Secondary Growth in Dicotyledonous Stems (With Diagram)

Secondary Growth in Dicotyledonous Stems (With Diagram)!

Introduction:

The dicotyledonous stems show distinct secondary growth in thickness, as they us­ually possess collateral open vascular bundles.

The cambium present in the bundle, other­wise known as a fascicular cambium, mainly produces secondary tissues at the initial stage.

Normally a few methods of secondary increase have been found in the dicotyle­donous stems. The first type is noticed in the stems where vascular bundles remain arranged in rings and narrow medullary rays occur in the interfascicular regions.

Due to the differentiation of interfascicular cambium in the parenchymatous cells of the medullary rays, a complete cambium ring is formed, and the secondary tissues are added on either side of the cambium ring. The secondary vascular tissues thus have the form of a continuous cylinder.

This method is common in erect stems like those of members of sun­flower family and, in fact, in many dicotyledonous stems. In some dicotyledons the pri­mary vascular bundles remain arranged in a continuous cylinder, and the secondary vascular tissues are also added in the same form.

This type is found in Tilia, Nicotiana (tobacco), etc. The other method prevails in climbers like Aristolochia where broad medullary rays are present between the vascular bundles in the primary condition.

During secondary growth in this case the interfascicular cambium, a secondary meristem, goes on producing only parenchyma cells, and as a result a continuous ring of secondary tissues is not formed. Thus the secondary tissues also occur in strands.

In climbers like Cucurbita secondary growth is confined to the individual bundles and does not extend to the interfascicular regions. Besides these, anomalous secondary growth occurs in a good number of families.

Here is an account of the process of secondary growth in thickness in a typical dico­tyledonous stem. It starts in the intrastelar region and then extends to the extrastelar portion.

Cambium:

In dicotyledons and gymnosperms a portion of the procambium re­mains undifferentiated during the progressive maturation of the tissues leading to the formation of the primary body. This undifferentiated part is the cambium of the vas­cular bundle which occurs between xylem and phloem. It is called fascicular cambium (fascicle =bundle).

A few living parenchymatous cells of the medullary rays, which re­tained the potentialities of cells division, now resume meristematic nature, and thus new strips of meristems are formed in a line with the fascicular cambium.

They are known as interfascicular cambium, which may be formed either simultaneously with or after laying down of the primary body. The interfascicular cambium which is secondary in origin joins up with the fascicular cambium and thus a distinct continuous cambium ring or cylinder is formed.

The vascular cambium consists of two types of cells—-spindle-shaped elongated ones, known as fusiform initials, and comparatively smaller and isodiametric ones, called ray initials (Fig. 628).

The fusiform initials produce the secondary tissues which remain arranged parallel to the long axis of the organ, in fact forming the vertical or longitudinal system. The tracheary elements of xylem, fibres, parenchyma, and the sieve tubes, companion cells of phloem belong to this system.

The ray initials give rise to the ray cells—the xylem and phloem rays which remain arranged horizontally and thus form the transverse or horizontal system. Thus secondary vascular tissues are composed of two systems—the vertical and horizontal, originating from fusiform and ray initials respec­tively of the cambium.

The cambium, strictly speaking, is one layer in thickness. But during active growth the cambium cells and their immediate derivatives- form a zone, referred to as cambium zone, where the actual initials can hardly be distinguished from their immediate deriva­tives.

The cells possess conspicuous vacuoles; primary pit-fields with plasmodesmata are present. On the basis of the arrangement of the cells as found in tangential views, the cambium is of two types (Fig. 628).

In one type the fusiform initials occur in horizontal tiers, where ends of short fusiform cells of one tier remain more or less at the same level with another. This type of meristem is known as storied or stratified (Fig. 628A) cambium.

In the second case comparatively longer fusiform initials do not occur in horizontal tiers but overlap at the ends. This is called non- storied or non-stratified (Fig. 628B) cambium. In all highly developed dicotyledons die cambium is of storied type.

The cambium cells divide tangentially, and secondary tissues, secondary xylem and secondary phloem are added on the internal and external sides respectively. A cambium cell divides into two daughter cells, one of the derivatives becomes either a xylem mother cell or a phloem mother cell depending on its position, while the other one remains meristematic (Fig. 629).

The xylem or phloem mother cell by further differentiation and perhaps on further division ultimately develops into secondary xylem or secondary phloem element. The cambium cells continue to divide in this fashion pro­ducing secondary tissues on either sides. That is how the cambium perpetuates itself and thus remains active in plants for pretty long time.

Usually the cambium produces more xylem than phloem, so that the cambium ring with all the tissues in front of it is pushed more and more towards the periphery. Due to formation of secondary tissues the primary xylem and primary phloem which were lying side by side in the primary condition, are gradually pushed apart from each other.

Usually in the plants growing in tropical regions cam­bium is active throughout the year. In temperate regions cambial activity may cease with the onset of unfavourable period and enter into a dormant state till favourable conditions are available.

Secondary Xylem:

It constitutes the major portion of the secondary vascular tissues, and, in fact, the main bulk in woody plant. This tissue serves a number of important functions, viz., conduction of water and dissolved mineral matters, mechanical support; and the living cells provide space for storage of food.

As already stated, this compact tissue mostly made of thick-walled cells, consists of two systems—vertical and horizontal as derived from the fusiform and ray initials of the cambium. The vertical system is made of elements occurring parallel to the long axis of the organ.

They are the tracheary elements, tracheae and tracheids, xylem parenchyma and fibres. The horizontal system consists of xylem rays arranged at right angles to the long axis. The living cells of the rays and the xylem parenchyma of vertical system form an interconnected continuous system which is in communication with the living cells of the pith, phloem and cortex.

So far as the elements are concerned, secondary xylem is more or less similar to those occurring in primary xylem. Vessels or tracheae are most abundant here and they are usually shorter and much larger than those of primary xylem and are mostly of pitted types.

Annular and spiral thickenings of the vessel members are absent. The xylem paren­chyma cells may be long fusiform ones, or more commonly, they are short cells. These living cells are particularly meant for storage of food—-starch or fat. Tannins, crystals, etc., are also frequently present in these cells.

Secondary wall may be absent; if present, the pits between the parenchyma and tracheary elements are bordered, half-bordered or even simple. Xylem parenchyma may occur abundantly in some species, may be comparatively smaller in amount or entirely absent.

Much importance has been attached to this tissue in recent years in taxonomic considerations. They may remain either in association with the vessels or independent of them. In the former case they are called paratracheal, and in the latter, apotracheal.

On the basis of distribution paratracheal paren­chyma has been found to be of different types. Often they form entire sheaths, appearing circular or elliptical in cross-section, around the vessels. This type is termed vasicentric parenchyma. Apotracheal parenchyma may also be of different types.

It is thought that apotracheal type is primitive and paratracheal advanced, highest development being reached in vasicentric type. The fibres of secondary xylem have thick walls and’ reduced bordered pits. The two main types of fibres are the fibre tracheitis, and the libriform fibres.

The fibre tracheids have bordered pits, but the borders are poorly developed, whereas the libriform fibres resembling the phloem fibres have simple pits. These are the impor­tant supporting elements of xylem.

The xylem rays forming the horizontal system remain arranged at right angles to the long axis of the organ.

They originate from the ray initials and run radially as sheets of tissue and continue as band to the secondary phloem through the cambium, thus form­ing a continuous system of conducting tissue (Fig. 630).

The expression medullary rays has often been used for this band, obviously on the basis of their similarity or supposed homology with the pith rays of herbaceous dicotyledonous stems.

But, strictly speaking, they should be called vascular rays, as they are rays of vascular tissues, partly of xylem and partly of phloem, formed by the cambium.

The xylem rays establish transverse communication in the living cells of the vascular tissues. They make the gaseous interchange with the outer atmosphere easy through the intercellular spaces.

Apart from providing space for storage of food, the ray cells are also responsible for the movement of water from the xylem to the cambium and phloem, and that of food from the phloem to the cambium and living cells of Xylem.

The ray cells in the dicotyledons are usually parenchymatous. They vary in width —may be one-layered or uniseriate or two- to many-layered or multiseriate. In the angio-sperms usually rows of ray cells occur at the margins which are different from the rest both in character and functions.

They are always living and shorter than other ray cells. Special secretions are often present in these cells. Ray parenchyma cells may be upright or procumbent. Cells with their long axes oriented radially are procumbent; and those with long axes oriented vertically are the upright ray cells.

If both the types combine, the tissue is called heterogeneous; whereas if it consists of only type of cell, it is known as homo­geneous. Both uniseriate and multiseriate ray cells may be homogeneous or heterogen­eous.

Uniseriate type comprising vertically elongated cells and multiseriate heterogeneous rays are considered to be primitive and homogeneous rays to be advanced. In nearly all gymnosperms the rays are uniseriate. The cells of the xylem rays are usually living with rather thick walls.

In gymnosperms the rays may be composed of only living parenchyma cells and tracheids. The terms homocellular and heterocellular rays have been used for the former and latter respectively. The tracheids, called ray tracheitis, are dead, having lignified wall with bordered pits. They exhibit tendency of elongation.

Annual Rings:

The activity of the cambium is often periodical, and as a result, dis­tinct growth layers are formed in the xylem. In transverse views these growth layers appear as rings, and hence are referred to as growth rings. In most of the cases the perio­dicity in the activities of the cambium is seasonal, so the secondary xylem formed in a year constitutes the growth ring.

Hence it is also an annual ring. Formation of annual rings is characteristic in plants growing in temperate climates with pronounced seasonal variations. In a quite good number of plants the annual or growth rings are found in tropical and subtropical countries as well.

The immediate reason for the visibility of the annual ring is the formation of different types of wood or secondary xylem in different parts of the growing season (Figs. 631 & 632). The wood formed in spring, which is the most favourable season for plant growth, is less dense and is made of vessels with wide diameters.

The vigorous rate of assimilation in spring necessitates huge quantity of water, and so large vessels preponderate in the wood formed in that season. This is called early wood. In summer the rate of assimilation grad­ually decreases, and thus the secondary xylem formed is more dense and made of compact lignified elements.

This part is called late wood. In temperate countries in particular the terms, spring wood and summer wood, are used for early wood and late wood respectively. But because the two parts are not strictly seasonal in some plants even in temperate climates, it is better to designate them as early wood and late wood.

The winter is the resting season when cambial acti­vities remain suspended. The early wood-gradu­ally merges with the late wood of the season. But there exists a sharp line of demarcation between the late wood of one season and the early wood of the next season.

That explains the formation of concentric annual rings, each of which consists of two parts—-inner early wood and outer late wood. Annual rings are found in the deciduous and evergreen plants. Though they are very common in temperate countries with distinct growth and dormancy (resting) periods, they may also occur in plants of tropical and subtropical countries.

In some tropical trees distinct annual rings are formed due to periodic activity of the cambium in alternatingly wet and dry seasons. Well-marked rings are found in the important timber-yielding plants—Tectona grandis (teak), Dal- bergia sissoo (sissoo), Albizzia lebbeck (sirish), Salmalia malabarica (silk cotton), etc.

The seasonal activity of the cambium may be distributed by external influences, viz., adverse climatic conditions, drought, defoliation and disease. In that case normal deve­lopment of the wood is checked, and growth is resumed later in the same season.

As a result, additional layers, called false annual rings, are formed. Thus two or more false ring may be formed in a year, all of them constituting what are known as multiple annual ring.

Normally large vessels occur in the early wood, making it more conspicuous than the late wood. Here die vessels are of unequal diameters, the largest ones being restricted to the early wood.

As they exhibit a ring-like arrangement in transverse section, this type of wood is called ring porous (Fig. 632B). But in some plants the vessels are found to be of more or less equal diameters and they remain rather uniformly distributed throughout the wood or when there is only a gradual change in size and distribution throughout the growth ring.

This type of wood is called diffuse porous (Fig. 632A). Intermediate forms may occur between the two types. From phylogenetic point of view ring-porous wood is considered more advanced than the diffused-porous one.

In many plants the xylem parenchyma and ray parenchyma cells develop balloon-like protrusions into the tracheary elements. Such ingrowths are called tyloses (Figs. 632 & 633). They are characteristics of secondary xylem, though they may also develop in the primary xylem.

Tyloses are formed by the enlargement of the pit membranes of the half-bordered pits occurring between a living parenchyma cell and a tracheary element—a vessel or a tracheid. They are often pretty large, so much so that the lumen of the elements is almost blocked.

The nucleus with a part of cytoplasm flows from the living cells to the tylose. In fact, the delicate membrane grows and forms the large balloon-like protrusion. Starch, resin and other matters may be present in the tyloses. The walls of the tyloses are initially thin, but they may undergo lignification later. They may occur singly, or may divide and form a multiple structure. Tyloses are of common occurrence in many angiospermic families.

Normally they develop in the heart wood and add to the durability of the wood by blocking the lumen, Furthermore, they prevent the flow of fluids and movement of hyphae of fungal enemies through the lumen of the vessels.

But tyloses have been reported in some herbaceous plants like Coleus, Canna, Cucurbita. Tylose-like intrusions called tylosoids are common in the wood of some gymnosperms with resin ducts in vertical and horizontal systems. In fact, the epithelial cells enlarge and block a resin duct and look like a tylose intrusion. They basically differ from the tyloses in that they never grow through pits.

Sap wood and Heart wood. After formation of considerable quantity of secondary Xylem, two types of wood appear in the stem (Fig. 634). The outer portion consisting of recently formed xylem is called sap wood, and the centrally located portion which was formed earlier is referred to as heart wood.

The sap wood, also known as alburnum, is of light colour and contains some living cells, in addition to the tracheary elements and fibres. This part actually carries on the physiological functions—-primarily conduction of water and solutes, support and storage of food. The hard central part, the heart wood, also known as duramen, mainly consists of dead elements and is usually of dark colour.

It simply serves as a solid mechanical column. The sap wood is gradually transformed into heart wood. During this transformation a number of changes occur in the wood—the living cells lose protoplasts, water contents are reduced, food matters are removed from the cells, tyloses are frequently formed and thus the lumens of the elements are blocked, the walls of the parenchyma cells undergo considerable lignification, and sub­stances like gums, resins, oils and particularly tannins and colouring matters are infiltrated both on the walls and lumen of the ele­ments.

As a result, the heart wood becomes more compact, strong and durable and dark-coloured. From commercial point of view heart wood is obviously more valuable than sap wood; but the latter is more important from biological point of view, being really responsible for the physiological functions.

Secondary Phloem:

The cambium cells by tangential divisions produce secondary phloem elements on the outer side. Usually the number of phloem elements is less than that of xylem elements. In most of the dicotyledons the primary phloem gets crushed with increase in thickness and the secondary phloem takes over the physiological functions, and it remains active for considerable length of time.

The arrangement of cells in secondary phloem is similar to what prevails in secon­dary xylem—the elements being arranged in two systems, vertical and horizontal. Those in the vertical system are the sieve, tubes, companion cells, phloem parenchyma and fibres.

They derive their origin from the fusiform initials of the cambium, and are chiefly responsible for vertical conduction of elaborated food matters. The horizontal system
consists of the ray parenchyma cells formed by the ray initials. Like secondary xylem storied and non-storied arrangement is noticed.

There is no fundamental difference in the primary and secondary phloem as regards the elements, but the latter is a more complex tissue (Fig. 635). All the cells here remain arranged in radial rows; the sieve tubes are usu­ally shorter and larger in number and the fibres are more abundant.

The complex tissue as a whole has a longer functioning life. The sieve tubes usually have simple sieve plates, but com­pound plates may also be present in some plants. They do not show any regularity of distribution in the tissue. The companion cells accompany the sieve tubes, as they develop from the same mother cell.

There may be one companion cell extending along the entire length of the sieve tube or there may be a few ones. They are often quite distinct to be readily recognised in trans­verse views. In gymnosperms vertical system consists of sieve cells, parenchyma and aluminous cells.

Parenchyma cells are abundantly present in the secondary phloem of the dicotyle­dons, but with gradual increase and maturity their number is decreased. The nature of the cells is variable, they may be elongated with pointed ends like the cambium cells from which they have originated, or may be short rectangular or slightly elongated cylindrical ones.

Apart from their role in vertical conduction of elaborated food matters, the parenchyma cells also serve in storage of starch, crystals and other materials. Sclerenchyma is characteristic of secondary phloem. They occur abundantly either as discrete tangential bands, or as isolated patches, or even singly.

In certain dicotyledons the ‘fibres occur in patches regularly alternating with the bands composed of sieve tubes, companion cells and parenchyma. Sclereids are also of frequent occurrence, either independently or in association with the fibres.

Some phloem elements become functionless in course of time. In non-functioning phloem the number of sclereids increases, mainly because the parenchyma cells undergo sclerosis there.

Elongate fibre-like elements formed by sclerosis of parenchyma cells have been called fibre-sclereids. The fibres together with the sclereids provide mechanical strength. In fact, many fibres of commercial importance belong to this tissue.

Phloem Rays:

The phloem rays (Figs. 630 & 635) originate from the ray initials of the cambium and constitute the horizontal system of the tissue. With xylem rays occur­ring on the inner side these cells (phloem rays) form the vascular rays which establish communication in the living cells of the vascular tissues through the cambium, and are responsible for horizontal conduction to and from the xylem and cambium.

Near the cambium xylem rays and phloem rays are of equal size, with maturity of the plant phloem rays have increased width. Like xylem rays they may be uniseriate with only parenchyma cells, as in conifers, or multiseriate, and homogeneous, composed of either upright or procumbent cells, or heterogeneous, made of both upright and procumbent cells.

Phloem ram are smaller than their counterpart in xylem, because of the facts that the vascular cambium as a rule produces more xylem than phloem and that the outer part of phloem is periodically sloughed off.

Growth rings which are so characteristic of secondary xylem are practically absent in phloem. The elements of secondary phloem, as already stated, remain arranged in regular tangential series and thus exhibit something like concentric rings.

But unlike the early wood and late wood of xylem, phloem formed in the early and late part of the growing season does not show any distinction. So a line can hardly be drawn between the elements formed in consecutive seasons.

Something like rings have, however, been noticed in some species of dicotyledons which are formed due to differences in the cells produced at the beginning and end of the season. Tangential bands of fibres have also been observed in some plants. They at any rate do not indicate the age of secondary phloem.

Periderm:

Due to continued formation of secondary tissues by the cambium cylinder in the intrastelar region, considerable pressure is exerted on the epidermis and other extrastelar tissues. The epidermis gets more and more stretched and ulti­mately tends to rupture.

The protective function discharged by them in the primary condition is now taken over by a new secondary tissue, called periderm, formed in the extrastelar region.

The periderm consists of three tissues, viz., (i) a meristem known as phellogen or cork cambium, (ii) cells cut off by the phellogen on the outer side, called phellem or cork cells, and (iii) cells formed by the phellogen on the inner side, called phelloderm.

As stated previously, when considerable pressure comes from the internal portion and the epidermis tends to rupture, the phellogen or cork cambium is usually formed in the superficial layers of cortex.

It is a second­ary meristem arising from the living permanent cells which retained the potentialities of cell division. It ori­ginates as a single layer of initiating cells either in the subepidermal portion, in the epidermis itself, in the deep-seated layers of cortex or may even extend up to the phloem.

Phellogen is rather simple in comparison to the cambium, as it consists of only one type of initials. In transverse view the cells appear rectangular in shape and remain radially flattened.

They are compactly arranged without having intercellular spaces. The cells have vacuolate protoplasts. This lateral meristem behaves like the cambium cells. They divide tangentially producing new tissues both centrifugally and centripetally (Figs. 636 & 637). The derivatives usually remain arranged in radial rows.

Normally the phellogen produces much more phellem on the outer side than phelloderm on the inner. The cells constituting phellem, otherwise called cork cells, are uni­formly formed ones.

They remain arranged in distinct radial rows due to their origin from a tangentially dividing meristem. The cork cells are compactly set and have no inter­cellular spaces. They lose protoplasts after differentiation and become dead. The cells remain filled with air and coloured organic matters.

The wall characters of the cork cells are most significant. The primary walls are mostly of cellulose or partly suberised or even lignified. A thick layer of suberin is deposited next to the primary walls. So the cork cells become impervious to water and gases. In some cases the cork cells may be thick-walled, where the cavities of the cells remain filled up with resinous and other materials as in Eucalyptus.

But commonly they are rather thin-walled, radially elongate empty cells. Two types of phellem may occur in the same plant in alternating layers, as in Betula, where cork is peeled off like sheets of paper.

Periderm can serve as an effective secondary pro­tective tissue against desiccation and mechanical injuries mainly due to strong suberisation of the cork cells and their compact arrangement.

The phellogen produces phelloderm on the inner side. The cells constituting this tissue practically resemble those of cortex in their nature, wall structure and contents. These are living, more or less isodiametric cells with intercellular spaces.

But they remain arranged in definite radial rows. These cells may carry on photosynthesis and serve in storage of food. Some authors use the expression secondary cortex for the phelloderm. But it is unhappy and confusing, because the same term has been applied to the whole of periderm by others.

Thus it is evident from the process stated above that secondary growth in a dicoty­ledonous stem is initiated in the intrastelar region with the activities of the cambium, fascicular and interfascicular. Secondary vascular tissues are formed by the cambium ring. Lastly, for withstanding the pressure from the inside periderm arises in the extra­stelar region (Fig. 638).

Bark:

It has been stated that phellogen first originates in the superficial portion—either in the epidermis itself or in the subepidermal cells. The phellogen cells go on dividing and produce cork cells and phelloderm on the outer and inner sides respectively. But the formation of secondary tissues continues in the intrastelar region.

The periderm formed at the superficial region of the organ may not be adequate to withstand the pressure coming from inside. In that case additional layers of periderm are formed in the pro­gressively deeper regions of the stem.

So new phellogen layers may arise in the deeper regions of cortex, pericycle and even phloem. Once phellogen arises in the deeper region and it produces phellem or cork cells on the outer side, all the living cells present on the outer side of the phellogen do not get any supply of water and food.

Eventually all of them become dead. All these dead tissues lying outside the active phellogen constitute the bark of the trees. This term bark has been rather loosely used and has created a good deal of confusion. In non-technical sense bark includes many, tissues occurring at the outer part of the stem.

The term rhytidome was used for the outer bark in the latter part of the nineteenth century. Some anatomists prefer to retain the term rhytidome to cover all the tissues external to the innermost phellogen (outer bark); the term bark to be applied to all tissues external to the vascular cambium.

Commonly successive layers of periderm are formed in the progressively deeper regions of the stem. In that case bark is formed in concentric rings (Fig. 634) surrounding the entire stem.

This type is known as ring bark. In some plants the periderm is formed as overlapping scale-like layers, so that the outer tissues break up and are sloughed off in patches. This is known as scale bark.

Lenticels:

During secondary increase in thickness the periderm replaces the epidermis in the protective function. The dead cork cells with suberised walls are partly impervious to gases. Thus gaseous interchange between the internal living cells and outer atmosphere becomes difficult.

Some pores which look like lens-shaped raised spots on the surface of the stem, now develop and take up the function of gaseous exchange. These pores are known as lenticels. Only a few plants, mostly climbers, do not possess lenticels though periderm is formed.

Lenticels are first formed just beneath the stomata. They usually ori­ginate either just before, or simulta­neously with or after the initiation of the periderm. The parenchyma cells near about the substomatal chamber lose chlorophyll and divide in various planes, forming a mass of colourless loose cells.

Sooner or later normal phel­logen is differentiated in the adjoining deeper region (Fig. 639). The phellogen also produces loosely-arranged ceils on the outer side, instead of giving rise to normal cork cells.

All these cells—those formed by the division of the parenchyma in the substomatal region and those formed by phellogen on the outer side, are together known as com­plementary cells.

With increase in the number of complementary cells the epidermis is ruptured, and the former protrude above and thus appear like so many raised spots (Fig. 640). The outermost cells often die due to exposure to outer atmosphere and are replaced by the cells cut off by the phellogen.

Often massess of more dense and compact cells, known as closing cells, alternate with loosely-arranged complementary cells. The closing cells together form a layer referred to as closing layer which is helpful in keeping the loosely-arranged complementary cells in position. Continued formation of the complementary cells may cause rupture of the closing layer. This condition actually prevails in the growing season, whereas at the end of the same the formation of closing layer is more pronounced.

These complementary cells are characterised by thin walls and absence of suberin and loose arrangement due to presence of profuse intercellular spaces. The latter estab­lish communication with the spaces of the internal tissues.

Like the stomata the lenticels are thus concerned with gaseous exchange between the outer atmosphere and the internal tissues.

In some cases lenticels are formed independent of the stomata. Even it may happen that phellogen has produced cork cells for a while then it started forming loose comple­mentary cells which would ultimately break through the cork and give rise to a lenticel. The number of lenticels formed in stem is variable.

They may remain scattered or arran­ged in vertical or longitudinal rows. Rows of lenticels may occur opposite to the multi- seriate rays, suggesting free interchange of gases.