Secondary Growth in Dicotyledonous Stems (With Diagram) | Botany

Let us make an in-depth study of  Secondary growth in Dicotyledonous Stems. After reading this article you will learn about: 1. Introduction to Secondary Growth 2. Secondary Growth in Various Parts of Dicotyledonous Stems.

Introduction to Secondary Growth:

The primary body of the plant is developed from the apical meristem. Sometimes as in monocotyledons and pteridophytes, the primary plant body is complete in itself and does not grow in thickness by cambial activity.

However, in dicotyledons, the primary permanent tissues make the fundamental parts of the plant, and the further growth in thickness is completed by cambial activity, called secondary growth in thickness.

The tissues, formed during secondary growth are called secondary tissues. Secondary tissues may be two types—the vascular tissues that are developed by the true cambium, and cork and phelloderm, which are formed by phellogen or cork-cambium.

In a typical dicotyledonous stem, the secondary growth starts in the intra- and extrastelar regions.

Secondary Growth in Various Parts of Dicotyledonous Stems:

Cambium:

The vascular bundles of dicotyledonous stems are collateral and open, and arranged in a ring. They contain a single layer of cambium cells, which separate the xylem from the phloem, called fascicular cambium, i.e., the cambium of the vascular bundle, (fascicle = bundle).

When the primary xylem and primary phloem are first differentiated there is no cambium across the pith rays or medullary rays to connect the edges of the cambium within vascular bundles.

As soon as the differentiation of the first xylem and phloem of the bundles takes place, the cells of the pith or medullary rays which lie in between the edges of the cambium within the bundles, divide accordingly and form a layer of cambium across the medullary rays.

The newly formed cambium connects the fascicular cambium found within the vascular bundles, and thus a complete cambium ring is formed. The newly formed cambium strip which occurs in the gaps between the bundles is called inter-fascicular cambium, i.e., the cambium in between two vascular bundles. Thus a complete cambium ring is formed.

The cambium layer consists essentially of a single layer of cells. These cells divide in a direction parallel with the epidermis. Each time a cambial cell divides into two; one of the daughter cells remains meristematic, while the other is differentiated into a permanent tissue.

If the cell that is differentiated is next to the xylem it forms xylem, while if it is next to phloem it becomes phloem towards the outer side of the cambium. 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 the phloem, and the vascular bundles increase in size.

While there is more or less alternation in the production of xylem and phloem cells from a cambium cell, more cells are formed on the xylem side than on the phloem side. The cells formed from the cambium in the region of the pith rays become pith-ray cells. The activity of the cambium thus increases the length of the pith rays grow equally.

The formation of new cells from the cambium result in an enlargement of the stem that is known as the secondary thickening. The formation of new cells in secondary thickening continues throughout the life of the plant. It is in this way that the trunks of trees continue to grow in diameter. The cambium perpetuates and remains active for a considerable long period of time.

The thin-walled cells of the vascular cambium are highly vacuolate and in this respect are unlike most other meristematic cells. The electron microscopic structure reveals their highly vacuolate nature. Many ribosomes and dictyosomes, and well developed endoplasmic reticulum, are present (Srivastava, L.M., 1966).

Secondary Xylem:

The cambium ring cuts off new cells on its inner side are gradually modified into xylary elements, called the secondary xylem. This tissue serves many important functions, such as conduction of water and nutrients, mechanical support, etc. The secondary xylem of tree trunks is of great economic value, since it constitutes the timber and wood of commerce.

The secondary xylem consists of a compact mass of thick-walled cells so arranged as to form two systems—a longitudinal (vertical) and a transverse radiating system. The longitudinal system consists of elongate, overlapping and interlocked cells—tracheids, fibres and vessel elements—and longitudinal rows of parenchyma cells.

All these cells possess their long axes parallel with the long axis of the organ of which they are a part.

The secondary xylem consists of scalariform and pitted vessels, tracheids, wood fibres and wood parenchyma. These elements of secondary xylem are more or less similar to those occur in primary xylem. Vessels or tracheae are most abundant and are usually shorter than those of primary xylem. Mostly the vessels are pitted. Annular and spiral tracheids and vessels are altogether absent.

Xylem parenchyma cells may be long and fusiform, but sometimes they are short. They are living cells and usually meant for storage of food material (starch and fat) in them. Tannins and crystals are frequently found in these cells. Xylem parenchyma may occur either in the association of the vessels or quite independently. The fibres of secondary xylem possess thick walls and bordered pits.

Distribution of Wood (Xylem) Parenchyma:

Wood parenchyma is distributed in three ways:

(i) Terminal wood parenchyma;

(ii) Diffuse or metatracheal wood parenchyma and

(iii) Vasicentric or paratracheal wood parenchyma.

Terminal Wood Parenchyma:

In some gymnosperm woods, wood parenchyma is absent; in other (e.g., Larix and Pseudotsuga), and in some angiosperm woods (e.g., Magnolia and Salix), wood parenchyma cells occur only in the last-formed tissue of the annual ring. Such woods have terminal wood parenchyma.

Diffuse or Metatracheal Wood Parenchyma:

Where parenchyma occurs not only in this region, but also remains scattered throughout the annual ring, some of the cells lying among the tracheids, and fibre-tracheids the plant has diffuse or metatracheal wood parenchyma (e.g., in Malus, Quercus, Diospyros, etc.).

Vasicentric or Paratracheal Wood Parenchyma:

Where parenchyma occurs at the edge of the annual ring and elsewhere only about vessels and does not occur isolated among tracheids and fibres, the plant possesses vasicentric or paratracheal wood parenchyma (e.g., in Acer, Fraxinus, etc.).

Xylem Rays:

The xylem rays or wood rays extend radially in the secondary xylem. They are strap or ribbon like. They originate from the ray initials. The xylem rays run as a continuous band to the secondary phloem through the cambium, thus forming a continuous conducting system. All vascular rays are initiated by the cambium and, once formed, are increased in length indefinitely by the cambium.

Commonly these rays are known as medullary rays, or pith rays, on the basis of their similarity and parenchymatous nature with the pith rays of herbaceous dicotyledonous stems. These radial rays may be best called vascular rays, as these rays are of vascular tissue partly of xylem and partly of phloem.

The xylem rays traverse in the secondary xylem and establish communication with the living cells of the vascular tissue. In gymnosperm wood where no wood parenchyma is present, every tracheid is in direct contact with at least one ray. Vessels also in their longitudinal extent come into contact with many rays.

In herbaceous stems, such as of Ranunculus, where vascular bundles are separated by projecting parenchymatous wedges, and in vines, such as Clematis, where the bundles are separated by bands of secondary parenchyma, vascular rays are not found. The xylem rays help in the exchange of gases. They also aid in the conduction of water and food from phloem to the cambium and xylem parenchyma.

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 are called annual rings or growth rings.

They are commonly termed as annual rings because in the 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 width of growth rings varies greatly and depends upon the rate of the growth of tree. Unfavourable growing seasons produce narrow rings, and favourable seasons wide ones. Annual or growth rings are characteristic of woody plants of temperate climates.

Such rings are weakly developed in tropical forms except where there are marked climate changes such as distinct moist and dry seasons. Annuals and herbaceous stems show, naturally, but one layer.

In regions with a pronounced cold season, the activity of the cambium takes place only during the spring and summer seasons thus giving rise the growth in diameter of woody plants. The wood of one season is sharply distinct from that of the next season. In spring or summer the cambium is more active and forms a greater number of vessels with wider cavities.

As the number of leaves increases in the spring season, additional vessels are needed for the transport of sap at that time to supply the increased leaves. In winter or autumn season, however, there is less need of vessels for sap transport, the cambium is less active and gives rise to narrow pitted vessels, tracheids and wood fibres.

The wood developed in the summer or spring season is called spring wood or early wood, and the wood formed in winter or autumn season is known as autumn wood or late wood. However, the line of demarcation is quite conspicuous between the late wood of one year and the early wood of next year. An annual ring, therefore, consists of two parts—an inner layer, early wood, and an outer layer late wood.

Dendrochronology:

Each annual ring corresponds to one year’s growth, and on the basis of these rings the age of a particular plant can easily be calculated. The determination of age of a tree by counting the annual rings is known as dendrochronology.

Sometimes two annual rings are formed in a single year, and in such cases the counting of the annual rings does not show the correct age of the tree. This happens perhaps because of the drought conditions prevailed in the middle of a growing season.

Tyloses:

In many plants, the walls of the xylem vessels produce balloon like outgrowths into the lumen of the vessels are called tyloses. Usually these structures are formed in secondary xylem but they may also develop in primary xylem vessels. Tyloses are formed by the enlargement of the pit membranes of the half-bordered pits present in between a parenchyma cell and a vessel or a tracheid.

Usually they are sufficiently large and the lumen of the vessel is almost blocked. The nucleus of the xylem parenchyma cells along with cytoplasm passes into this balloon like outgrowth. The delicate pit membrane forms the balloon like tylosis inside the lumen cavity. In fully developed tyloses, starch crystals, resin gums and other substances are found, but they are not found very frequently.

The wall of tylosis may remain thin and membranous or very rarely it becomes thick and even lignified. The tylosis may remain very small or sufficiently large in size as the case may be. They may be one or few in number (e.g., in Populus) in a single cell or many (e.g., in white oak) and may fill the complete cell.

They are commonly found in many angiospermic families. Normally they develop in the heart wood of angiosperms and block the lumen of the vessels, and thus add to the durability of the wood. Tyloses also occur in the vessels of Coleus, Cucurbita, Rumex, Asarum and Convolvulus. Tyloses prevent rapid entrance of water, air and fungus by blocking the lumen of the vessel.

Tyloses are said to undergo division in some plants and form multicellular tissue, which fills the lumen compactly, as in Robinia and Madura. The tyloses are characteristic of certain species, and always absent in others.

In many plants the development of tylosis takes place by means of wounding. They may be present in the inner part of leaf traces after the leaf has fallen. Such tyloses occur rarely; they are irregular in shape and size.

In the wood of conifers there is also found a closing of the cavity of resin canals by the enlargement of the epithelial cells. These enlarged cells are commonly known as tylosoids.

Sapwood and Heartwood:

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 also in the association of vessels and fibres. This part of the stem performs the physiological activities, such as conduction of water and nutrients, storage of food, etc.

The central region of the old trees, which was formed earlier, is filled up with tannins, resins, gums and other substances which make it hard and durable, is called heartwood or duramen. It looks black due to the presence of various substances in it. Usually the vessels remain plugged with tyloses. The function of heartwood is no longer of 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 contents of cell walls are reduced; food materials are withdrawn from the living cells; tyloses are frequently formed which block the vessels; the parenchyma walls become lignified; oils, gums, tannins, resins and other substances develop in the cells.

In certain plants—for example, Ulmus and Malus pumila, the heartwood remains saturated with water; in other plants, for example, in Fraxinus the heartwood may become very dry. The oils, resins and colouring materials infiltrate the walls, and gums and resins may fill the lumina of the cells.

In Diospyros and Swietenia, the cell cavities are filled with a dark-coloured gummy substance. The colour of heartwood, in general, is the result of the presence of these substances. Generally the heartwood is darker in colour than sapwood. However, in some genera, such as Betula, Populus, Picea, Agathis the heartwood is hardly darker in colour than the sapwood.

The proportion of sapwood and heartwood is highly variable in different species. Some trees do not have clearly differentiated heartwood (e.g., Populus, Salix, Picea, Abies), others possess thin sapwood (e.g., Robinia, Moras, Taxus), the still others possess a thick sapwood (e.g., Acer, Fraxinus, Fagus).

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 formation of resins, oils and tannins, and the blocking of the vessels by tyloses and gums, render the wood less susceptible to attack by the organisms of decay. 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.

Secondary Phloem:

The cambial cells divide tangentially and produce secondary phloem elements towards outside of it. Normally, the amount of secondary phloem is lesser than the amount of secondary xylem. In most of the dicotyledons; usually the primary phloem becomes crushed and functionless and the secondary phloem performs all physiological activities for sufficiently a long period of time.

This is a complex tissue made up of various types of cells having common origin in the cambium. These cells are quite similar to the cells of primary phloem. However, the secondary phloem possesses a more regular arrangement of the cells in radial rows. The sieve tubes are comparatively larger in number and possess thicker walls.

The elements of secondary phloem are sieve tubes, companion cells, and phloem parenchyma and phloem ray cells. Sometimes sclerenchyma is also found. Presence of sieve tubes is characteristic of angiosperms, however, they are not found in gymnosperms. In gymnosperms, sieve cells are present.

The companion cells are not found in gymnosperms but probably they are present in all types of angiosperms. The companion cells are usually found accompanied with the sieve tubes. Phloem parenchyma cells are also found in the secondary phloem of all plants except few primitive types.

Phloem parenchyma cells are formed directly from parenchyma mother cells, which are formed from cambial cells. Sclerenchyma is also found in the secondary phloem of several plants. Usually the fibres occur in tangential bands. In certain plants which possess a hard or tough bark, the fibres consist the greater part of the secondary phloem and surround the softer tissues.

Sieve tubes are series of sieve-tube elements attached end to end with certain sieve areas more highly specialized than others. The sieve tubes of the secondary phloem of dicotyledons are of many types as regards the shape and nature of the end and side walls.

In many woody species (e.g., Carya cordiformis), the oblique end walls of the sieve tube elements frequently extend for about half the length of the element.

These oblique walls possess many areas which together make compound sieve plates. The other type, i.e., simple sieve plate is found in Robinia, Madura and some species of Ulmus. Here the terminal walls of the sieve tube elements are transverse and there is a single specialized sieve area. In the majority of species, the sieve tube elements of the secondary phloem possess simple sieve plates.

Sclerenchyma of one type or another is a characteristic of the secondary phloem of several species. Fibres occur frequently in definite tangential bands (e.g., in Liriodendron and Populus). In Cephalanthus, the fibres are found singly. However, in Carya cordiformis, the fibres constitute the greater part of the secondary phloem and surround the groups of softer tissues.

All conditions have been reported in gymnosperms. The phloem of Pinus strobus lacks sclerenchyma; well developed tangential bands of fibres are found to be present in Juniperus, and large masses of sclereids are present in Tsuga. In Thuja occidentalis, the fibres are arranged in uniseriate tangential rows. These rows of fibres alternate with rows of sieve cells and phloem parenchyma.

In Platanus and Fagus sclereids are the only type of sclerenchyma present in the phloem. The sclereids are found abundantly in the older, living, but non-conducting phloem of the woody plants.

Phloem Rays:

The phloem rays are usually present in the vascular tissues developed by the cambium. The vascular rays are formed in the cambium and develop on either side of it with the secondary xylem and secondary phloem of which they are a part. The phloem rays may be one to several cells in width. Normally they are of uniform width throughout their length.

They may increase in width outwardly, the increase being due to the multiplication of the cells or to the increase in size of cells toward the outer end of the ray. The phloem rays may be one cell wide (e.g., in Castanea and Salix), two or three cells wide (e.g., in Malus pumila) or many cells wide (e.g., in Robinia and Liriodendron).

However, in oaks there are two types of phloem rays—one very broad and the other uniseriate.

Commonly the phloem ray cells in woody plants, as seen in transverse section, are rectangular and radially elongated. In herbaceous plants, commonly the ray cells are globose. In Cephalanthus, Agrimonia and Potentilla the ray cells closely resemble the phloem parenchyma cells. All phloem ray cells are parenchymatous with active protoplasm, but as they become older many of them become sclereids.

A special type of ray cell known as albuminous cell is found in gymnosperms. These albuminous cells are found to be situated at the upper and lower margins of the phloem rays. The albuminous cells differ from the ordinary ray cells both structurally and functionally.

They are joined directly with the sieve cells by sieve areas. They do not contain starch, and are of much greater vertical diameter than the normal ray cells. They retain their protoplasts as long as the sieve cells with which they are connected function. It is thought that they function like companion cells of angiosperms.

Seasonal Rings in Secondary Phloem:

The tissues of the secondary phloem are generally arranged in definite tangential bands. These layers of tissue have the appearance of annual rings. However, these ring like bands do not possess definite seasonal limits like those of secondary xylem, because there is no sharp distinction between the phloem cells formed in the early and late growing season.

Seasonal formation of sclerenchyma bands may exist, but this is not constant feature. In tropical plants new layers of phloem and xylem are formed with each period of new growth.

Function:

The functions of secondary phloem are normally the same as that of primary phloem. The various cells of secondary phloem are structurally adapted for the function of translocation of food. The sieve tubes, companion cells and some phloem parenchyma cells are especially adapted for lengthwise conduction, and certain phloem rays help in horizontal conduction to and from the xylem and the cambium.

Some of the phloem parenchyma cells in some plants act as storage tissue of starch, crystals and other organic materials.

Economic Importance:

The secondary phloem of various trees and shrubs of the Malvaceae, Tiliaceae, Moraceae has provided bast fibres for economic purposes. The tapa cloth of Pacific islands is composed of mainly of phloem fibres.

Tannin obtained from the secondary phloem of various plants is utilized for the preparation of spices and drugs. Secretory canals are abundantly found in the secondary phloem, and the secretions are of much economic value—such as rubber is obtained from the latex of Hevea brasiliensis, and resins from various gymnosperms.

Periderm:

Due to continued formation of secondary tissues, in the older stem, and roots, however, the epidermis gets stretched and ultimately tends to rupture and followed by the death of epidermal cells and outer tissues, and a new protective layer is developed called periderm. The formation of periderm is a common phenomenon in stems and roots of dicotyledons and gymnosperms that increase in thickness by secondary growth.

Structurally, the periderm consists of three parts:

1. A meristem known as phellogen or cork cambium,

2. The layer of cells cut off by phellogen on the outer side, the phellem or cork, and

3. The cells cut off by phellogen towards inner side, the phelloderm.

The periderm appears on the surface of those plant parts that possess a continuous increase in thickness by secondary growth. Usually the periderm occurs in the roots and stems and their branches in gymnosperms and woody dicotyledons. It occurs in herbaceous dicotyledons, sometimes limited to the oldest parts of stem or root.

Phellogen:

In contrast to the vascular cambium, the phellogen is relatively simple in structure, and composed of one type of cells. The cells of phellogen appear rectangular in cross- section, and somewhat flattened radially. Their protoplasts are vacuolated and may contain tannins and chloroplasts, except in the lenticels, intercellular spaces lacking.

When we consider the place of origin of the meristem forming the periderm, it becomes necessary to distinguish between the first periderm, and the subsequent periderms, which arise beneath the first and replace it as the axis increases in circumference. In most stems the first phellogen arises in the sub-epidermal layer.

In a few plants the phellogen arises in the epidermal cells (e.g., Nerium, Pyrus). Sometimes only a part of the phellogen is developed from epidermis while the other part arises in sub-epidermal cells (e.g., Pyrus). In some stems the second or third cortical layer initiates the development of periderm (e.g., Robinia, Aristolochia, Pinus, Larix, etc.).

In still other plants the phellogen arises near the vascular region or directly in the phloem (e.g., in Caryophyllaceae, Cupressaceae, Ericaceae, Punica, Vitis, etc.). If the first periderm is followed by the formation of others, these are formed repeatedly, in successively deeper layers of the cortex or phloem.

At the time of the beginning of the development of a phellogen in epidermal cells, the protoplasts lose their central vacuoles and the cytoplasm increases in amount and becomes more richly granular. As soon as this initial layer develops, it divides tangentially and, to a lesser extent radially, in the similar way as division takes place in true cambium. The derivative cells are normally arranged in radial rows.

Generally, several to many times as many cells are cut off toward the outside (phellem-cork cells) as toward the inside (phelloderm). Phelloderm cells are few or absent; rarely phelloderm is greater in amount than phellem.

Phellem (Cork Cells):

The cells that constitute phellem are commonly known as cork cells. They are like the phellogen cells from which they are derived. As seen in tangential section, they are polygonal and uniform in shape, and often radially thin as seen in cross section of the stem.

The cells of the commercial cork (Quercus suber) are radially elongated as seen in transverse section. In the periderm of Betula and Prunus, the cork cells are elongated tangentially as seen in cross-section. There are no intercellular spaces among cork cells.

Commercial Cork:

The development of the periderm layers in the cork oaks (Quercus suber) is of special interest. The ability of the plant produce phellogen in deeper layers when the superficial periderm is removed is utilized in the production of commercial cork from the cork oak (Quercus suber).

At the age of about twenty years, when the tree is about 40 cm in circumference, this outer layer, known as virgin cork is removed by stripping to the phellogen. The exposed tissue dries out to about 1/8 in. 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 nine or ten years the new cork layer has attained sufficient thickness to be commercially valuable and is in turn removed. Of course, this cork is of better quality than the virgin cork, but of inferior quality than the cork obtained at the third and subsequent stripping’s. These stripping’s take place at intervals of about nine years until the tree is 150 or more years old.

After the successive stripping’s the new phellogen layers develop at greater depth in the living tissue. The cortex is lost after few stripping’s and the subsequent cork layers are formed in the secondary phloem. 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. In most plants, they resemble cortical cells in wall structure and contents. Their shape is similar to that of phellogen cells. They may be distinguished from cortical cells by their arrangement in radial series resulting from their origin from the tangentially dividing phellogen.

In some species they act as photosynthetic tissue and aid in starch storage. They are pitted like other parenchyma cells. Occasionally, the sclereids and other such specialized cells occur in phelloderm. The term secondary cortex is sometimes applied to phelloderm, which does not seem to be appropriate.

Bark:

The term bark is commonly applied to all tissues outside the vascular cambium of the stem, in either primary or secondary state of growth. In this way, bark includes primary phloem and cortex in stem with primary tissues only, and primary and secondary phloem, cortex and periderm in stem with secondary tissues.

This term is also used to denote the tissue that is accumulated on the surface of the stem as a result of the activity of cork cambium.

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. The term bark in restricted sense is applied to these dead tissues together with the cork layers. In wider sense the term is applied to denote the tissues outside the vascular cambium. However, the term bark is loose and non-technical.

Rhytidome:

In most of plants, as soon as the first phellogen ceases to function, second phellogen develops in the tissue below the first one. In this way additional layers of periderm are formed in the progressively deeper regions of the stem, thus new phellogen layers arise in deeper regions of the cortex which may exceed even upto phloem.

As the phellogen arises in deeper region and cuts cork cells or phellem towards outside, all the living cells outside the phellogen do not get water supply and nutrients, and become dead. These dead tissues formed outside the phellogen constitute the rhytidome.

In some rhytidomes parenchyma and soft cork cells predominate whereas others contain large amounts of fibres usually derived from the phloem. The manners in which the successive layers of periderm originate possesses a characteristic effect upon the appearance of the rhytidome.

When the sequent periderms develop as overlapping scale-like layers, the outer tissue breaks up into units related to the layers of periderm, and thus formed outer bark is termed scale- bark. On the other hand, if the phellogen arises around the whole circumference of the stem, a ring bark is formed, which shows the separation of hollow cylinders or rings from the stem.

Lenticels:

Usually in the periderm of most plants, certain areas with loosely arranged cells have been found, which possess more or less raised and corky spots where the underneath tissues break through the epidermis. Such areas are universally found on the stems of woody plants. These broken areas are called the lenticels.

Wutz (1955) defined a lenticel as a small portion of the periderm where the activity of the phellogen is more than elsewhere, and the cork cells produced by it are loosely arranged and possess numerous intercellular spaces.

These areas are thicker radially than rest of the periderm because of the presence of loose complementary cells. The lenticels perform the function of exchange of gases during night or when the stomata are closed.

Lenticels are first formed immediately beneath the stomata or group of stomata and the number of lenticels, therefore, depends upon the number of stomata or groups of stomata. The lenticels may be scattered on the stems or they may be arranged in vertical or horizontal rows. The lenticels also occur on the roots.

The lenticels originate beneath the stomata, either just before or simultaneously with the initiation of the first layer of the periderm. In most of plants, lenticel formation takes place in the first growing season and sometimes previous to the growth in length has stopped.

As the lenticel formation begins, the parenchyma cells found near about 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.

Such cells are also produced by phellogen towards outside instead of cork cells. As the complementary cells increase in number, pressure is caused against the epidermis and it ruptures. Very often, the outer most cells die due to exposure to outer atmosphere and are replaced by the cells cut off by cork cambium or phellogen.

The thin walled loose complementary cells may alternate with masses of more dense and compact cells called the closing cells. These cells together form a layer called closing layer. With the continuous formation of new loose complementary cells, the closing layers are ruptured.

The lenticels are filled up with complementary cells completely in the spring season whereas in the end of the spring season the lenticel becomes closed by the formation of closing layer.

The 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.

Sometimes, lenticels develop independent of the stomata. In such cases the phellogen cuts for sometime the cork cells and then loose complementary cells which ultimately break the cork and rise to a new lenticel.