region.
The intra-fascicular cambium has primary phloem on the outside and primary xylem inside. During secondary growth certain cells of the interfascicular region at the level of intra-fascicular cambium become meristematic by dedifferentiation. These secondary meristematic cells are the interfascicular cambium. The interfascicular cambia join with intra-fascicular cambia and thus a continuous cambium ring is formed.
All segments of cambium ring do not function normally. The intra-fascicular part of cambium divides tangentially. The inner derivative cells are xylem mother cells. The peripheral derivative cells are phloem mother cells. Xylem and phloem mother cells together with intra-fascicular cambium form a cambial zone within vascular bundles.
Later phloem mother cell differentiates into secondary phloem and xylem mother cell differentiates into secondary xylem. As a result of production of secondary vascular tissues the primary xylem is gradually pushed towards the centre and the primary phloem is gradually pushed towards the periphery. The interfascicular part of cambium ring forms rays of parenchyma only.
The interfascicular cambium donates parenchyma both inside and outside. Production of secondary vascular tissues is restricted to individual vascular bundles only. Each vascular bundle with primary and secondary vascular tissues remains separated from other vascular bundles by wide interfascicular regions composed of primary and secondary parenchyma rays. Thus a fluted stele is formed.
The above type of anomalous secondary growth is also observed in Clematis (Fig. 22.4C) and Vitis etc. It is to note that in Clematis, Vitis, Bignonia, Aristolochia and Tinospora the anomalous secondary growth starts from a single normal cambial layer with abnormal activity.
Prestonia macrocarpa, Bauhinia divaricata and B. sericella exhibit flat and ribbon like stem at maturity. The stele is flattened strap shaped (Fig. 22.5). At the time of secondary growth a normal cambium ring arises and it forms secondary xylem and phloem towards inner and peripheral side respectively in normal way.
At early stage when little amount of secondary tissues are formed, the vascular cylinder is more or less round and so the shoot is also round. The secondary xylem consists of small tracheary elements all around the pith uniformly. After a period of activity two opposite sides of cambium becomes more active, and the alternate sides of it becomes less active.
The secondary xylem produced by the alternating segments of cambium is different. The secondary xylem, present at the less active region of cambium, shows small tracheary element. The xylem at the more active region of cambium exhibits vessels with large lumen.
As growth continues the stele gradually becomes flattened strap or band shaped and the shoot becomes flat and ribbon like. This type of stem is usually observed in those species of Bauhinia, which are vines. By becoming flat, they maintain the ability to flex and increase conductivity by forming vessels with large lumen.
Figure 22.5 Transverse section of bauhinia stem (diagrammatic).
Reason # 2. Abnormally Situated Cambium Forms Normal Secondary Vascular Tissues:
Cambia of this category produce secondary vascular tissues in normal fashion, but their positions are anomalous. These cambia form discrete vascular cylinders and their arrangements differ according to species.
Most of the climbing species of Serjania exhibit a principal vascular cylinder surrounded by small peripheral vascular cylinders. The vascular cylinders and the pericycle compose the stele. Endodermis delimits the stele on the peripheral side.
In Serjania stem several cambial layers are present from the first. In the cross- section of stem the cambia are arranged in various ways. Different species of Serjania (Fig. 22.6) exhibit the various arrangements of cambia. In the stele of Serjania caracasana (Fig. 22.6D) stem a central principal cambial cylinder occurs.
Several minor cambial cylinders surround the principal cambial cylinder. In Serjania corrugata (Fig. 22.6B) there is no central cambium. In the stele five to seven peripheral cambial cylinders occur in a circle and the cambia are approximately equal. In all species individual cambial layer behaves in normal fashion, i.e. donates secondary xylem on the inside and secondary phloem on the peripheral side.
As a result several cylinders of secondary vascular tissues are formed. Each vascular cylinder consists of a cambial cylinder, secondary phloem on the peripheral side of cambium and secondary xylem on the inside of cambium that encloses parenchymatous pith. The vascular strands remain separated by parenchyma.
All species of Serjania have a continuous ring of sclerenchyma enclosing all vascular strands. The ring is composed of fibrous cells and is wavy in climbing species. The ring occurs at the outer limit of bast. This wavy fibrous cylinder is regarded as pericycle.
Procambial strand differentiates into primary vascular bundles. Normally primary bundles differentiate in a circle. In case of Serjania it is observed that primary leaf-trace bundles at certain points of circle are indented. These bundles tend to be abstricted from the circle of vascular bundles.
Thus individual groups of primary leaf-trace bundles are formed. During secondary growth a continuous cambial cylinder develops linking the absricted groups of primary leaf-traces. As a result several separate cambial cylinders develop in the developing stele. This happens at a very early stage of development.
The individual cambial layer behaves in normal fashion and ultimately gives rise many vascular cylindricalstrands in the stele. The principal cambial cylinder forms the central vascular strand of the stem. The peripheral cambial cylinders form vascular cylinders that surround the principal vascular strand. It is to note that like Bignonia the cleavage of wood in Serjania is not a secondary process.
The formation of several cylinders of wood is predetermined. Apical meristem forms procambial strands that differentiate into primary vascular bundles where cambial cylinders develop. So the stele of Serjania consists separate woody cylinders from the beginning.
The lianas, belonging to the family Sapindaceae, show the following four types of anomalous structure depending upon special arrangements of vascular bundles at their origin:
i. The cleft xylem mass:
Ex. Urvillea laevis (Fig. 22.6A) and Serjania piscatoria. The mature stem shows either deeply lobed stele or separated vascular strands, each possesses a vascular cambium and discrete pith. The primary body of these stems shows normal structure but superficially grooved at certain regions due the presence of depressions in the cortex.
At the onset of secondary growth, strips of cambia appear below the grooves and the axis subsequently split into portions corresponding to the number of grooves. Each of the split portions possesses its own ring of cambium, through the activity of which the stem grows in thickness. Usually the stem is split into three or more portions. At maturity, the axis becomes ribbed due to unequal development of xylem at five or more points.
ii. The compound xylem mass:
Ex. Serjania Juscifolia, Paullinia etc. The stem or branch exhibits a central ring of vascular bundles present at the middle. There are several peripheral rings of vascular bundles present around the central ring. Small amount of cortical parenchyma occurs between the central and peripheral ring of vascular bundles.
The number of peripheral bundle ring varies from three to ten and they are placed closely side-by-side. The central and peripheral bundle rings of vascular bundles possess pith and have their own cambium ring, through the activity of which the stem or branch grows permanently in thickness.
This cambium ring forms secondary xylem and phloem towards inner and peripheral side in normal way. The vascular bundles of central and peripheral rings join each other at the nodes. The leaf traces at first run through the central ring and subsequently pass into one of the peripheral rings.
At maturity, the stem or branch appears to be a cable like structure. This is an adaptive type of anomaly where the plant is benefited when exposed to stretching or torsion. In an estimate, it is recorded that this type of anomaly occurs in 91 out of 172 species of Serjania and 16 out of 122 species of Paullinia.
iii. The divided xylem mass:
Ex. Serjania corrugata (Fig. 22.6B). The young stem possesses five to seven bundle rings and they lie side-by-side in a circle. Each bundle ring is in the form of incomplete circle and encloses pith at the centre. In young stems, the peripheral pith is continuous with central pith.
The bundle rings possess permanent growth in thickness by a ring of cambium present in them. The divided xylem mass, at maturity, encloses pith. In contrast to compound, the divided xylem mass exhibits no central ring vascular cylinder.
iv. The corded xylem mass:
Ex. Thinouia (Fig. 22.6C). The stem exhibits a central and around it several peripheral rings of vascular bundles. Each vascular cylinder possesses ring of cambium with bast and wood that encloses pith. At young stage, the stems grow in thickness with central vascular cylinder only, i.e. by the appearance of a normal cambium ring with normal activity.
This normal growth in thickness occurs up to the fifth or sixth year after which new strands of accessory cambia originate at the cortex external to the original vascular ring. The central vascular cylinder still continues to grow. The new strands of accessory cambia, though abnormal in position, produce normal phloem and xylem towards exterior and interior respectively. The ring of vascular bundles encloses pith at the centre.
The newly formed peripheral ring of vascular bundles is connected with one another but remains separated from the original vascular ring. This anomaly also occurs as a secondary complication in older stems of a few species of Serjania and Paullinia, which exhibit compound or divided xylem mass. The newly formed secondary vascular cylinder may be cylindrical (e.g. Thinouia) or flat transversely (e.g. Paullinia). At maturity, the stems appear as cords.
Reason # 3. Formation of Secondary Tissues by Accessory Cambium:
Baugainvillea, Amaranthus, Boerhaavia, Achyranthes, Celosia etc. exhibit accessory cambia that form vascular and non-vascular tissues. This type of anomaly is illustrated below taking the example of Boerhaavia diffusa.
In Boerhaavia diffusa (Fig. 22.7) endodermis delimits stele of stem. All segments of endodermis are not always distinct. Just below the endodermis there occurs the pericycle. It is narrow and consists of one or two cell layers. In older stems just below the endodermis a few scattered fibres are present.
The position of fibres is of special interest because they locate the endodermis when the latter is indistinct. The other structures of stele include parenchymatous ground tissue, conjunctive tissues and primary vascular bundles.
A cross-section of young stem shows that the primary vascular bundles are of different sizes. The bundles are arranged in three rings. All vascular bundles are derived from procambial strands.
A young stem of Boerhaavia diffusa exhibits (in t. s.) two large central vascular bundles in the innermost ring. These are the largest bundles in the stem. The bundles have a tangential diameter about twice as long as the radial. Each vascular bundle is conjoint, collateral and has intra-fascicular cambium that behaves in normal fashion. The cambium divides tangentially and the inner derivative cells differentiate into secondary xylem.
The peripheral derivative cells form secondary phloem. The secondary vascular elements are arranged in axial and radial rows. Production of secondary vascular tissues is restricted to individual vascular bundles only. Each vascular bundle has limited amount of secondary vascular tissue formation.
As secondary growth continues the primary phloem cells become crushed. The dead remnants of crushed phloem appear as cup over the later formed secondary phloem. No interfascicular cambium develops in the inner ring of vascular bundles.
The middle ring consists of six to fourteen loosely arranged primary vascular bundles. The bundles are conjoint, collateral and open. Little amount of secondary vascular tissues are produced in them by intra-fascicular cambium. Interfascicular cambium is not formed in the middle ring of vascular bundles.
The outermost ring consists of 15-20 or more primary vascular bundles. The bundles are small, even minute and occur at the periphery of stele. Each bundle is conjoint, collateral and open. These bundles are also formed from procambial strands but are belated in development in comparison to vascular bundles of other two rings.
Each vascular bundle has intra-fascicular cambium. Interfascicular cambium develops at interfascicular region by dedifferentiation. Interfascicular cambium joins with intra-fascicular cambium on both of its lateral sides. This results in the formation of a complete cambial cylinder. It is this cambial cylinder that is responsible for all subsequent anomalous secondary growth.
The cambial cylinder is very active. It divides tangentially. The intra-fascicular part of cambial cylinder forms secondary vascular tissues only. The peripheral derivatives form secondary phloem. The inner derivatives of intra-fascicular cambium differentiate into secondary xylem. The interfascicular part of cambial cylinder also divides tangentially.
The peripheral derivatives form parenchyma cells. The inner derivatives differentiate into conjunctive tissue and storage parenchyma. The conjunctive tissues are always formed internally. It consists of elongated living cells that later are transformed to sclerenchyma by lignin deposition on their walls. These sclerenchyma cells are similar to fibres and serve for food storage.
With further secondary growth the cambial cylinder donates a broad zone of anomalous wood inwardly. The anomalous wood consists of secondary xylem formed by intra-fascicular cambium, lignified conjunctive tissue and lignified adjacent cells of pith. In mature stele lignified conjunctive tissues and secondary xylem become confluent forming a zone of wood.
The activity of original vascular cambial cylinder after a period declines. Another cambial cylinder arises at the peripheral parenchyma cells formed by the original cambial cylinder. This new cambium donates conjunctive tissues inside and parenchyma outside.
Certain segments of new cambium form secondary xylem inside and secondary phloem outside. These secondary vascular tissues are always formed opposite to each other; as a result collateral vascular bundles are formed.
After a period of activity the function of new cambium ceases. Then another new additional cambium cylinder arises from the peripheral parenchyma cells produced by its predecessor.
The function of new additional cambium is similar to that of the previous cambial cylinders. In this way four to five additional cambial cylinders originate and accordingly growth rings are produced. All the supernumerary cambia have the same function. As a result the anomalous wood broadens.
It is to note that in the stele of Boerhaavia diffusa the original cambial cylinder does not develop de novo in the pericycle during secondary growth. Instead the cambial cylinder is formed by the union between intra-fascicular cambium of peripheral small bundles and interfascicular cambium formed at interfascicular region of peripheral bundles. Later additional/accessory cambia originate from parenchyma produced by original cambial cylinder or from the parenchyma of their products.
In Chenopodium, isolated phloem patches termed phloem islands are found embedded in the conjunctive tissues, usually above the clusters of vessels. These phloem patches are formed centrifugally by the accessory cambium. Later there arise arcs of new cambium that forms conjunctive tissues to the inside and buries the phloem.
Reason # 4. Formation of Interxylary Phloem (Fig. 22.9):
The secondary phloem strands or layers that remain surrounded by secondary xylem are referred to as interxylary phloem. Interxylary phloem is also termed as included phloem/interxylary soft bast as it remains embedded in wood. Eames and MacDaniels illustrated the following two methods by means of which interxylary phloem becomes embedded in secondary xylem.
In Combretum, Leptadenia (Figs. 22.8B & 22.9A-D) and Entada, during secondary growth a normal cambium ring is formed by the union of intra-fascicular and interfascicular cambium. The cambial ring functions normally, i.e. produces secondary xylem inside and secondary phloem outside.
Subsequently certain small segments of cambial ring donate secondary phloem toward the inside for a brief period. Normally these segments produce secondary xylem. During the production of interxylary soft bast these segments form secondary phloem in place of secondary xylem.
The production secondary phloem continues and after a brief period of such activity these abnormal segments of cambial region regain their normal activity, i.e. donate secondary xylem inside. As a result the inwardly formed phloem gets buried in the secondary xylem. It is to note that the islands of soft bast are developed from the inner derivative cells of cambium.
The other form, e.g. Strychnos (family: Strychnaceae) has siphonostele in the stem. The primary vascular bundles are conjoint, open and bicollateral. The inner phloem of the vascular bundle is referred to as intra-xylary phloem or intra-xylary soft bast. To avoid confusion with interxylary phloem the term internal phloem replaces intra-xylary phloem.
Internal phloem is normally primary in origin and develops from procambial strands along with primary outer phloem and primary xylem. The internal phloem of Strychnos is in the form of isolated strands that form a ring completely surrounding the pith. In certain species the isolated bast occurs opposite all or some of the vascular bundles of stem.
During secondary growth a normal cambial ring is formed by the union of intra-fascicular and interfascicular cambium. The cambial ring has normal function and forms secondary xylem toward inside. Secondary phloem is produced toward outside of stem. The formation of secondary tissues continues for some time.
The interxylary phloem appears at a later stage of the growth of wood. During secondary growth certain small segments of cambial ring form the peripheral phloem strands toward the outside as a part of their normal function. Later the phloem strands become embedded in the secondary xylem in the following way (Fig. 22.9E-H).
Such segments of cambium cease to be active. The rest of the cambial segments continue to produce secondary vascular tissues. The derivative cells of once active cambial segments differentiate into secondary vascular tissues. Thus the original cambial ring becomes interrupted. Later it is repaired by the production of new complementary strips of cambia on the outer side.
Strips of cambia arise opposite the cambial segments that cease to function. The new cambial strips develop as secondary meristem. They originate either at the pericycle or in the phloem that is few rows away from the original cambium. Strips of cambia and the active segments of original cambia unite with their edges thus forming a complete cambial ring.
This cambial ring has normal activity. The secondary xylem produced by this cambial ring encloses the secondary phloem formed by the once active original cambial segment. This process is repeated several times and as a result numerous scattered strands of interxylary phloem are formed embedded in secondary xylem.
It is to note that in Strychnos both internal phloem and interxylary phloem are formed. The former is primary in origin and is regarded as primary anomalous structure. Interxylary phloem is formed as a result of anomalous secondary growth and the islands of soft bast in the wood are formed by the cambium on its outer side. So the interxylary phloem is the normal secondary soft bast.
Interxylary phloem is also formed in several species of Thunbergia stem. The islands of soft bast occur in the form of bands in the wood (Fig. 22.8A). The characteristic of stem is that the bands of soft bast only arise in the interfascicular region.
The bands remain surrounded by secondary xylem. The characteristic of wood is that vessels only occur at intra-fascicular region. The wood present at interfascicular region above and below of interxylary phloem consists of fibres and tracheids only.
Interxylary phloem of Thunbergia is formed either from the inner derivative cells of cambium (like Combretum) or from the outer derivative cells of cambium (like Strychnos). In Thunbergia coccinea, T. grandiflora, T. parva, T. sinuata, T. mysorensis etc. the islands of soft bast are developed from the inner side of the cambium.
In T. capensis, T. cyanea, T. hispida, T. hirta etc. wedges of bast are formed and from these some of the islands arise. ‘These wedges are partly formed by tissue produced from the inner side of the cambium and partly by tissue given off externally by the cambium’-Solereder. It is to note that in the latter case the interxylary phloem is the normal secondary soft bast.
Reason # 5. Formation of Intra-Xylary Phloem (= internal phloem):
The cylinder of phloem strands or isolated phloem strands that occur below xylem strands towards pith are termed as intra-xylary phloem. Committee on Nomenclature (1957) suggests the term internal phloem in place of intra-xylary phloem. Isolated phloem strands occur in Calotropis, Strychnos etc. Cylindrical phloem strand is present in Asclepias curassavica.
Normally internal phloem is primary in origin, i.e. it develops from provascular strands. Exceptions are observed in Campsis radicans (=Tecoma radicans) and Campsis grandiflora (=T. andrepens) where internal phloem is secondary in origin. In the above mentioned species internal phloem is developed as a result of anomalous secondary growth.
The stele of Tecoma radicans is siphonostele. The primary vascular bundles are arranged more or less in a ring and each vascular bundle is collateral, open with protoxylem endarch. During secondary growth intra-fascicular cambium and interfascicular cambium unite to form complete cambium ring. The cambium ring has normal function, i.e. produces secondary phloem on the peripheral side and secondary xylem towards inner side, the secondary xylem being in excess of secondary phloem.
As a result primary vascular bundles are pushed towards the centre. After a period of activity two strips of additional cambia originate below secondary xylem on two opposite sides of pith. Each strip of cambium functions abnormally. Each cambial strip donates secondary phloem towards the centre or pith side and secondary xylem towards the peripheral side.
As growth continues, two arcs of secondary vascular bundles are formed at the margin of pith. The bundles show inverse orientation of wood and bast. The bast that develops towards pith side is referred to as intra-xylary phloem/internal phloem. Subsequently the two patches of intra-xylary phloem crush the pith cells to a narrow band.
In developing stems the primary vascular bundles are conspicuous towards the inner margin of secondary xylem formed by the normal cambial ring. A few layers of parenchyma cells are observed between the wood formed by the normal cambial ring and additional cambium/accessory cambium.
It is to note that anomaly in the stele of Tecoma is due the development of two arcs of medullary bundles with inverse orientation of wood and bast in contrast to normal vascular bundles. Anomaly in the stele is also owing to the formation of internal phloem/intra-xylary phloem by additional/accessory cambium during anomalous secondary growth.
Anomalous secondary growth in Dracaena stem:
Dracaena is arborescent in habit, and belongs to the monocotyledonous family Agavaceae. The vascular bundles of monocotyledons are closed, i.e. intra-fascicular cambium is absent. So monocotyledons lack normal secondary growth from a vascular cambium. In Dracaena the stele is atactostele.
The primary vascular bundles are distributed over the ground tissue without having any definite arrangement. The primary vascular bundles are not compactly arranged on the ground tissue and the interfascicular region is moderately wide. The ground tissue is composed of parenchyma cells and the parenchyma shows no radial seriation of cells.
Each primary vascular bundle is leptocentric/amphivasal, i.e. the xylem completely surrounds phloem and there exists no intra-fascicular cambium. Each vascular bundle is circular or oval in transverse section. The xylem consists of tracheids only. Protoxylem with annular and spiral thickening is present.
The cross-section of mature stem of Dracaena, where certain amount of secondary growth has occurred, exhibits secondary vascular bundles and ground tissue commonly termed as conjunctive tissue. Conjunctive tissues are parenchymatous, the walls of which are thin and may sometimes become thickened or even lignified.
The conjunctive tissue exhibits radial arrangement of cells and thus aids in differentiation from the primary interfascicular ground tissue. The secondary vascular bundles are to some extent arranged in radial rows. The vascular bundles are more or less compactly arranged and anastomose in some regions in contrast to primary vascular bundles.
Each vascular bundle is oval in t. s. and leptocentirc/amphivasal like primary vascular bundles. The secondary phloem is small in amount in comparison to primary phloem. The secondary phloem elements are sieve tubes, companion cells and phloem parenchyma. The sieve tubes are short with transverse end walls and the sieve plate is simple.
The tracheary elements are composed of tracheids and xylem parenchyma. The tracheids are long and the associated xylem parenchyma may be lignified. The tracheids exhibit scalariform thickening and lack annular and spiral protoxylem. The secondary bundles appear to lie embedded in the conjunctive tissues. The secondary tissues (secondary vascular bundles and conjunctive tissues) surround the primary vascular bundles and ground tissue.
Internally the stele of Dracaena stem before secondary growth remains surrounded by many layered parenchymatous cortex. The peripheral layer of stele is the pericycle that cannot be easily distinguished. The secondary growth of Dracaena is brought about by a special type of vascular cambium, also termed as secondary thickening meristem.
This cambium originates in the older regions of stem that consists of parenchyma cells situated outside the vascular bundles. Cambium originates either from cortical parenchyma or pericycle. The cambium contributes to the growth of stem in diameter. The cambium is active in the part of axis that has ceased to elongate. The cambium does not function like the vascular cambium of dicotyledons.
Each cambial cell, as seen in I. s. may be fusiform with both ends tapered or rectangular. Sometimes one end of a cambial cell may be truncated and the other end is tapering. The cambial cells divide tangentially. Initially the cambial cells donate cell towards inside only. Later a small amount of tissue is produced towards outside.
Tangential divisions in individual cambial cell continue resulting in the formation of radial series of derivatives. The peripheral derivative cells may continue tangential divisions. Some of the derivatives may cease to divide. These cells then transform to secondary cortex. The inner derivate of cambial cells forms a radial series of cells.
Some of the derivative cells differentiate into thin walled parenchyma cells that form ground tissue. The ground tissue is termed as conjunctive tissue. Later the thin walls of ground tissue may be thickened or lignified. The other derivatives of cambial cells differentiate into vascular strands. The cambial initials form longitudinal files of single cell where vascular bundles develop.
Anticlinal, periclinal and irregular longitudinal divisions in the files of cells result in the formation of leptocentric vascular bundles. The vascular bundles interrupt the radial seriation of conjunctive tissues that are interfascicular secondary parenchyma. Each cell of parenchyma has moderately thick and pitted wall. So each cell of secondary parenchyma may be regarded as conducting parenchyma.
It is to note that in Dracaena anomalous secondary thickening is brought about by a special cambium—termed secondary thickening meristem. Due to the activity of this meristem conjunctive tissue and secondary vascular bundles originate. The continuous activity of the meristem results in the formation of indefinite amount of secondary tissues. Sometimes weakly developed growth rings are observed.
But the relation between rings to annual growth is yet to be established. Continuous formation of secondary tissues results in the increase of diameter in Dracaena stem. It is reported that in Dracaena draco, the stem attained a girth of 45 feet. The height of the tree was 70 feet and the age was estimated to be of six thousand years old.
Haberlandt grouped all the anomalous forms of secondary growth into two categories, namely adaptive and non-adaptive. The adaptive anomaly is an adaptation to definite external conditions. In contrast the non-adaptive anomaly is not an adaptation to definite external condition (ex. Amaranthus, Mirabilis, Boerhaavia, Chenopodium etc.). It merely represents variations of design.
The adaptive anomalies are the characteristic of plants that are lianas [ex. Aristolochia, Tinospora, Bignonia, Serjania and Bauhinia (Fig. 29.16B & C) etc.], plants that have fleshy roots (ex. beet root, Fig. 29.17) which principally serve for storage and plant with submerged stem where cambium devotes most of its derivatives to produce tissues that add buoyancy (ex. Aeschynomene aspera, A. indica and Herminiera etc.).
Inextensibility, inflexibility and incompressibility are the essential mechanical requirements of a liana. A climbing plant is often thrown into a number of folds or frequently becomes extensively twisted over a living supporting organ.
It hangs freely and so is exposed to pulling strain. As a result a twiner becomes inextensible. Sometimes the hanging stems are shaken by violent winds. So a liana becomes inflexible. The twiners are often exposed to radial compression. This happens when supporting organ grows in thickness. All the above mentioned mechanical requirements of Iiane-stems are achieved by adaptive anomalous secondary growth.
The stelar organization of liane-stems is constructed like a twisted rope or cable. This cable-like structure possesses high degree of strength combined with pliancy. In a liana pliancy is often accomplished through splitting of the lignified xylem into isolated strands.
Pliancy is also achieved by the interpolation of softer tissues like secondary phloem into lignified xylem. The anatomical features like splitting of xylem and interpolation of softer tissues into xylem is common to all liane-stems, but they are brought about in different ways.
In the stele of Bignonia four wedges of phloem split the xylem cylinder. The resulting lobation of secondary xylem increases the pliancy of stem. There are eight lateral sides where wedges of phloem and secondary xylem are in contact with each other.
These sides are the planes of weakness of stele but they are not detrimentally weak. Because parallel islands of bast fibres are differentiated in the wedges of phloem and these mechanical cells add strength to the planes of weakness.
In Aristolochia stem a fluted vascular cylinder is formed. In the young stem pliancy is brought about by the development of broad medullary rays at interfascicular region. In mature stems the vascular strands are split by the development of rays of parenchyma.
In Serjania several vascular strands are formed from the beginning and they are held together by parenchymatous ground tissue. The presence of parenchyma between the vascular strands increases the pliancy of stem.
In Strychnos, Thunbergia, pliancy is brought about by the formation of interxylary phloem. It is to be noted that the development of inter-xylary-leptome- strands is not confined to liane-stems only. They do occur in a number of woody plants that are not lianas. So Schenck (Haberlandt) concluded that the formation of interxylary phloem is not an adaptive anomaly. It merely represents a variety of design.
Gnetum, a gymnospermous liane, also shows adaptive anomalous secondary growth (Fig. 22.10A), In this case, in addition to primary cambium ring, some accessory cambium develops in succession encircling the primary cambium.
These cambium-rings function normally, i.e. produce xylem at inner face and phloem at outer face. Thus certain amount of parenchyma and secondary phloem become embedded between the xylem, formed by two successive cambium rings. Due to the presence of softer tissues Gnetum becomes flexible.