Growth in plants occur by the activity of the apical meristems present in the apices or tips of stem, branches and roots. Apical meristems divide and form new cells. In due course such tissues become permanent and lays down the basic or fundamental body of plants. Thus the tissues which derive their origin from the apical meristem are called primary permanent tissues and the plant body made of primary tissues is called primary body. Thus, the apical meristem is responsible for the formation of the primary growth of the plants.
Thus, apical meristems cause linear growth.
In monocotyledons and pteridophytesthis primary structure remain as such throughout the life of the plants. It is structurally and functionally both self-sufficient.
In Dicotyledons and Gymnosperms the primary growth is not able to bear the load of plant – branches, flowers, fruits. Therefore, there is required additional strength to the stem and branches.
Thus, growth in thickness is called secondary growth. The increase in thickness due to the addition of secondary tissues cut off by the vascular cambium and the cork cambium in the stellar and extra-stelar regions respectively, is called secondary growth.
Secondary growth is common in gymnosperms and dicotyledons, but is normally absent in monocotyledonous plants. However, in some exceptional genera of monocotyledons such as Dracaena and Yucca, anomalous secondary growth can be observed.
Secondary Growth in Dicot Stems
Secondary growth in dicot stems is initiated in the intrastelar region with the activities of cambium (fascicular or interfascicular cambium). In the extrastelar region cork cambium gives rise to the periderm.
Intrastelar Secondary Growth:
The secondary growth occurring in the stellar region is called intrastelar secondary growth. It includes the following:-
1. Formation of Cambial Ring:
In young dicotyledonous stem limited number of vascular bundles are arranged in form of a ring. Each vascular bundle is conjoint, collateral, open and endarch. They are called as open because a single layer of cambium cells called the intra fascicular cambium (fascicle – bundle) is present between the xylem and phloem.
In between the vascular bundles there are medullary rays, the cells of which become mertistematic and form a cambium strip called the interfascicular cambium, i.e., the cambium in between the two vascular bundles.
The fascicular and inter fascicular cambium join laterally to form a complete ring of cambium.
Activity of Cambium:-The vascular cambium consists of two types of initiating cells – the fusiform initials and ray initials. The fusiform initials are elongated and spindle shaped. They produce the elements of secondary xylem and secondary phloem. The ray initials are small and isodiametric. They produce phloem rays to the outside and the xylem rays to the inside. These rays are called the vascular rays.
Formation of Secondary Tissues:- The cells of vascular cambium divide repeatedly by periclinal method i.e., in a direction parallel with the epidermis. Each time a cambial cell divides into two, one of the daughter cells remainmeristematic, while the other is differentiated into a permanent tissue.
Those cells which are produced outside the cambial ring differentiate into secondary phloem and those produced to the inner side of the cambial ring differentiate into secondary xylem.
The cambium cells divide continuously in this manner producing secondary tissues on both sides of it. In this way, new cells are added to the xylem and phloem, and the vascular tissues increase in size.
Normally the cambium produces more amount of secondary xylem to the innerside and less amount of secondary phloem to the outerside.
The cell formed from the ray initials of cambium in the region between the vascular bundles become the secondary medullary rays. They extend from pith to the secondary xylem and phloem. The portion of the ray present in the xylem region is called xylem ray or wood ray and the portion of the ray in the phloem is called phloem ray. These rays help in radial conduction of water, salts and food materials.
The formation of new cells from the cambium results in an enlargement of the stem that is known as secondary thickening. As the stem increases in thickness, the primary phloem and primary xylem become obliterated and replaced by secondary phloem and secondary xylem.
Extra Stelar Secondary Growth:
Due to the formation of secondary vascular tissues in the stellar region, an outward pressure is exerted on the epidermis. Due to this, epidermisgets stretched and ultimately tends to rupture exposing the living cells.
At this stage, a new protective layer called the periderm is produced in the cortical region.
Periderm is formed by the activity of a secondary lateral meristem called phellogen or cork cambium.
Secondary growth in cortex begins with the appearance of a meristematic layer either sub-epidermal or epidermal (e.g., Teak, Azadirachta) or in the cortex (e.g., Aristolochia).
In contrast to the vascular cambium, the phellogen is relatively simple in structure and composed of one type of cells. They are rectangular and have vacuolated protoplasts and may contain tannins and chloroplasts.
The cells of phellogen divide vertically and cut off many cells toward the outside and toward the inside. The cells formed towards the innerside develop into secondary cortex or phelloderm and those cells formed towards the outer side develop into phellem or cork. Usually more amount of cork is produced than the secondary cortex. The phellogen (cork cambium), phellem (cork) and phelloderm (secondary cortex) together constitute periderm.
Phellem or Cork:
Phellem arises towards the outerside of the phellogen. They are polygonal and uniform in shape. The cells are closely arranged without intercellular spaces and with thin cellulose cell walls. The cells later become dead by losing their protoplasts and their walls become thicker due to the deposition of suberin. The cells are impervious to water and gases. They give protection to inner parts of the organ.
Commercial Cork:
The phellem of Quercussuber (oak tree) is the source of commercial cork. In this plant, the phellogen arises in the epidermis, which forms extended masses of cork tissues. At the age of twenty years, when the tree is about 40 cm in circumference, this outer layer, known as virgin cork.this cork is stripped off for the quick formation of commercial cork.
The exposed tissue dries out to about 1/8 inch in depth. A new phellogen is established beneath the dry layer and rapidly produces a massive cork of a better quality than the first. After 9 or 10 years the new cork layer of formed with sufficient thickness to be commercially valuable and is in turn removed.
The stripping of the cork take place at intervals of about nine years until the tree is 150 or more years old. The commercial cork cells have thin walls and cells are filled with air. Due to suberin, it is impervious to water and resistant to oil. Because of air filled lacuna, the cork is light in weight, and has thermal insulator qualities. The important properties of the commercial cork are its imperviousness, its lightness, toughness and elasticity.
Phelloderm:
The phellogen cuts off the phelloderm cells towards inner side. The phelloderm cells are living cells with cellulose walls. The cells contain vacuolated cytoplasm and shows a conspicuous nucleus. In most plants, they resemble cortical cells but they are arranged in radial rows because they arise from the tangentially dividing phellogen.
In some species, they act as photosynthetic tissue and aid in starch storage.
Bark:
All the tissues outside the vascular cambium of the stem is called as bark. Thus, it includes the secondary phloem and periderm.
As the periderm develops, it becomes separated by a non-living layer of cork cells from the living tissues. The tissue layers thus separated become dead.
When the cork cambium arises from the inner layers of the cortex, the bark is thick; e.g., Thuja. If it is formed from the outer layers, the bark is thin; e.g., Psidium guajava.
When the cork cambium is organized in the form of a complete ring, the bark that is produced also develops in the form of a ring, which can be stripped easily. This is known as ring bark. E.g., Betula, Clematis. Whereas in Psidium, Eucalyptus, the cork cambium originates in strips, the bark is in form of overlapping strips. The bark is removed as strips or scales. Such bark is known as scale bark.
Bark in Cinchona (yields quinoine) and Cinnamomum (source of Dalchini) are commercially important.
Lenticels:
Due to secondary growth, the periderm develops in place of the epidermis. Since the cork tissue is composed of closely arranged, dead, suberin coated cells, gaseous exchange between the internal tissues and the external atmosphere is obstructed.
So, to carry out the gaseous exchange, small openings composed of mature cells develop. These openings are called as lenticels. They are located opposite to stomata and carry out their function in the secondary body of the plants.
The lenticels originate beneath the stomata, either just before, or simultaneously with the initiation of the first layer of the periderm. As the lenticel formation begins, the parenchyma cells found near the sub-stomatal cavity lose their chlorophyll and divide irregularly in different planes giving rise to a mass of colourless, rounded, thin walled, loose cells called complementary cells.
As the complementary cells increase in number, pressure is caused against the epidermis and it ruptures. The thin walled loose complementary cells alternate with masses of more dense and compact cells called the closing cells. These cells together form a layer called closing layer.
Complementary cells are thin-walled, rounded and loose with sufficiently developed intercellular spaces among them. Their cell walls are not suberized. Due to the presence of profuse intercellular spaces, the lenticels perform the function of exchange of gases between the atmosphere and internal tissues of the plant.
Annual Rings or Growth Rings:
The secondary xylem in the stems of perennial plants commonly consists of concentric layers each one of which represents a seasonal increment. In transverse section of the axis, these layers appear as rings, and called annual rings or growth rings.
They are commonly termed as annual rings because in woody plants of temperate regions and in those of tropical regions where there is an annual alternation of growing and dormant period, each layer represents the growth of one year.
The cambium exhibits its activity as periodical or seasonal due to climatic variation. During spring season, the plant has to translocate more water and mineral because they develop new buds, leaves and flowers. Therefore, the cambium becomes more active in this season and forms xylem vessel with wider cavities. The xylem formed during spring season is called early wood or spring wood.
On the other hand, during winter the rate of assimilation is decreased and there is less need of vessels for sap transport, the cambium is less active and gives rise to narrow vessels, tracheids and wood fibres. The xylem formed during winter is called late wood or autumn wood.
Thus spring wood with wider vessels and autum wood with narrow vessels formed during one year together make an annual ring or growth ring. Thus, the periods of active growth alternate with the periods of slow growth.
Generally, the late wood is more denser and harder than the early wood.
By counting the total number of annual rings, the age of the plant can approximately be determined. Thus, determination of age of a tree by counting the annual rings is known as dendrochronology.
Porous wood and Non Porous wood:
Gymnosperms (conifer and cycads) lack vessels and their wood is made of only tracheids. Therefore, their wood is called non-porous and soft wood. On the other hand angiosperm wood is made of tracheids and vessles both. Therefore, their wood is called porous and hard wood. Hard wood or soft wood have no relation with physical hardness of wood.
In porous woods, when large vessels of unequal diameter are arranged more or less in a ring, the wood is called ring porous wood. E.g., Castanea ring porous vessels conduct more water. On the other hand, when vessels of equal dimensions are found uniformly distributed, the wood is called diffuse porous wood, e.g., Acer, Betula.
Heart Wood and Sap Wood:
The outer region of the old trees consisting of recently formed xylem elements is sapwood or alburnum. This is of light colour and contains some living cells in association with vessels and fibres. This part of the stem performs the physiological activities, such as conduction of water and minerals, storage of food, etc.
The central region of the old trees, which was formed earlier is whose cells are inactive, non-fucntional without any living cells is called as heart wood or duramen. The secondary xylem in this region is filled up with tannins, resins, gums and other substances which make it hard and durable and it is dark in colour. Their vessels are plugged with tyloses.
The function of heartwood is no longer conduction, it gives only mechanical support to the stem.
The sapwood changes into heartwood very gradually. During the transformation a number of changes occur – all living cells lose protoplasts, water content of cell walls are reduced, food material are withdrawn from the living cells, tyloses are formed.
From economic point of view, heartwood is more useful than sapwood. Heartwood, as timber is more durable than sapwood, because the reduction of food materials available for pathogens by the absence of protoplasm and starch.
The haemotoxylin is obtained from the heartwood of Haematoxylon campechianum.
Because of the absence of resin, gums and colouring substances, sapwood is preferred for pulpwood, and for wood to be impregnated with preservatives.
Tyloses:
In many plants, axial and ray parenchyma cells located next to the vessels form ballon-like outgrowth through pit cavities into the lumen of the vessels. These outgrowths are called as tyloses.
The parenchyma cells, adjoining the half-bordered pits of vessels, penetrate into the vessel in the form of short protuberances. These protuberances gradually enlarge to form ballon-like structuresThe nucleus and part of the cytoplasm of the parenchyma cell commonly migrate into the tyloses. The tyloses are filled with starch, resins, gums and other substances.
Usually, they are sufficiently large and the lumen of the vessel is almost blocked. They add to the durability of the wood. Tyloses prevent rapid entrance of water, air and fungus by blocking the lumen of the vessel. In many plants the development of tyloses takes place by means of wounding.
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