Secondary growth can be defined as a phenomenon where, after the completion of primary tissue formation, more secondary tissues are developed to supplement dermal, vascular and mechanical tissue system in certain plants.
The meristem of primary tissue divides. The daughter cells, after differentiation and maturation, form primary tissues of plant body. Primary tissues include epidermis, cortex, primary phloem, primary xylem and pith, which are observed in the cross section of young stems and roots.
In many plants, vegetative development is completed after the maturation of primary tissues. However, in many herbaceous and woody dicotyledons, formations of new tissues continue even after the maturation of primary tissues.
The production of these new tissues is attributable to the lateral meristem, which includes cork and vascular cambium. These cambia produce new tissues for effective protection, conduction and mechanical strength – a phenomenon termed secondary growth.
Secondary tissues are formed by the cambium, which is normally present in dicotyledonous roots and stems. Though some monocot stems possess nonvascular cambia, which produce secondary tissues, it is regarded as anomalous. So normal secondary growth occurs in dicots only, and it causes increase in thickness both in intrastelar and extrastelar region of roots and stems.
Intrastelar secondary growth in dicot stems:
In dicotyledonous stem, the vascular bundles are arranged more or less in a ring. The bundles originate from procambium strand. The procambium cells divide; the inner derivatives are differentiated into primary xylem and the peripheral ones form primary phloem.
A strip of procambium remains in between primary xylem and phloem and this undifferentiated procambium is known as cambium. The cambium, along with peripheral primary phloem and inner primary xylem, forms collateral open vascular bundle of dicot stem.
At the onset of secondary growth, the union of fascicular and interfascicular cambium forms a normal vascular cambium ring. The cambium, which is normally present in between primary xylem and primary phloem is the fascicular cambium, which originates from procambium and so primary in origin.
The interfascicular cambium originates from primary permanent medullary ray cells, situated between the primary vascular bundles and so secondary in origin. The interfascicular cambium differentiates along the line of fascicular cambium and so their union produces a normal cambium ring (Fig. 20.1). The cell lineages during secondary growth in dicotyledonous stem are illustrated in Box 20.3.
The cambium ring divides tangentially and produces daughter cells on the peripheral and inner side. The peripheral cells are differentiated into secondary phloem, while the inner derivatives form secondary xylem (Figs. 20.2 & 20.3). In between the two secondary tissues, i.e. xylem and phloem, the continuously dividing cambium is present.
Secondary tissues are produced both at fascicular and interfascicular region. Normally equal proportion of secondary xylem are differentiated in all segments of the cambium ring on the inner side while the secondary phloem are formed towards periphery in equal amount in all the segments of the cambium ring but they (secondary xylem and phloem) are not of equal quantity.
Generally, more amount of secondary xylem is differentiated in contrast to secondary phloem. The cells of cambium ring also divide radially to produce ray cells or medullary cells. The ray parenchyma cells are arranged radially in secondary xylem and phloem.
The cells of cambium possess all the properties of meristematic cells, which are:
(i) Compactly set with little intercellular spaces,
(ii) Cell wall is thin,
(iii) Rich cytoplasm,
(iv) Cytoplasm contains less reserve food, ergastic substances and crystals,
(v) Proplastid present,
(vi) Vacuoles present, and
(vii) Nucleus large.
During secondary growth a large amount of secondary tissues are produced within the stele by the activity of cambium ring. As more and more secondary tissues are produced, the primary xylem gradually approaches to the centre and appears as small patches below the secondary xylem. The primary phloem occupies the peripheral position above the secondary phloem (Fig. 20.4).
Extrastelar secondary growth in dicot stem or periderm formation:
In intrastelar secondary growth a considerable amount of secondary vascular tissues are produced. As a result a pressure is developed within the stele and it is transmitted to extrastelar region when the endodermis is ruptured, cells of cortex are crushed; the epidermis is stretched and tends to be ruptured.
So to withstand this pressure a special protective tissue is formed usually replacing the peripheral tissues of stems. These protective tissues are known as periderm, which is formed as a result of extrastelar secondary growth.
Periderm originates from the permanent cells present in the epidermis, cortex or phloem and so secondary in origin. In these permanent tissues certain cells become meristematic, which is termed as cork cambium or phellogen. The cells of phellogen are uniseriate, polygonal or rectangular in shape, compactly set with little intercellular spaces.
They contain large nuclei, vacuoles and sometimes chloroplastid and tannin. In contrast to vascular cambium with ray and fusiform initials, phellogen consists of one type of cells. The cork cambium divides mainly tangentially or less commonly radially. The entire cork cambium ring may divide or the division may be restricted to certain cells only. The daughter cells, thus produced, lie at radial rows.
The peripheral derivatives of cork cambium are known as phellem. These cells are rectangular, compactly set without any intercellular spaces, lie at radial rows whose number ranges from two to twenty, and it varies according to species.
Phellem, also known as cork, is dead at maturity. The cell walls may contain suberin or lignin in addition to cellulose. It is the suberin and wax, the fatty substances, which make the phellem or cork impervious to water and gases, thus conferring upon it the protective properties. Sometimes non-suberized cells occur in the phellem —termed phelloids.
In plants different types of cork cells occur.
The two common types are:
(i) Radially elongated, thin walled, empty and very light cork and
(ii) Radially thin, thick walled, cell lumen is impregnated with dark staining materials like tannin or resin, ex. Eucalyptus. These two types may occur in the same plant, ex. Betula, Arbutus.
The cork cells protect the inner cells from any mechanical injury and owing to their suberized cell wall the inner cells are protected from desiccation. Phellem or cork cells are dead. They are resistant to pressure, acids and other chemicals. They are resilient and impermeable to gases and liquids.
The above features and the strength, elasticity and lightness give much commercial value to the cork. The commercial cork is chiefly obtained from Quercus suber. The bottle corks that are used as stoppers are made sizes by tangential unit to stop leakage from vertically oriented lenticels (Fig. 32.3).
The inner derivatives of cork cambium are known as phelloderm. They are living cells and resemble the parenchyma of cortex, but they can be easily differentiated from the cortical parenchyma by their radial arrangement. The cell wall of phelloderm is thin and composed of cellulose only. In some plants phelloderm possesses chloroplastids, can photosynthesize and store starch.
Phellem, phellogen and phelloderm are collectively known as periderm, which forms a protective covering on the peripheral side and is impervious to air and water (Fig. 20.5). Intrastelar secondary growth may continue and the sole periderm may be unable to withstand the pressure of the expanding stele. Then, in the deeper layers of cortex, successive layers of periderm may be formed with the help of several cork cambia.
a. BARK:
The cork or suberized tissue present at the periphery of stems and roots, which originates from the secondary lateral meristem or phellogen, is termed as bark. It is the protective tissue of roots and stems and now a days this term is used to designate all tissues present outside the vascular cambium. In this sense it includes periderm, cortex, primary and secondary phloem. In mature barks a large quantity of sclerenchyma, phloem fibres and sclereids (e.g. Alstonia) are present.
Cork cambium or phellogen originates in the living epidermal parenchyma or hypodermal collenchyma cells of stems. Periderm is developed from this meristem and it may be followed by the inception and development of another phellogen and subsequently by other periderm. The subsequent periderm may be in the form of complete cylinders around the stem parallel to first formed periderm.
The barks, thus developed, are known as ring bark (Ex. Vitis, Clematis etc.). In Ficus, Pyriis etc. additional phellogen develops in the form of separate overlapping arcs and so the periderm assumes the form of scales or shells. These barks are known as scale bark.
Barks are composed of dead cells. When the subsequent inner periderm develops, the outer layers are sloughed off. In ring bark the peripheral dead cells are sloughed in the form of hollow cylinders where as in scale bark they are peeled off as scales.
The barks protect the inner tissues from desiccation, mechanical injury, entry of pathogen and the strain set up due to the formation of secondary tissues at the stele.
b. RHYTIDOME (Fig. 20.6):
It is a special type of protective tissue composed of successive bands of periderm with enclosed cells, which are either cortical parenchyma or secondary phloem. At the time of extrastelar secondary growth a periderm is formed at the peripheral region of stems. In Robinia this may be followed by successive inception and development of phellogen in more and more deep region of stem which gives rise to bands of periderm.
The cells of phellem eventually become suberized. The successive bands of periderm enclose either cortical tissues or secondary phloem. Due to the formation of inner periderm, the peripheral periderm bands and the enclosed cells die. The peripheral successive bands of periderm together with the enclosed cortex or secondary phloem and all the tissues present external to innermost phellogen are collectively referred to as rhytidome.
The term bark is applied to all tissues present external to the vascular cambium. Sometimes the term outer bark is applied to rhytidome and the living part of bark inside the rhytidome is referred to as inner bark.
Figure 20.6 Diagrammatic representation of rhytidome in transverse section.
c. POLYDERM:
It is a special protective tissue consisting of uniseriate suberized endodermis-like cells alternating with multiseriate non-suberized cells. It occurs in underground stems and roots of Rosaceae, Onagraceae, Myrtaceae and Hypericaceae. It consists of twenty or more layers of cell that are suberized and non-suberized. The peripheral cells are dead and the inner cells including the suberized cells contain living protoplasts.
At the time of formation, a special phellogen is differentiated at the pericycle. The phellogen forms tissues centrifugally by tangential divisions. The tissues, thus formed, consist of thin walled non-suberized parenchyma cells, which alternate with uniseriate endodermoid cells. The latter becomes cork cells.
During the formation of cork casparian strips appear on the wall that later undergoes more extensive suberization. Polyderm is formed at the pericycle and it is exposed to surface after the death of the cortical tissues. It protects the inner tissues. The inner tissues including the suberized cells remain alive. The non-suberized cells are concerned with food storage.
d. LENTICEL (Fig. 20.7):
It is the raised opening or pore on the epidermis or bark of stems and roots through which atmospheric gases diffuse into and out of the living cells. They occur on young branches and on the surface of fruits (ex. apple and pear).
In some plants like Anabasis, Haloxylon, Vitis etc., though periderm is formed lenticels are not developed. They may appear as dots, little flecks, rough dark patches, lens-shaped raised or dome shaped corky spots or fissure on the fruit, root and stem surfaces. The orientation of fissure recognizes the transverse and longitudinal arrangement of lenticels.
In cross sectional view lenticels appear as loose aggregate of cells which are protruded above by rupturing the epidermis of roots and stems. These cells may be suberized or non-suberized and are termed as complementary cells or filling cell, which are loosely arranged, more or less spherical, thin walled and larger in size than the neighbouring cells.
Below the complementary cells phellogen is present and it is continuous with the rest of phellogen, which is situated between phelloderm and phellem of periderm. Beneath the phellogen, phelloderm is situated and it is also continuous with the rest of periderm.
In some plants, within the complementary cells some cells are produced which are compact and suberized. These cells are termed as closing layer. It is believed that the loose aggregate of complementary cells is held in position by the closing layer. The closing layer may be broken due to the formation of new complementary cells. This layer may be formed again at the end of growing season.
At the region of lenticels, complementary cells with profuse intercellular spaces interrupt the continuity of phellem. The closing layer, though compact contains intercellular spaces.
The phellogen also possesses similar gaps. There is continuity of the intercellular spaces between complementary cells, closing layers, phellogen, phelloderm and inner tissues. So it is believed that gaseous exchange between inner tissues and atmosphere occurs through lenticels like stomata.
Three types of lenticels are recognized in dicotyledonous stems on the basis of structure of complementary cells and the arrangement of closing layers:
(i) In the first type (ex. Pyrus, Populus, Magnolia etc.) the complementary cells are suberized with air spaces. These tissues exhibit annual variation of loose aggregate of thin walled cells and compact thick walled tissue,
(ii) The second type (ex. Tilia, Quercus etc.) exhibits a loose aggregate of non-suberized complementary cells below which suberized cells are formed to form closing layer;
(iii) In the third type (ex, Prunus, Betula, Fagus etc.), the complementary cells regularly alternate with closing layers.
The former is several cells wide and consists of loose aggregate of non-suberized cells. The cells of closing layers are suberized, compact and narrow, which hold the loose complementary cells in places. The closing layers may be ruptured due to the formation of new complementary cells.
The lenticels, present on fruit surfaces of apples, are atypical consisting of areas of corky cells. Scan electron microscopic study of the lenticel of potato tubers reveals the presence of some thread like waxy outgrowths from the sides of the pores; it is suggested that these outgrowths regulate the gaseous exchange.
The number of lenticels occurring per unit area is not constant. Initially and usually lenticels arise beneath stomata. They may develop under each or a group of stomata or irrespective of them.
Lenticels may arise from deeply seated phellogen and from successive phellogen, which has no relation to the position of stomata. Generally lenticels occur irregularly scattered over the surface; sometimes transverse and longitudinal arrangement are also noted.
Usually lenticels develop from the phellogen. The phellogen originates at the sub-stomatal region. The sub-stomatal cells divide in all directions and lose chlorophyll. The derivatives towards the cortex form the phellogen of lenticel, which gradually becomes continuous with the rest of the phellogen of periderm.
The phellogen of lenticel divides and the cells produced towards the exterior, as well as those formed during the divisions of sub-stomatal cells, are termed as filling cells or complementary cells. As more and more complementary cells are formed, they exert a pressure on the epidermis and as a result the epidermis ruptures. The complementary cells, protrude above through the ruptured epidermis.
The exposed cells gradually dry up, die and the phellogen of lenticel forms the closing layers centrifugally in addition to complementary cells. The phellogen of lenticel, by repeated divisions, forms phelloderm towards the interior. This phelloderm becomes continuous with that formed by the phellogen of periderm.
When lenticel originates from the deep-seated successive phellogen, certain localized region functions as phellogen of lenticel irrespective of the position of stomata. In some cases lenticel may be formed from the phellogen, which produced phellem for a while. In this case the phellogen, after forming cork for sometime forms complementary cells.
Secondary growth in dicot root:
Radial vascular strands with exarch protoxylem are the characteristic of roots, i.e. primary xylem and phloem lie separate and alternate to each other. In dicot roots secondary tissues are formed both at intrastelar and extrastelar region.
Intrastelar secondary growth occurs by the secondary cambium, as the primary cambium is absent in roots. Secondary cambium originates from the permanent tissues present in the stele. Strips of cambia differentiate below each primary phloem. Cambia are also formed above the protoxylem, near or at the pericycle.
These strips of cambia extend laterally and join with each other. As a result a wavy cambium ring with ridges and furrows appear. The ridges lie overarching the protoxylem whereas the furrows (Fig. 20.8) are present below the primary phloem. So according to the number of xylem and phloem present in the stele the number of ridges and furrow appear.
The wavy cambium ring divides in all segments and produces secondary tissues at the peripheral and inner side. The peripheral derivatives are differentiated into secondary phloem while the secondary xylem is formed from the inner cells. Normally cambial cells at the furrowed region divide more in contrast to ridged region.
As a result the wavy cambium ring becomes more or less circular and it lies in between the secondary xylem and phloem. Usually more secondary xylem is differentiated in comparison to phloem. All elements of xylem (tracheids, trachea, xylem fibre and xylem parenchyma or xylem ray) and phloem (sieve tube, companion cells, phloem fibre and phloem parenchyma or phloem ray) are differentiated in the secondary vascular tissues.
Periderm is formed in roots. Soon after the formation and division of vascular cambium some cells of the pericycle become meristematic. As a result another cambium is formed called cork cambium or phellogen. Phellogen divides and the peripheral derivatives are differentiated into phellem or cork cells whereas the inner cells form phelloderm.
After the differentiation of periderm all tissues, which lay outside the cork, die. They are ultimately sloughed off. Periderm is the protective tissue and is impervious to air and water. At some regions lenticels may appear for gaseous diffusion.
As a result of activities of vascular cambium the primary vascular tissues are widely separated. The cork cambium forms periderm. So after the secondary growth an old root resembles a stem where secondary growth has occurred. But the root nature is revealed from the exarch protoxylem, which is pushed towards the centre (Fig. 20.9).
Secondary xylem:
Secondary xylem is a complex tissue, known also as wood. The study of wood by preparing sections for microscopic observations is defined as xylotomy. Secondary xylem is derived from the vascular cambium.
It develops in stems and roots of gymnosperm and angiosperm-dicotyledonous plants as a consequence of secondary growth. The cells composing wood are mainly thick walled. Cell cytoplasm deposits the thickening materials during differentiation. Later cytoplasm dies and leaves the cells of wood devoid of any living contents.
Woods are divided into two main groups —porous and non-porous based on the presence or absence of vessels or pores. Non-porous wood predominate in gymnosperm where predominantly tracheids with a small amount of parenchyma compose the wood. Example: pines, spruces and firs where the texture of wood is uniform and a carpenter works with ease.
So non-porous wood or gymnospermous wood is referred to as softwood. In contrast angiosperm dicotyledonous wood exhibits a variety of cell types including thick-walled fibres. It is sometimes difficult to work with such type of wood. So porous wood or angiosperm-dicotyledonous wood is referred to as hardwood.
These two terms hardwood and softwood are traditionally used in timber trade. But the terms, however, are misleading. Woods with soft and hard texture can be found in both groups of plants. The terms have no relation to the relative hardness or softness of timber. These terms —hardwood and softwood are applicable only to porous and non-porous wood respectively.
As for example the wood of Ochroma, Bombax ceiba and Pterocymbiurn tinctorium are soft to very soft. These plants belong to angiosperm and their wood is referred to as hardwood by definition as the wood is porous. On the other hand the woods of Cedriis deodara and Pinus roxburghii are very hard. These plants belong to gymnosperm and their wood is referred to as softwood by definition as the wood is non-porous.
Wood contains all the types of cells that are observed in primary xylem but no new ones. The only difference between primary- and secondary xylem is the origin and arrangement of elements of xylem. Secondary xylem originates from vascular cambium whereas procambium gives rise to primary xylem.
The arrangement of wood cell types is either ‘along-the-axis’— called axial system or ‘across-the-axis’ — termed radial system. The arrangement reflects to that of the fusiform- and ray initials of cambium that form wood cell types. Fusiform initial gives rise to elements of axial system and ray initial forms the cells present on the radial system (Box 20.1).
The cell types of wood can be easily studied in transverse section (t.s. = cut across the axial plane) and in longitudinal section (l.s. = cut along the axial plane), Figs. 20.10 & 20.19. The images observed in the sections of wood through an optical microscope reveal the shape of the cells present in wood. The knowledge of shapes in t.s. and l.s. provides one to envision the three dimensional shape of a cell (Fig. 20.10A).
The cambium that forms wood is composed of spindle shaped fusiform initials and the isodiametric ray initials. The ray initials form the ray parenchyma cells, which compose the horizontal or radial tissue system.
The fusiform initials lie parallel to the long axis and give rise to tracheary elements, fibres and axial parenchyma that form the vertical or axial tissue system. The living cells of axial and radial systems are interconnected to form a continuous system.
Ray parenchyma:
Ray parenchyma cells are of various shapes and the two most common forms are where the longest axis of the cell is either vertical or radial. The former one, where the longest axis is oriented vertically, is called erect or upright ray cells. The latter one is termed as procumbent ray cells where the longest axis is oriented radially.
Ray cells may also be square or isodiametric. Square cells are considered as morphologically equivalent to erect cells. In a radial longitudinal section the parenchyma cells appear as fine horizontal lines. So a transverse or radial longitudinal section reveals the longitudinal axis of ray cells. The transverse section of ray cells is obtained from tangential longitudinal section (TLS).
In TLS ray parenchyma appears variously, for instance:
(i) Uniseriate: the ray cells are entirely one cell in width, Ex. Salix, Pinus etc.
(ii) Biseriate: any portion of the ray masses is two cells wide, and
(iii) Multiseriate: any portion of the ray masses is more than two cells in width, e.g. Quercus.
The TLS reveals that the biseriate and multiseriate ray gradually become uniseriate at both its upper and lower edges.
Uniseriate and multiseriate rays both may be either homocellular or heterocellular (Fig. 20.11).
In dicotyledons the homocellular rays consists of:
(1) Erect cells only,
(2) Procumbent cells only,
(3) Square cells and
(4) Erect and square cells only.
In heterocellular type procumbent and square or procumbent and erect cells occur. The entire ray system may consist of either homocellular or heterocellular types, or of combinations of two.
Figure 20.11 Diagrammatic figures of ray parenchyma cells in tangential and radial longitudinal sectional view.
In gymnosperm xylem rays are almost exclusively uniseriate.
The ray cells may be:
(i) Homocellular: rays consist of parenchyma cells only, ex. Thuja and
(ii) Heterocellular: where the ray cells comprise parenchyma cells and ray tracheids, ex. Pinus.
The ray tracheids (Fig. 20.12B) have bordered pits and are devoid of protoplasts in contrast to ray parenchyma. They are multiseriate only if they contain resin canals. Ray tracheids are horizontal, rectangular cells that look somewhat like parenchyma cells. They have lignified walls and in Pinus the lignifications may be in form of teeth or band that project in the cell lumen.
In angiosperm ray cells may be homogeneous (ex. Populus) where all the cells are procumbent, i.e. the longest axis is oriented in radial direction or heterogeneous (ex. Olea) where the ray cells are procumbent and square or oriented vertically. These two terms — homogeneous and heterogeneous are more or less similar to homocellular and heterocellular ray cells respectively of gymnospermous wood.
The ray cells of Pinus have very large pits with very narrow border that appears as more or less circular or rectangular areas throughout the width of the cells. These are known as fenestriform pits (Fig. 20.12A) whose surface view is obtained in radial longitudinal section.
Initially ray parenchyma cells are living and serve in storage and aeration. They store carbohydrates and other nutrients. These substances are transported radially within the wood over short distances. The upright cells have direct connection with axial cells. The ray/axial interface exhibit many forms. When the upright ray parenchyma cell is in contact with axial parenchyma plasmodesmata exist between them.
The upright parenchyma cell may be in contact with the axial tracheary elements (tracheid or vessel). In such case the ray cells have very thin walls while pits occur on the walls of tracheary elements. Pits are the routes of transport of carbohydrates and other nutrients stored in the upright parenchyma cells.
The stored starch in the procumbent parenchyma cell is first digested into sugar and then transferred to axial conducting cells. In some plants there exists no connection between procumbent parenchyma cells and cells of axial system. In such cases the transfer of nutrients is routed through upright parenchyma cells.
Axial parenchyma:
The distribution of axial parenchyma is very characteristic in dicotyledonous wood. Parenchyma may lie independent of vessels or they are distinctly associated with them. These two forms are known as apotracheal and paratracheal respectively.
The common apotracheal forms are (Fig. 10.2):
(i) Diffuse parenchyma: parenchyma cells appear as single cell or as small uniseriate band throughout the growth ring (ex. Quercus);
(ii) Banded or metatracheal parenchyma: parenchyma cells appear concentric bands (ex. Hicoria) and
(iii) Boundary parenchyma: parenchyma may appear at the beginning or end of the growth ring and accordingly termed as initial parenchyma (ex. Ceratonia, Zygophyllum) and terminal parenchyma (ex. Magnolia, Salix etc.)
Paratracheal parenchyma may be (Fig. 10.2):
(i) Scanty (ex. Acer):
The parenchyma cells do not form a continuous sheath surrounding the vessel;
(ii) Vasicentric parenchyma (ex. Tamarix):
The parenchyma cells form a continuous sheath around the vessel of different width;
(iii) Aliform parenchyma (ex. Acacia):
Vasicentric parenchyma extends laterally as wings.
Annual ring:
Most trees and shrubs of temperate origin show characteristic growth layers, often called growth rings or annual rings of secondary xylem. Growth rings appear as concentric or eccentric rings seen in transverse section of stems; each ring represents a year’s growth of secondary xylem.
The concentric rings are formed at the straight parts of stems under uniform conditions whereas the eccentric rings appear as a result of adverse natural calamities. The activity of cambium is a seasonal phenomenon. The periodic activity and quiescence of cambium lead to the formation of annual or growth rings.
These rings are distinguishable with an unaided eye because of the differences in structure and colours between the secondary xylem formed at the early and late parts of a growth season. As each ring represents a year’s growth, the approximate age of a plant can be ascertained by counting the number of growth-rings (Fig. 20.13).
In temperate perennial plants, the activity of cambium begins in the spring. The wood produced during this period is the early wood, often called spring wood. The spring wood is less dense, consists of thin-walled elements and possesses vessels with large lumen. The cells of wood, produced at the late parts of growth season, possess thicker walls and vessels with smaller diameter.
These are late wood, also known as summer or autumn wood. The early wood and late wood, formed in one growth season, together constitute the growth ring or annual ring. The early wood and late wood of the same season gradually merge with each other and with adjacent growth rings also. But a sharp line of demarcation exists between the late wood of one growth season and early wood of next season.
In Tilia, Ceratonia, Zygophyllum etc. a few layers of wood parenchyma is present between the growth rings and these parenchyma form the division line between the rings. It was possible to determine accurately the age of Pinus longifolia, Tectona grandis, Terminalia tomentosa, Acacia catechu and Bombax malabaricum.
The annual rings were carefully counted and the false rings, which occasionally develop in the above species, were properly located and omitted from counting. There was a common belief that growth rings are formed only in deciduous trees and the evergreen trees are without them.
However, Chowdhury (1939) reported the formation of growth rings in both evergreen (e.g. Michelia champaca) and deciduous trees (e.g. Cedrela toona, Albizia lebbeck and Dalbergia sissoo). Annual rings are also formed in tropical genera namely, Bursera (Burseraceae), Citharexylum (Verbenaceae), Rapanea (Myrsinaceae) and Swietenia (Meliaceae).
In some plants woods are formed without growth-rings, e.g. Baccharis (Asteraceae), Laguncularia (Combretaceae), Rhizophora (Rhizophoraceae), Manilkara (Sapotaceae) and Pisonia (Nyctaginceae) etc.
Ring porous and diffuse porous wood:
In a growth ring (Fig. 20.14), the vessels may or may not be of equal diameter. When the vessels, sometimes called pores in commerce, are more or less of same diameter and uniformly distributed in early and late wood, the wood is said to be diffuse porous wood; ex. Acer, Populus, Olea, Albizia lebbeck, Dalbergia sissoo and Michelia champaka etc.
In the ring porous wood, the vessels are not of equal in diameter throughout the growth ring. The vessels with larger diameter are present on early wood and the late wood shows vessels with smaller diameter.
These variations in diameter can be easily being observed in the transverse sections of stem of Cedrela toona, Tectona grandis, Quercus, Fraxinus etc. It is observed that ring porous woods are few in number in comparison to diffuse porous wood. From phylogenetic point of view ring porous wood is considered to be more advanced.
Usually the number of growth rings denotes the approximate age of a tree. However, this is not always accurate due to the formation of additional growth rings in the same season. In Tamarix aphylla two growth rings are developed in a single season. More than one growth rings formed per annum is also reported in Avicennia.
Additional growth rings may be developed due to adverse natural calamites like draught, frost, defoliation, flood, and mechanical injury or due to infection, and biotic factors that disturb the plants when the activity of cambium is either checked or stimulated; when stimulated additional rings are formed, termed false annual ring.
When two or more rings are formed in one season they are often called as double or multiple annual rings respectively. False rings may be formed both in temperate and tropical species. Researches reveal that day length, temperature, hormones etc. influence the activity of the cambium. It is not very difficult to distinguish the false growth marks from the true ones.
In Cedrela toona the growth marks, either true or false, are always associated with concentric parenchyma bands. Vessels occur on either side of the growth marks of the parenchyma bands. In true marks, the vessels occurring on different sides of parenchyma bands are markedly of different diameter while the vessels on either side of parenchyma bands of false marks are more or less of same diameter.
In most trees of tropical origin except a few genera there is no distinguishable growth ring. The plants like Acacia tortilis, Thymelaea hirsuta, where there are no growth rings, produce same type of secondary wood throughout the growth season. In the South Californian pine Pinus radiata the cambium is not fully dormant even in winter.
The Indian evergreen and deciduous trees do not always show distinguishable growth rings. The concentric rings in wood, as seen in Shorea robusta and Eugenia jambolana, have no relation with annual growth activity and so not indicative regarding the age of the tree.
The arrangement and pattern of distribution of vessels differ and these are easily observed in transverse sections of wood. For instance, Eucalyptus shows single vessel; vessel clusters are present in Pistacia. The vessels whose walls are appressed with each other are often called multiples. They are present in Salix. There are more types that are illustrated in Fig. 20.15.
The size, shape, arrangements and the relative amount of constituent elements of secondary xylem of most trees are genetically labeled. Thus secondary xylem provides characters to identify an unknown timber. With the aid of these characters a key can be formulated by which timbers can be identified. A key can be made by using punched-card system and the computer.
Usually a dichotomous key is formulated using the same principle as is followed in plant taxonomy. Dichotomous key is very popular and is used all over the world. But it has limited scope and is applicable only to a small group of timber. Fahn illustrated a dichotomous key to identify woods. Rao and Juneja (1971) proposed a dichotomous key to identify fifty important timbers of India.
They schematically represented a dichotomous key for eight timbers only and commented that ‘even a novice may not have difficulty in following the principle and working of a dichotomous key’. The dichotomous key is illustrated in Fig. 20.16. It is customary to present a dichotomous key in a running form and it is illustrated in Box 20.2. For details consult Fahn and Rao and Juneja.
Importance of studying growth rings:
Analysis of growth rings, also referred to as dendrochronology, has many uses and a few are mentioned below:
i. Foresters often estimate the age of a tree by counting the numbers of growth rings.
ii. From the growth-ring-patterns the quality of timber can be ascertained.
iii. Study of growth-rings is used as a means of dating of wood, e.g. oak. Oak wood is used in house construction and in the backing of old pictures. A chronology of at least one thousand years has been established in oak wood.
iv. Growth-ring analysis is also used as a check of radio-carbon dating.
v. It has become a tool in the study of past climate and archaeological dating.
vi. Study of growth rings provides evidences in forensic investigation.
Tylosis:
In many plants several axial or ray parenchyma surround the tracheary elements. Some of them protrude into tracheary elements through the pit. These ingrowths are called tyloses (singular – tylosis or tylose). Several tyloses may be formed in the vessel formed by the surrounding parenchyma. The tyloses increase, come in contact with one another, completely fill and block the vessel.
As a result the tracheary elements become inactive. The nucleus and some amount of cytoplasm flow to the tyloses. The walls may remain thin or lignin may deposit. The lumens of tracheary element with numerous tyloses (Fig. 20.17) appear as network (ex. Quercus).
Tylosoid:
In gymnosperm, the epithelial cells, i.e. resin producing parenchyma surrounding the resin ducts, sometimes enlarge as tylosis-like intrusions and block the duct. These ingrowths are termed as tylosoids (ex. Pinus). In angiosperm (ex. Vitis, Bombax etc.) parenchyma proliferates into the neighboring sieve tubes in a tylose- like manner. These proliferations are also known as tylosoids. Tylosoids of gymnosperm and angiosperm differ from tyloses in not protruding through pits.
Sapwood and heartwood:
The wood of some old trees where considerable amount of secondary xylem has developed, becomes differentiated into two regions-the peripheral region, called sapwood and the inner zone, termed heartwood.
The sapwood also called alburnum, in contrast to heartwood, is of lighter colour and consists of the active outer secondary xylem which is mainly concerned with translocation and storage of food. These are the youngest formed wood and are softer in texture than the heartwood. The sapwood contains living parenchyma and water-filled tracheary elements; it is full of ‘xylem sap’-from where the name is derived.
The inner heartwood also termed as duramen, in contrast to sapwood, is dark colored, hard in texture and contains inactive elements of primary and secondary xylem. Tyloses develop in the tracheary elements of some wood and block the lumen. The lumen may also be blocked by another method referred to as gummosis (ex. Primus).
In this phenomenon the paratracheal parenchyma cells produce gum that flows through the pits and fill the lumen. In gymnosperm, non-active aspirated pits are present. The wood loses cell sap, water and reserve materials. Xylem parenchyma becomes heavily lignified. Various organic compounds like oils, resins, gums, tannins etc. accumulate in the cell lumen, which imparts the dark color.
Due to impregnation of these substances, the heartwood becomes more durable and resistant to decay. The living xylem elements of sapwood in course of time die become inactive and gradually converted to heartwood. However, in Sequoia sempervirens the ray parenchyma of heartwood may remain alive for hundred years.
The heartwood has enormous commercial value. The heartwood yields good quality of timber due to its durability and resistance to decay. The dye haematoxylin is obtained from the heartwood of Haematoxylum campechianum.
The ratio between the quantity and the degree of difference of heartwood and sapwood varies which is due to different conditions of growth. In Abies, Picea the heartwoods are not well differentiated. Taxus, Morus possess thin sapwood while Fagus and Acer etc. consist thick sapwood.
Reaction wood:
Reaction wood is developed in stems and branches of a tree that are under stress. Gravity causes lateral stress. In response to such stress dicots and gymnosperm plants produce reaction wood and thus prevent the branches from drooping and becoming pendant. Reaction wood appears in three different forms according to the nature of plants. They are referred to as tension wood, compression wood and contrasting wood.
i. Tension wood:
Tension wood, as the name implies, develops under tension. It is formed in dicotyledonous deciduous trees on the upper side of leaning hardwood tree branches. It is also formed on the curved side of the stem.
A cross- section of a branch having tension wood reveals that the wood is lighter in colour than normal wood. The growth rings are eccentric and much wider on the upper side of a branch than normal wood. Gelatinous fibres compose tension wood.
The cell wall of these fibres have little or no lignin and high cellulose content (Fig. 20.18) that fills most of the volume of lumen of cells. The content of hemicellulose is also different from that of normal wood. The tension wood has fuzzy appearance. This is due to the fact that when tension wood is sawed the fibres tend to pull out giving a fuzzy or villous surface.
ii. Compression wood:
The reaction wood that is formed in conifers is referred to as compression wood. Compression wood is formed on the underside of softwood tree branches in response to compression stress. A cross-section of a branch having compression wood reveals that the wood is darker in colour than normal wood. The growth rings are wider than normal.
In this wood the tracheids are short. In cross-sectional view the tracheids are round and spaces occur between the corners of them. The structure of the secondary wall of tracheid is noteworthy.
The secondary wall is composed of S1 and S2 (=outer & middle layer respectively) layers only. S3 (the innermost layer) layer is absent. The compression wood has less cellulose (Fig. 20.18). It is enriched in lignin; as a result it has higher ductility than the normal soft wood.
iii. Contrasting wood:
The wood that develops on the opposite side of reaction wood is referred to as contrasting wood. In softwood conifer trees it develops on the outer side and in hardwood deciduous trees it develops on the underside of the leaning branch. The contrasting wood exhibits thin growth rings, long tracheids with square or rectangular cross-section-walls and thick S3 (inner layer) layer in the structure of the secondary wall.
Secondary phloem:
The vascular cambium ring produces secondary phloem on the peripheral side. The elements of secondary phloem are arranged in vertical and horizontal manner and thus compose the axial and radial system of plants. In some dicotyledonous plant, growth rings composed of phloem, are observed but the demarcation lines are not prominent.
Sieve elements, phloem fibre and phloem parenchyma represent the vertical system. They are developed from the fusiform initials of cambium. The ray initials in the cambium divide and produce xylem rays inwardly and phloem ray on the peripheral side. The phloem rays compose the radial system and are continuous with the xylem rays.
In conifers sieve cells, albuminous cells, parenchyma cells and fibres represent the vertical system. Uniseriate phloem rays, which consist of parenchyma cells only, comprise the radial system and in some cases albuminous cells are found in association with parenchyma cells. The ray cells may be in contact with sieve cells lengthwise.
The sieve cells overlap one another at their ends and in these regions, there are more sieve areas. The parenchyma cells comprising the vertical system may store starch, resins, fats, tannins and sometimes crystals. Calcium oxalate crystals are found in Larix. Fibres are absent from secondary phloem of Pinus.
Resin ducts are frequently observed in secondary phloem of conifers. The resin, obtained from Abies balsaynea, is popularly known as Canada balsam and is commonly used as mounting medium in microscopical preparation because it possesses the same refractive index of a glass.
In dicotyledons, the vertical system of secondary phloem consists of sieve tubes, companion cells, phloem fibres, phloem parenchyma and the horizontal system comprises uniseriate or multiseriate ray parenchyma cells only.
In both systems sclereids, laticifers and crystals are observed. The secondary phloem of Pyrus malus contains fibres that do not develop from the fusiform initials of cambium; instead they arise from non-functioning phloem. These fibres are termed as fibre-sclereids.
Sclereids are present in functioning and nonfunctioning phloem. They may be the sole sclerified element of the secondary phloem (e.g. Fagus), or they may be associated with fibre (e.g. Prunus).
In the sieve tubes the sieve areas are well developed in the secondary phloem. Vitis, Populus, in their sieve tubes, possess oblique end walls with compound sieve plates. Simple sieve plates with slightly oblique end walls are found in Acer.