Read this essay to learn about the growth in intrastelar and extrastelar region of dicotyledonous stems.
Essay # 1. Growth in Intrastelar Region of Dicotyledonous Stems:
Cambium is a lateral meristem the cells of which are in a layer or strips or cylinder at stelar or extrastelar region with chiefly periclinal mode of cell division.
Cambium is formed from the procambium strand which remains undifferentiated during the progressive process of maturation leading to the formation of the primary body and ultimately occurs between xylem and phloem. It is called fascicular cambium (fascicle means bundle).
During the course of secondary growth a few living parenchymatous cells of the medullary rays, which remain in the G0 stage of the cell cycle, resume the meristematic activity and, thus, new strips of meristems are formed in the same line of circumference with the strips of fascicular cambia. Thus a continuous cambium ring or cylinder is formed. The newly formed cambium strips in-between the vascular bundles are known as interfascicular cambia.
The fascicular cambium consists of spindle- shaped elongated fusiform initials, and comparatively smaller and isodiametric ray initials. The former ones produce the secondary tissues forming the vertical or longitudinal system parallel to the long axis such as all the elements of xylem and phloem except the horizontally arranged ray cells. The xylem and phloem rays forming the transverse or horizontal system are produced by the ray initials.
Initially, the cambium remains single- layered in thickness. With the onset of the active secondary growth the cambium cells indistinguishable with their nascent derivatives together form a zone, referred to as cambium zone. The cells of this zone possess conspicuous vacuoles and primary pit-fields with plasmodesmatal connections.
In tangential views the cambium cells appear in two ways. In one way, the fusiform initials appear in horizontal tiers in which the lengths of the fusiform cells are equal and their ends remain more or less at the same level without overlapping. This type of cambium cells is known as storied or stratified cambium.
In other way, the comparatively longer fusiform initials do not appear in horizontal tiers but overlap at their ends. This is known as non- storied or non-stratified cambium. In all highly developed dicotyledons the cambium is of storied type.
The plane of division of the cambium cells is tangential adding secondary xylem and secondary phloem to the internal and external sides, respectively. Each cambium initial divides into two daughter derivatives, one of which becomes either a xylem mother cell or a phloem mother cell depending on its position, while the other one remains meristematic.
The xylem or phloem mother cell is finally differentiated through further division into secondary xylem or secondary phloem elements. Thus the cambium cells continue secondary growth and perpetuate themselves as active for a longer period.
Usually, the cambium produces more xylem than phloem thus pushing all the tissues external to the cambium ring more and more towards the periphery. Due to this mode of growth of secondary tissues the conjoint primary xylem and primary phloem are gradually pushed apart from each other as well as from the cambium.
Usually, in the plants growing in tropical regions cambium is active throughout the year. In temperate regions cambial activity may cease with the onset of unfavourable winter and enter into a dormant state till favourable summer conditions are available.
b. Secondary Xylem:
Secondary xylem constitutes the major portion of the secondary vascular tissue in woody plants performing a number of important functions, viz., conduction of water and dissolved mineral matters, mechanical support, storage of food etc. Secondary xylem is mostly made of thick-walled cells consisting of both vertical and horizontal systems as derived from the fusiform and ray initials of the cambium.
The vertical system consists of the tracheary elements, tracheae and tracheids, xylem parenchyma and fibres. The horizontal system consists only of xylemrays arranged at right angles to the long axis. The living ray cells, xylem parenchyma, pith cells, living cells of phloem and cortex together form a symplastic system through plasmodesmatal connections.
The secondary xylem elements are more or less similar to those occurring in primary xylem with abundant larger pitted vessels and shorter fusiform xylem parenchyma cells meant for storage of starch or fat. Tannins, mineral crystals etc., are also frequently present in these cells.
Secondary wall may be absent in xylem parenchyma, if present, the pits between them and tracheary elements are bordered, half-bordered or even simple. Secondary xylem parenchyma may occur abundantly in some species or may be reduced in number or entirely absent. The characteristics of the secondary taxonomic characteristic in recent years.
If they occur in association with the vessels, they are called paratracheal and, if they occur independently, they are called apotracheal. The former are of different types based on their distribution. The most common type is the vesicentric paratracheal parenchyma which forms entire sheaths, appearing circular or elliptical in cross- section, around the vessels.
Apotracheal parenchyma is considered as primitive and, through its different evolutionary types, advanced to the paratracheal forms culminating in highest developed vasicentric type.
The secondary xylem fibres have thick walls with reduced bordered pits. The two main types of fibres are the fibre tracheids, and the libriform fibres. The former have pits with poorly developed borders, whereas the latter possess simple pits resembling the phloem fibres. Both these fibres are the important mechanically supporting elements of xylem.
The xylem rays in the horizontal system are placed at right angles to the long axis of the plant body or organ. Their pattern of origin from the ray initials is as radial sheets of cells and continues as bands to the secondary phloem through the cambium, thus forming a continuous system of conducting tissues.
These bands are called vascular rays, as they are rays of vascular tissues, partly of xylem and partly of phloem, formed by the cambium. They establish transverse communications in the living cells of the vascular tissues. They also make connections with intercellular space system of the whole plant body for gaseous exchange with intercellular spaces of their own.
Apart from storage function the ray cells also perform the function of conduction of water from the xylem to the cambium and phloem, and that of food from the phloem to the cambium and living cells of xylem.
The parenchymatous ray cells in dicotyledons may be uniseriate or multiseriate. The ray cells in angiosperms are always living and shorter than other ray cells. Ray parenchyma cells may be oriented upright or procumbent. In the former type the ray cells remain oriented vertically with their long axes, and, in the latter type, the ray cells orient their long axes horizontally.
If the secondary xylem contains both the types it is said to be heterogeneous; whereas if it consists of only single type, it is called homogeneous. Both uniseriate and multiseriate ray cells may be homogeneous or heterogeneous. Uniseriate vertically elongated ray parenchyma and multiseriate heterogeneous ray parenchyma are considered to be primitive and homogeneous ray parenchyma to be advanced. In almost all gymnosperms the rays, are uniseriate.
The ray cells may be of single type, usually living with comparatively thicker walls called homocellular rays. In gymnosperms, tracheids may be present in addition to the living parenchyma in xylem rays called heterocellular rays. The dead ray tracheids have lignified walls with bordered pits.
c. Annual Rings:
Due to periodical activity of the cambium distinct growth layers may be formed in the xylem appearing as rings in transverse view and are usually referred to as growth rings. In most of the cases the activity of the cambium shows per year seasonal periodicity and, therefore, the secondary xylem formed in a year constitutes the growth ring and called annual ring. Formation of such rings is characteristic in plants growing in temperate climates with pronounced seasonal variations. In tropical and subtropical plants the annual growth rings may also be formed.
The annual ring is formed due to the formation of different types of wood or secondary xylem in different periods of the year. Spring is the most favourable season for plant growth and the wood formed in this season is less dense and is made of vessels with wider diameters.
Due to the rapid rate of photosynthesis in this favorable season there appears a high demand of water for photosynthate transport and, therefore, larger and wider vessels in secondary xylem are formed in that season. This is called the early wood or spring wood.
In summer, the rate of photosynthesis gradually decreases mainly due to high temperature and water scarcity, and thus the secondary xylem formed in this season is dense and made of narrower, compact and lignified vessel elements. This part is called late wood or summer wood.
The terms spring wood and summer wood are well suited in case of temperate plant where this type of growth pattern coincides with the seasons. But in some plants, however, the formation of late wood and summer wood is not strictly seasonal even in temperate climates. They are designated simply as early wood and late wood. Winter is the resting season when cambium shows minimum activity.
In-between the early wood and late wood of one season a sharp line of demarcation exists. Thus concentric annual rings each consisting of inner early wood and outer late wood are observed in the transverse view of the trunks of temperate deciduous and evergreen plants.
In some tropical trees distinct annual rings are formed due to periodic cambial activity in alternate wet and dry seasons. Well- marked rings are found in the important timber yielding plants like Tectona grandis (teak), Dalbergia sissoo (sissoo), Albizzia lebbeck (sirish), Salmalia malabarica (silk cotton), etc.
The seasonal activity of the cambium is influenced by adverse climatic conditions like drought, defoliation and diseases. In such situation, wood development is temporarily stopped and growth is resumed later in the same season. As a result, additional layers, called false annual rings, are formed. Thus more than two false rings may be formed. This is known as multiple annual rings.
The early wood is more conspicuous than the late wood due to the formation of wider and larger vessels. The vessels in the early wood are of unequal diameter and the largest ones show a ring-like arrangement in transverse section. This type of wood is called ring porous wood.
Sometimes the vessels of uniform diameter are found to be distributed uniformly throughout the wood. But when there is a gradient of distribution in size from smaller to larger ones throughout the growth ring it is called diffuse porous wood. Phylogenetically ring-porous wood is considered more advanced than the diffuse porous wood as intermediate forms are observed between them.
Tyloses (singular: tylosis) are the balloon-like protrusions into the tracheary elements of the secondary xylem in certain plants. These are developed by the adjacent xylem parenchyma and ray parenchyma cells. Such ingrowths may be developed in the primary xylem also. Tyloses are the enlarged protrusions of the pit membranes of the half-bordered pits from a living parenchyma cell into a vessel or a tracheid lumen.
Often they are so large that the lumen of the vessel or tracheid is almost blocked. A tylosis contains a nucleus with a certain amount of cytoplasm coming from the living cell. Starch, resin and other matters may also be present in it. The initial thin walls of tyloses may undergo lignification later leading to the formation of sclereids. A single tylosis may divide and form a multiple structure.
Tyloses are commonly found in many angiospermic families and abundantly found in the species of Tinospora, Morus, Catalpa, Quercus, Juglans etc. Normally they develop in the heart wood (previously formed centrally located portion of wood) to give extra mechanical support and to prevent from collapsing the vessel elements by the pressure developed by the secondary growth by blocking the lumen.
Tyloses not only prevent the flow of sap through the vessels but also the movement of fungal hyphae and prevent from spreading of diseases. The sap movement, however, takes place through the newly formed secondary vessel elements. Tyloses are also known to occur in the sap wood beneath the wounds as well as in connection with some diseases.
Occurrence of tyloses has also been reported in some herbaceous plants like Coleus, Carina, Cucurbita etc.
The epithelial cells in certain gymnosperms may block the resin duct by the enlargement and intrusions into them looking like tyloses, called the tylosoids (e.g., Pinus).
In angiosperm (e.g., Vitis, Bombax etc.) parenchyma proliferates into the neighboring sieve tubes in a tylose-like manner. These proliferations are also known as tylosoids. These tylosoids of gymnosperm and angiosperm differ from tyloses in not protruding through pits.
Sap Wood and Heart Wood:
Two types of wood formation are prevalent in woody species after secondary growth – the outer nascent secondary xylem is the sap wood or alburnum, and the centrally placed older hard portion is referred to as heart wood or duramen. The former is light- colored containing some living cells in addition to the tracheary elements and fibres.
This part actually carries on the water and solute conduction in addition to mechanical support and storage of food. The hard central dark colored heart wood mainly consists of dead elements representing merely a solid mechanical column.
The sap wood is gradually transformed into heart wood through certain changes like:
1. The loss of protoplasts in the living cells.
2. Reduced water contents.
3. Withdrawal of the food matters from the cells.
4. Formation of tyloses to block the lumens of the tracheary elements.
5. Considerable lignification of the walls of the parenchyma cells.
6. Infiltration of substances like gums, resins, oils and particularly tannins and colouring matters on both the walls and lumen of the elements.
Finally, the heart wood becomes more compact, strong and dark-colored. The heart wood is commercially more valuable than the sap wood but the latter is functionally more important. The heart wood yields good quality of timber due to its durability and resistance to decay. The dye haematoxylin is obtained from the heart wood of Haematoxylum campechianum.
The living xylem elements of sap wood in course of time become dead and finally converted to heartwood. However, in Sequoia semper- virens the ray parenchyma of heart wood may remain alive for hundred years.
The ratio between the quantity and the degree of difference of heart wood and sap wood varies with different conditions of growth. In Abies, Picea etc. the heart woods are not well differentiated. Taxus and Morus possess thin sap wood while Fagus and Acer contain thick sap wood.
f. Secondary Phloem:
Due to the tangential divisions of the cambium cells secondary phloem elements are produced on the outer side. Usually the amount of secondary phloem tissue is comparatively less than that of the secondary xylem tissue. In majority of the dicotyledons the primary phloem is crushed due to pressure originated by the secondary growth in thickness and subsequently the secondary phloem takes over the physiological functions for a considerable period.
The secondary phloem elements again are oriented in vertical and horizontal systems. The sieve tubes, companion cells, phloem parenchyma and fibers together form the vertical system. They are the derivatives of the fusiform initials of the cambium, chiefly responsible for vertical conduction of food matters in different directions.
The ray parenchyma cells formed by the ray initials are the only components of the horizontal system. These ray parenchyma cells may also remain arranged in storied and non-storied form like the secondary xylem.
The secondary phloem is more complex than the primary phloem though both are fundamentally similar on the basis of elemental analysis. The secondary phloem elements remain arranged in radial rows. The sieve tubes and phloem fibers are usually more numerous. The sieve plates are usually simple, but compound plates may also be present in some plants.
The companion cells and the sieve tubes together form the sieve elements as they are derived from the same mother cell. Single companion cell may accompany the sieve tube along its entire length or a few companion cells may be associated with it.
In gymnosperms the vertical system consists of sieve cells, phloem parenchyma and albuminous cells. In dicotyledons, phloem parenchyma are very numerous in the secondary phloem, but with maturity their number decreases. These parenchyma cells may be elongated with pointed ends or may be short rectangular or oblong.
They play a role in vertical conduction of solute as well as storage of starch, crystals and other ergastic substances. Abundant presence of sclerenchyma as discrete tangential bands, or as isolated patches, or even singly is the characteristic of secondary phloem. Patches of fibers may also be present in regular alteration with the bands of sieve tubes, companion cells and parenchyma.
Sclereids are one of the components of the secondary phloem in association with the fibres, the number of which increases in the non-functional phloem through sclerosis of phloem parenchyma. Elongated fibre-like elements formed by sclerosis of parenchyma cells have been called fibre- sclereids. The fibres and the sclereids give the mechanical strength. Many fibres of commercial importance belong to this secondary phloem.
g. Phloem Rays:
From the ray initials of the cambium phloem rays originate and constitute the horizontal system of the phloem. The phloem ray cells are the components of this system. They establish connections with the xylem rays and other living cells of the vascular tissues cent to the cambium are of equal size.
With maturity of the plant phloem ray cells gradually increase in width. They may be (a) uniseriate and parenchymatous, as in conifers, or (b) multi- seriate and homogeneous, composed of either upright or procumbent cells, or (c) heterogeneous, made of both upright and procumbent cells.
No growth ring is formed in secondary phloem. The elements of secondary phloem remain arranged in regular tangential series and thus exhibit very faint concentric rings. But unlike the secondary xylem there is no seasonal variation in the growth pattern and, therefore, no line of demarcation is observed between the elements formed in consecutive seasons.
In some species of dicotyledons, however, very faint rings are observed due to differences in the cells produced at the beginning and end of the season. Tangential bands of fibres have also been observed in some plants. But, in any way, they do not indicate the age of secondary phloem.
Essay # 2. Growth in Extrastelar Reions of Dicotyledonous Stem:
Continued secondary growth in the intrastelar region by the activity of the cambium cylinder large amount of pressure is exerted on the extrastelar tissues, which migrate ultimately to the epidermis.
As a result, the epidermis gets more stretched and ultimately tends to rupture exposing the internal tissues to the outside. In this situation, to protect the inner tissues, a new dermal tissue is formed secondarily, called periderm, in the extrastelar region.
It consists of three tissues:
1. A meristem known as phellogen or cork cambium.
2. The phellogen derived cells on the outer side, called phellem or cork cells.
3. The phellogen derived cells on the inner side, called phelloderm.
The phellogen or cork cambium is a secondary meristem arising from the living cells staying in the G0 stage of the cell cycle.
It originates as a single layer of initiating cells either:
(a) In the subepidermal portion, or
(b) In the epidermis itself,
(c) In the deep-seated layers of cortex or
(d) May even extend up to the phloem.
Phellogen consists of only one type of compactly arranged initials which appear rectangular in shape and flattened radially in transverse section. The cells divide tangentially producing new tissues in both centrifugal and centripetal directions. The derivatives usually remain arranged in radial rows.
The amount of phellem produced by the phellogen on the outer side is much more than that of phelloderm on the inner side. The cork cells or phellem are uniformly produced being arranged in distinct radial rows as they originate from a tangentially dividing meristem.
The cork cells are compactly arranged and have no intercellular spaces. After differentiation they become dead and become air-filled and accumulate coloring matters. The primary walls of these cells are mostly made of cellulose, partly suberin and lignin.
A thick layer of suberin is deposited next to the primary walls. So the cork cells become impervious to water and gases. In Eucalyptus the cork cells are extremely thick- walled and the cell lumens are completely filled up with resinous and other materials.
But usually they remain radially elongated, thin-walled empty cells. In Betula cork cell layer is peeled off like sheets of paper and the periderm serves as an effective secondary protective tissue against desiccation and mechanical injuries due to suberisation of the compactly arranged cork cells.
The phelloderm cells on the inner side resemble those of cortex. These cells are living, more or less isodiametric with intercellular spaces arranged in definite radial rows. They may photosynthesise and store food.
In conclusion, it can be said that secondary growth to increase in girth in a dicotyledonous stem is initiated in the intrastelar region by the activity of the cambium ring formed by the joining of the fascicular and interfascicular cambia and to cope up with pressure generated in the intrastelar region, periderm is formed in the extrastelar periphery.
In large trees, due to continued formation of secondary tissues in the intrastelar region the periderm may not be adequate to withstand the inwardly generated pressure. In that case additional layers of periderm are formed successively in the deeper regions of the cortex, pericycle and even phloem.
The outwardly cut-off phellem or cork cells by the phellogen ultimately get devoid of water and food supply and eventually become dead. All these dead tissues lying outside the active phellogen constitute the bark of the trees.
The term bark is loosely used and the term rhytidome is used for the outer bark covering all the tissues external to the innermost phellogen. Actually, the term bark is given to all tissues external to the vascular cambium.
Due to the formation of successive layers of periderm in the deeper regions of the stem the bark is formed in concentric rings surrounding the entire stem, which is known as ring bark. In some plants the periderm is formed as overlapping scale-like layers, known as scale bark. In Eucalyptus, Platanus, etc. the bark is intermediate between these two types, where the outer layers of the bark peel off in the form of sheets.
The secondary growth in thickness makes the epidermis to be replaced by the periderm in performing the protective function. The suberised walls of the dead cork cells are partly impervious to gases and thus for gaseous exchange between the internal living cells and the atmosphere some lens-shaped pores develop on the surface of the stem. These pores are known as lenticels. Only a few plants, mostly climbers, do not form lenticels though periderm is formed.
Lenticel formation takes place for the first time just beneath the stomata present in the epidermis.
They usually start to form either:
(a) Just before the commencement of periderm formation, or
(b) Simultaneously with the periderm initiation, or
(c) After the initiation of the periderm.
The parenchyma cells adjacent to the substomatal chamber lose chlorophyll and divide in various planes to form a mass of colourless loose cells. Sooner or later normal phellogen is differentiated in the adjoining deeper region, which further produces loose cells on the outer side. The whole loose cell mass formed by the divisions in the sub-stomatal parenchyma as well as in phellogen on the outside is known as complementary cells.
With the continued addition of the complementary cell mass the epidermis is protruded above and finally ruptured, thus appearing like so many raised spots. The outermost cells at the atmospheric interface often die and are replaced by the cells derived from the phellogen. Often mass of compact cells, known as closing cells, alternate with loose complementary cells.
The closing cells together form a layer referred to as closing layer which helps to keep the loose complementary cells in position. During the active growing season the closing layer ruptures due to the pressure generated by the continued addition of the complementary cells. At the end of the growing season the closing layer is again formed.
The loose complementary cells are thin- walled, non-suberised with profuse intercellular spaces to establish communications with the internal tissues. Like the stomata the lenticels are thus mainly concerned with gaseous exchange between the atmosphere and the internal tissues.
In some cases, lenticels are formed in the stomata free regions. After the formation of the cork cells by the phellogen for a while loose complementary cells are produced in localised areas which ultimately protrude above and rupture through the cork giving rise to lenticels.
The number of lenticels formed in stem is variable. They may remain scattered or arranged in vertical or longitudinal rows. Rows of lenticels may occur opposite to the multiseriate rays, suggesting free interchange of gases between them.
Based on the orientation and rupture of the epidermis the lenticels may be transverse or longitudinal.
In dicotyledonous plants three types of lenticels are observed:
1. The complementary cells are suberised with little intercellular spaces as in Magnolia, Pyrus, Salix, etc.
2. Loose, non-suberised complementary cells followed by the formation of compact and suberized complementary cells as in Tilia, Quercus, etc.
3. Alternately arranged loose non-suberised complementary cells and compact suberised cells to form a multilayered complementary tissue as in Betula, Fagus, etc.
Rhytidome is a special type of bark composed of successive layers of periderm as well as either the cortical parenchyma or secondary phloem. In Robinia periderm formation continues successively by the development of phellogen in the deeper regions of stem giving rise to periderm bands due to the death of the peripheral ones. The cork cells get suberised and the successive periderm bands enclose either cortical tissues or secondary phloem.
All the periderm bands together with the enclosed cortex or secondary phloem and all the tissues present external to the innermost phellogen are collectively referred to as rhytidome. The term bark is applied to all the tissues present external to the vascular cambium. Sometimes the term outer bark is applied to rhytidome and the living part of the bark inside the rhytidome is referred to as inner bark.
Polyderm is a special protective tissue consisting of twenty or more alternating layers of uniseriate suberised cells and multiseriate non- suberised cells. The peripheral cells are dead and the inner cells including the suberised cells contain living protoplasts. It is found in underground stems and roots of Rosaceae, Onagraceae, Myrtaceae and Hypericaceae.
At the time of its formation, a special phellogen is differentiated at the pericycle, which forms tissues by tangential divisions in centripetal succession. The newly formed tissues consist of thin walled non-suberised parenchyma cells alternating with uniseriate endodermoid cells.
The latter are differentiated into cork cells with the formation of casparian strips on their walls that later undergo more extensive suberisation. Polyderm is formed at the pericycle and it is exposed to outside after the death of the cortical tissues. It performs the function of protection of the inner living tissues. The non-suberised cells are concerned with food storage.