Plant Anatomy and The Secondary Plant Body
|Biology - Plant Anatomy|
The early growth of a seedling results from cell divisions and cell elongation in the apical meristems of the shoot and root. The young seedling is composed of primary tissue and was the subject of our presentation on the primary plant body.
As growth continues, additional layers of cells may differentiate along the older portions of the stem and root. These secondary cell layers increase the diameter of the stem and root and are the products of the lateral meristems of the plant. There are two lateral meristems: the vascular cambium that gives rise to the secondary xylem and phloem, and the cork cambium that gives rise to the new outer covering of the plant. The activity of the semeristems forms the secondary plant body.
From the viewpoint of both the plant and humans, secondary growth is of great importance. Without it, we'd not have wood for our homes or cork for our bottles. Without it, plants would be confined to heights of only a few meters, and the whole course of terrestrial evolution of both plants and animals would have been altered.
Let's start our examination of secondary growth by considering the vascular cambium. First, let's re-examine the apical meristem and follow the differentiation of the provascular strands. As differentiation progresses, phloem forms toward the outside while xylem forms toward the inside. In many species, a layer of cells in the centre of the strand remains meristematic. This layer is the vascular cambium. The cells of the vascular cambium can divide either parallel to the surface of the stem or parallel to the radius. If the division is radial, two cambial cells result.
If, however, the division is parallel to the surface, one of the cells produced differentiates into either a xylem or a phloem cell, while the other retains the properties of the cambium. Whether one of the daughter cells differentiates into a potential xylem or phloem cell depends on the position of the cell with regard to the centre of the vascular bundle. If the cell is toward the inside, it will eventually be xylem; if outside, phloem. We say eventually because the newly formed phloem and xylem cells frequently undergo additional divisions producing more phloem or xylem cells. The cambium and its immature derivatives are known as the cambial zone.
The immature derivatives are capable of both radial and periclinal divisions, and most of the xylem and phloem produced are the result of periclinal divisions of the derivatives of the cambial initials. There may, for example, be ten derivatives for each cambial initial. The cambium that develops within the vascular bundles is called the fascicular cambium. In most dicotyledons, the vascular bundles of the stem are separated by regions of parenchyma cells, known as the interfascicular regions. If a continuous cambium is to develop around the circumference of the stem, cambium must differentiate in this region also. The interfascicular cambium differentiates from the parenchyma cells between the vascular bundles.
The cambial cells that give rise to the vascular elements of the stem are longer than they are wide and are known as fusiform initials. They and their derivatives are the source of the vertical or axial system of the vascular cylinder, composed of the tracheids, vessels, and fibres of the xylem and the sieve tubes, companion cells, and fibres of the phloem.
But, there is a second system in which the cells form at right angles to the long axis of the stem. This is the radial system, consisting of the rays seen here in a cross section of the stem. The rays are files of parenchyma cells which extend through the vascular cylinder. The rays probably function to transport materials across the vascular cylinder. They are also frequently the site of starch storage. The parenchyma cells of the rays are elongate, but their long axis is at right angles to the axis of the stem. They are not the products of the fusiform initials, but are formed by the ray initials.
Now, let's look at the products of the vascular cambium. First, in most species there is much more xylem produced than phloem. Second, the cells that differentiate from the cambial derivatives reflect in complexity the structure of the xylem and phloem of the species. For example, in the case of the conifers, the products in the xylem are tracheids and parenchyma, while in the woody angiosperms they may be vessels, tracheids, fibres, and parenchyma cells. Third, the cambium demonstrates a periodicity of functioning that is most apparent in trees.
This periodicity of cell division and differentiation in the cambium results in the formation of growth rings. These rings are clearly seen in most woods and are a result of the re-initiation of growth by the cambium after a period of inactivity. In most temperate trees, this is in the spring when water becomes available and the temperature increases. Active growth continues into summer. The result is a growth ring, characterized by relatively large cells with thin walls produced as early or spring wood, and small cells with thick walls, produced as late or summer wood. In many temperate trees under normal environmental conditions, one ring is produced each year. Growth rings can be used to date the age of the tree. The large, thin-walled tracheids of the spring wood are weaker and wear easier than the thicker walled, small-diameter summer wood.
As the growth in height of a tree is always initiated at the apical meristem, the upper part of a tree is younger than the lower portions. Consequently, there are fewer growth rings toward the top of the tree. Under conditions of environmental stress, as during drought or unusual temperature changes or insect attack that causes defoliation, growth rings may not be produced or the amount of growth in the ring may be greatly reduced. In tropical and subtropical climates, there may be little periodicity in cambial activity, making age determinations difficult. However, in many temperate climates, not only can trees be dated by growth rings, but past climatic conditions can be deduced from their width and the size of their cells. Also, past events, such as fires, are recorded as scars.
These differences in growth rings can also be used to date archaeological ruins. In the case of this pueblo dwelling, the growth rings in the roof beam can be matched with those of trees that are still alive in the same area. The rings are counted back from the present to determine the time the tree used in the beam was cut down.
Different types of cells may be differentiated in the secondary xylem at specific times during the growth of the ring. In the gymnosperms, such differences are not striking, with tracheids and parenchyma being produced throughout the growth period, although the diameter of the tracheids varies at different times. Such wood is called non-porous. In angiosperms, however, the pattern is quite different. The large vessels of the secondary xylem may be uniformly distributed throughout the growth ring. Such wood is termed diffuse porous, and is found in such trees as maple and the tulip tree. In other angiosperms, such as some oaks and ash, the largest vessels are produced in the early wood and appear as a ring of pores. This type of wood is known as ring porous.
One of the many remarkable features of vascular cambia is that they are capable of reactivation of division year after year, three thousand to four thousand years in the case of the giant sequoia and the bristle cone pine. The activity of the cambium and the accumulation of xylem and phloem results in an increase in the diameter of the stem. As this occurs, the initials must undergo radial divisions, so that the cambium remains intact. The expansion of the xylem crushes the older phloem. This means that, while massive amounts of xylem accumulate in woody plants, there is little more than a year or two of functional phloem present.
As a woody stem continues to grow in diameter, the vessels and tracheids toward the centre cease to be active in transport. Tannins and crystals may be deposited in these non-functional cells, and their colour may gradually change to a darker, often more reddish hue. This wood is known as heartwood, in contrast to the active, functional sapwood close to the cambium.
So far, we have been describing the formation of the vascular cambium and secondary growth in stems. Now, let's look at what happens in roots. In some ways the initiation of the vascular cambium in roots is similar to that in shoots: cells that remain undifferentiated between the primary phloem and primary xylem form the vascular cambium that gives rise to the secondary phloem and secondary xylem. In the root, however, the protoxylem comes into direct contact with the pericycle at the poles. Here, divisions of the pericycle initiate the formation of the cambium. Thus, we have the cambium differentiating from the provascular tissue between the phloem and xylem and from the pericycle over the protoxylem poles.
Once the cambium is complete around the circumference of the whole root, the formation of secondary phloem and secondary xylem pushes the primary phloem out from between the protoxylem poles and rounds out the tissue pattern. Ray initials may be formed from the pericycle opposite the protoxylem poles in some species or in different parts of the vascular cambium in others. As the stem, or root, grows in diameter due to the activity of the vascular cambium, the phloem, cortex, and epidermal cells are pushed out away from the centre of the organ. In the process, they tend to be crushed, split, and sloughed off. This leaves the plant without an outer covering for protection. The development of a second cambium, the cork cambium, or phellogen, fills this need by providing the plant with a protective outer covering. Cork cambium usually differentiates from a layer of the cortex, rarely from the epidermis.
In long-lived organs, the cork cambium originates in the secondary phloem. In all cases, the cambial cells divide tangentially, producing a layer of cork cells to the outside. Usually, the cork cambium is not continuous around the stem; rather, it forms in overlapping arcs. Not only is cork produced toward the outside, but cork parenchyma or phelloderm, consisting of parenchyma-like cells, is produced toward the inside. Cork cambium, cork, and cork parenchyma are collectively called the periderm.
The walls of cork cells become impregnated with waterproof suberin and some die. It was these dead cells that Robert Hooke observed and named as cells in1665. Any living cells outside the cork cells no longer receive nutrients from the inside and die. Some cork cambia, initiated when the plant was young, persist throughout the life of the plant, with the number of cambial cells increasing as the plant increases in diameter. Trees like beech, birch, and some cherries have such cambia. In many trees, however, especially in the temperate zones, the first cork cambium ceases to divide after a few years, and successive layers of cork cambia are formed deeper within the stem, particularly in the secondary phloem.
Cork occurs not only on stems and roots, but also is present in some fruits, as some green apples, and various storage organs, such as the potato. Early in the development of the potato, a thin cork layer, the skin of the potato, is formed over the whole surface. When the potato is newly picked, this skin is thin, but as it is kept in storage, the cork becomes thicker and more highly suberised, becoming the potato peel. The cork layer makes potato storage possible.
Cork and bark are related but should not be confused. Bark usually refers to the region from the vascular cambium to the outside of the stem, and it includes more than the cork in most species. Bark can be divided into two layers: the outer layer, which usually includes the cork and cork cambium and is known as the rhytidome; and the inner layer, which consists of the tissue between the outer layer and the vascular cambium, frequently including large amounts of secondary phloem fibres.
The patterns we readily associate with bark are a result of the character of the rhytidome. In some rhytidomes, parenchyma and cork not highly suberised predominate, while in others, a large amount of phloem fibres may be present. Fibrous barks form a network pattern when they split, as in ash and basswood, while those lacking fibres break up into individual, scalelike units, as in pines and maple. Scale barks found in the pines also owe their appearance to the manner of origin of the successive layers of periderm. If the later periderms are formed as overlapping layers, the outer tissue breaks up into scale units clearly seen in the outer bark.
In other plants, such as redwoods and oaks, the periderm cells show considerable cohesion to one another, and the bark then becomes thick and only gradually wears away. In still other species, the periderm cells readily become loose and separate into long strips of rhytidome. An extreme example of this is in the eucalyptus. These trees, native to Australia, are widely introduced into North and South America, southern Africa, and southern Europe. Sheets of enlarged cork parenchyma break down, separating long strips of bark from the stem.
Bark has many commercial uses. For centuries the bark of oaks and other trees have been used in the tanning of leather because of the high tannic acid content of the cork cells. On islands of the South Pacific, the fibre-rich inner bark of the paper mulberry is beaten in a fabric and used as clothing. Strips of inner bark of a variety of species were used by many North American natives to weave baskets. The most important commercial use of bark involves the rhytidome of the cork oak.
In this plant, the phellogen forms first from the epidermis, and may remain on the plant indefinitely. For commercial use, however, the first periderm is removed from the tree after about twenty years. When this periderm is removed, the exposed cells die, and a new layer of phellogen forms within the cortex. About every ten years, the cork is carefully removed from the trunk, and new phellogen forms within the cortex. Later, phellogen forms in the secondary phloem. In places where living tissue is exposed to air as a result of a wound, phellogen will develop and a layer of cork will be produced. This is wound cork and can be produced on any part of the plant. It functions to reduce water loss and protect the plant against invasion by bacteria and fungi. Wound phellogen develops several layers deep beneath the wound. After cork is formed and suberised, the damaged cells are sloughed off.
In the normal course of development of the periderm, the stem or root is essentially sealed. Little diffusion of gases is possible, but the underlying tissue is alive and requires oxygen to survive. In these conditions, we find that special openings in the periderm, known as lenticels, are formed in stems and roots. Lenticels develop due to local activity of the cork cambium that results in the formation of a mass of parenchyma cells with intercellular spaces. The lenticel phellogen also has intercellular spaces. This porous tissue protrudes through the outer layer of the cork and provides a pathway for the diffusion of oxygen. In roots they are also a pathway for the uptake of water and minerals. Lenticels are visible as raised areas that may be bean-shaped, round, or elongate. They figure prominently in the bark of many trees, such as the cherry. Some fruits, such as pears and apples, also contain lenticels.
The most important product of secondary growth, in human terms, is secondary xylem, or wood. Millions of board feet of lumber are cut and used yearly for everything from houses to furniture to bowling pins to matches. There are dozens of different types of wood used for myriad purposes. The uses of various types of wood reflect the cellular composition of the species that produced them. The wood of gymnosperms, such as pines and fir, are commonly said to be soft, meaning that they are straight-grained, easily penetrated by nails, readily split.
In contrast, the wood of angiosperms, such as maple and oak, is called hard because it is more compact, frequently weighs more per unit volume, and is less easy to nail. Because of its more complex grain, angiosperm wood usually splits less readily and is easier to carve and turn into elaborate shapes. Many of these generalizations are true, but there are exceptions. For example, hemlock is a gymnosperm but has harder wood than some angiosperm species. Balsa, one of the lightest of all woods, comes from an angiosperm species. The grain of Philippine mahogany, an angiosperm, is as straight as that of pine, a gymnosperm.
Now, let's look at gymnosperm and then angiosperm wood in greater detail. Gymnosperm wood is composed of tracheids and parenchyma cells. The tracheids that make up the bulk of the wood are arranged in overlapping files parallel to the long axis of the tree. They compose the vertical system of the plant. The parenchyma cells are mainly in rays, one cell wide and three or four cells tall, oriented at right angles to the long axis of the trunk. They are the radial system of the tree. Rays may be more than one cell wide when they contain resin ducts. Resin ducts are lined with thin-walled parenchyma cells that secrete resin into the duct. Resin ducts may run either parallel or perpendicular to the long axis of the tree. Resin aids in protecting the tree against insect and fungal attack because it is toxic to these organisms. Amber is fossil resin secreted by both gymnosperms and angiosperms. Rich deposits of amber are found around the Baltic Sea in northern Europe. They are forty to sixty million years old and may contain insects in remarkable states of preservation.
To gain a better appreciation of the structure of gymnosperm wood, let's look at it in three planes: the transverse, which is at right angles to the long axis of the stem; the radial, which is along the radius of the stem; and the tangential, which is longitudinal to the long axis and at right angles to the radius of the stem. In the transverse plane, the cross sections of the tracheids are seen, and the rays are seen in longitudinal view. In the radial plane, the tracheids are viewed in longitudinal section, as are the files of ray parenchyma. The tangential plane shows the ray parenchyma in cross section and the tracheids in longitudinal section. Each plane gives a different view; only by studying all three can we analyse the structure of the wood.
From the study of sections, we can construct three-dimensional diagrams, such as the one shown, in their proper relationships. Such diagrams are an excellent means of understanding the characteristics of the wood. It is interesting to note that the cellular composition of the wood creates the character of the lumber derived from gymnosperm trees. Boards from straight-grained gymnosperm wood can be visually uninteresting for use in furniture; although where knots are plentiful, the wood makes attractive wall panelling. However, because of the uniformity of the grain, it is readily sawed, planed, and nailed, making it particularly useful in construction. In addition, conifers are the major source of plywood. In making plywood, logs are placed on huge lathes that peel thin sheets of wood from the log. The sheets are then glued together so that the grain in the various sheets runs at right angles. By this method, wide, thin boards with great strength are obtained. Plywood is extremely important in construction, serving as wall and floor covering and moulds for concrete.
One of the most valued of conifer woods comes from Sequoia sempervirens, the coast redwood. It is sought for its uniform grain, fine red colour, and resistance to insect and fungal attack. The colour and resistance are related, as the lumber is cut from the heartwood, in which the tracheids are filled with a variety of phenolic compounds toxic to many fungi and insects. The huge size of the tree means that there is an abundance of clear, knot-free lumber available per tree.
Knots in lumber are the remains of branches formed when the trunk was smaller in diameter. Two kinds of knots can be seen: tight knots, from the portion of the trunk where the growth rings of the branch are continuous with those of the trunk; and loose knots, where the trunk surrounds the branch but is not continuous with it. As the tree ages, the buried branch is located farther and farther from the surface of the trunk. Consequently, in a tree that reaches the age of a redwood, there is a vast amount of wood unmarred by knots.
Now let's look at angiosperm wood. In cellular terms, the secondary xylem of angiosperms is considerably more complex than that of gymnosperms. The cells of angiosperm wood include vessel members, tracheids, several types of fibres, axial parenchyma, as well as ray parenchyma. If we view oak wood in the same way we did the conifer wood earlier, we can gain an appreciation of the complex nature of angiosperm wood.
We can see differences in the cellular composition, specifically the presence of vessels and fibres in addition to the tracheids and parenchyma found in gymnosperm wood. We can also see the massive, multicellular rays and the seasonal distribution of the vessels that makes oak a ring-porous wood. We can also compare oak with diffuse porous wood, such as this maple. Here, the more even distribution of the vessels gives the wood a different appearance in all three planes. Here, too, the rays are less massive and more uniform than in oak. The complex nature of the secondary xylem in angiosperms strongly affects the physical properties of the wood.
Visually, angiosperm woods, such as oak and maple, have grains that exhibit interesting patterns when sawed into lumber. Stains and oils can be used to bring out the differences in the texture of the wood that results from the various mixtures of the wide-diameter vessels, the narrower tracheids, the almost solid fibres, and the thin-walled parenchyma cells. Logs can be cut in a number of ways to enhance either the aesthetic or the practical value of the lumber. Here we have diagrammed the way in which logs are cut to produce plain-sawed and quarter-sawed lumber.
Tangential cuts result in plain-sawed lumber. Plain-sawed lumber is characterised by patterns of strips; concentric, irregular parabolas; or ellipses caused by the differences in colour and structure of the early and late wood. Any wood with pronounced differences in the nature of the early and late wood will show marked patterns when cut this way. As the diameter of the growth ring gradually decreases from one end of a log to the other, a straight cut across them will yield cones of irregular shape and a range of interesting shapes. In quarter-sawed wood, the cut runs approximately parallel to the vascular rays along the radial plane. The rays appear as stripes or ribbons running across the surface.
When oak is quarter sawed, the large rays add a great deal to the texture of the wood. Quarter-sawed yellow pine or Douglas fir are used for flooring, because the wearing quality of these soft woods is increased by having the harder area of the late wood exposed vertically to wear. When North America was first settled, extensive forests covered the eastern half of the continent. The composition of these forests differed in various parts of the country, and the type of dwelling built by the early settlers reflected both the species that were common and the characteristics of the available wood.
Log cabins were built where conifers were abundant, because the straight trunks and highly linear character of the xylem facilitated squaring and notching the logs with an axe. Where large angiosperm trees, particularly oaks, were numerous, massive beams were fashioned by axe and saw. Such beams formed the skeleton of the building, with the areas between the beams filled with mud and wattle or bricks. Wooden pegs were used to hold the oak frame together because the dense wood resisted penetration by the handmade nails available at the time. Later, settlements reached the prairies, and lumber of any kind became scarce. The extensive conifer forests of the north were utilised for a different type of construction. In place of massive beams or whole logs, a frame was made of pine or fir boards, two inches by four inches, on which was nailed flat siding, also of pine. This is a major type of construction still in use today.
Finally, let's consider secondary growth in monocotyledons. Most monocotyledons have no secondary growth from a vascular cambium, as do dicotyledons, but some do show secondary growth by other means. This is true for the larger monocotyledons, such as palms and yucca. To understand secondary growth in monocotyledons, it is necessary to consider the shoot tip.
The monocotyledons possess, in addition to the apical meristem, a wide region of cells known as the primary thickening meristem. The distribution of this meristem can be seen here. Meristematic cells that function as a cambium are continuous with the primary thickening meristem. However, these meristematic cells are not located between the xylem and phloem, as in the dicotyledons, but are independent of the established bundles. Moreover, the cambium produces parenchyma toward the inside and usually only a small amount of parenchyma toward the outside.
In palms, secondary growth is known as diffuse secondary growth because it is not restricted to a limited area and occurs far from the apical meristem. We have introduced the concept of secondary growth in plants. Secondary growth allows plants to live longer and grow taller. Secondary growth occurs in both the vascular tissue and in the outer layers of the root and shoot. The vascular cambium produces new layers of xylem, phloem, and rays. The cork cambium produces the cork and cork parenchyma. Lenticels develop in the cork for gas exchange.
In trees, the massive accumulation of xylem forms wood, which is exploited as lumber by humans. Our lives would indeed be different without secondary growth in plants.
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