The term bark refers to all tissues of a woody stem or root occurring just outside of the vascular cambium, i.e., all tissues that could be stripped away from the woody core. The petiole or rachis of a long-lived leaf can theoretically also form patches of bark, but this normally is not observed. Stems of monocotyledons, especially trunks of arborescent types, often have an outer cover, sometimes appearing like a form of bark, but in these cases the origin of the cover is very different from that of dicotyledons and gymnosperms. Ferns and other seedless vascular plants never form bark of any type.
Where present, the bark most typically serves two very important functions. The outer, mostly dead tissues (outer bark) form a protective barrier between the plant axis and the abiotic and biotic environment. The inner tissue (secondary phloem), including living cells (inner bark), is where sugar transport for the plant occurs, and the inner bark also can have defenses against herbivores, such as cells with tough cell walls (secondary phloem fibers or sclereids) or cells and tubes filled with bitter or toxic chemicals.
Bark is a very complex structure, consisting of cells that formed from lateral meristems (cambia producing secondary tissues) but often containing some cells (living or dead) that remain from the first-formed plant (primary tissues, such as epidermis, cortex, and primary phloem). In general, the inner bark is defined very narrowly as the innermost sector consisting only of secondary phloem, which is produced to the outside of and by the vascular cambium. The outer bark always includes all tissues formed by the lateral meristem known as the cork cambium (phellogen) or a number of individual and often discontinuous cork cambia.
Each cork cambium produces a sector of cells called a periderm. The dividing cells (initials) of the cork cambium (phellogen) divide to produce cells, most of which are "pushed" toward the outside, and some may be pushed toward the inside. Cells pushed to the outside generally have layering of suberin in their walls, and thereby become cork cells (also called phellem). Suberin, often composed of suberic acid and phellonic acid, is a fatty and waxy substance, which makes the cells mostly impervious to water and unable to exchange gases and nutrients, hence these cells soon die and entrap air. In some species, cells pushed to the outside of the cork cambium can develop with thickened cell walls but lacking suberin (phelloids). Cells pushed from the cork cambium to the inside are called phelloderm, and these can remain alive because they have unthickened and unspecialized cell walls and, hence, can exchange gases and obtain nutrients.
In general, outermost bark layer is considered to be a replacement for the epidermis of the stem and root, as the plant axis becomes older and grows thicker. What would occur if the cells on the outer zone of the cylinder, increasing in radius, did not keep pace via new cell divisions? The circumference of the axis would increase greatly, and the likely outcome would be stretching and tearing of the outer cell layers. In fact, this can be observed. A plant could experience extra cell divisions of the existing surface layers, to thereby increase the surface area, but this generally is not the case. Instead, seed plants, especially dicotyledons and gymnosperms, have solved this problem by producing new layers of cells (periderm) to accommodate tangential growth (lateral expansion) of the stem or root while, at the same time, helping to reduce water loss from the stem or root by forming the water-repellent cork cells.
The functions of the outer bark have been speculated about, but there has been relatively little scientific testing. Bark, and cork in particular, are excellent for thermal insulation, and thick bark is particularly common on trees in cold biomes, whereas thin barks are more typical of tropical habitats. During fires, thermal insulation by thick bark, or bark that does not burn into inner bark, can be an adaptation for plant species inhabiting fire-adapted communities.
Cork is indigestible, creating a surface that is not very attractive to many animals, although many herbivores (mammals and boring insects) attack the inner bark, where the secondary phloem is loaded with sucrose. Cork, with its fat and wax, is fairly water repellent, and thus creates a resistance to water loss from the plant to a drier atmosphere.
Occurrence of dead cells, without nutritional value, on the outer surface likely is a strategy to discourage growth of fungi and bacteria. However, in very wet habitats, such as cloud forests and rain forests, algae, bryophytes, lichens, and fungi may grow luxuriantly on wet bark, and branches can support huge loads of epiphytes.
In typical woody plants, the first outer bark is seen by the observer late during the first year or in year two, but, of course, the cell divisions to produce that first outer bark commonly start earlier, and for viewing the early stages you would need a microscope. Outer bark often begins to form long before a stem or root experiences damage to surface cells, but after internodal elongation has ceased and as the adjacent leaves become aged. Stems of tomato, garden geranium, and green beans have periderm formation in their green, relatively young lower stems. There are even some species in which periderm forms on roots of seedlings within a week after germination! Likewise, inner bark, i.e., secondary phloem, is already being produced in relatively young stems, even long before outer bark is initiated. Nevertheless, bark is most frequently regarded as the protective tissue of old stems and roots, and as characteristic of shrubs, trees, and perennial climbers.
The first phellogen of stems (tissue called the initial periderm) most commonly forms in the outermost layer of cortex. A cell wall forms parallel to the surface of the organ (i.e., periclinal), initially in a group of cells and then expanding to neighboring cells until the sheet of cells is continuous around the stem. Each cell (initial) then divides to produce derivative cells to the outside and inside. To the outside, cork cells are usually formed. The epidermis remains intact upon the cork cells until they die, while files of cork cells accumulate beneath the epidermis. For a small percentage of plants, the initial periderm of stem arises in the epidermis. These two are sometimes called the superficial periderm.
Not every species forms a continuous initial periderm around the stem circumference. In some lineages the initial periderm instead forms as longitudinal strips, alternating with green strips of original stem tissues. Some of these may therefore continue stem photosynthesis in the green portions that have no periderm.
For some species, the initial periderm arises deep in the cortex or parenchyma cells of the phloem, for which the term superficial would not be appropriate.
This initial periderm normally functions for a short time, but in apple (Pyrus), for example, the initial periderm can persist for twenty years! The first phellogen can persist if either the stem is not increasing in diameter or the initials of the phellogen are multiplying to match the increase in axis circumference (i.e., anticlinal divisions, perpendicular to the surface; occurs in oak, fir, beech, and hornbeam).
The initial periderm of a root arises deep within the axis, not near the surface. Pericycle, which is located just inside the root cortex, is the place where the first phellogen of roots arises.
The initial periderm is interrupted at points around the stem by the occurrence of lenticels (Latin lentis, a lentil). These are blister-like, lenticular breaks in the surface. Most often, a lenticel on a stem forms where a stomate once occurred. The lenticel phellogen forms from cells interior to the stomate (lining the substomatal chamber) and is also connected with the adjacent cork cambium. From the lenticel phellogen cells are also produced to the outside and inside, but the outer cells tend to round up and thereby have intercellular air spaces (Phellem usually has no intercellular air spaces.), so that the tissue inside the lenticel is more loosely packed (filling tissue or complementary tissue). The cells of the lenticel also tend to expand outside the stem, yielding that blistered appearance. Each lenticel therefore becomes a pathway through which gases (especially oxygen) can diffuse to the living cells of the bark. Without sufficient oxygen, cells of bark can die.
Lenticels also can be found on fruits, e.g., the specks on apples and pears and warts on avocado.
Cells of the filling tissue are nonsuberized in some species (e.g., magnolia and cottonwood) and suberized in others (oak, elderberry, and ash). There are even species in which layers of nonsuberized cells alternate with layers of suberized cells, forming a banded pattern (e.g., cherry and birch).
Lenticels may be oriented either longitudinal or transverse. The transverse lenticels of some species (e.g., birch, jatropha, and cherry) become very elongated with age as axis circumference increases, and can exceed one centimeter in length.
Leaf scars, where leaves were attached, can be found on older stems transformed to appear superficially like lenticels.
After the cambium of the initial periderm begins to produce cork cells and phelloderm, new phellogens arise successively deeper in the axis, always from living cells, e.g., cortex, phloem, or phelloderm. In most species, the second and subsequent phellogens do not extend completely around the axis, but instead occupy only a portion of the circumference. This results in a set of successive periderms, arc- or shell-shaped segments that entrap pockets of living or dead cells. So forms the outer bark as a complex of these overlapping periderms. In former years, the outer bark was also called the rhytidome, but this term is now hardly ever used. Outer layers of living cells, entrapped by the segments of periderms, are blocked from nutrients and therefore die. The succession of partially overlapping periderms may mature as a patchwork design separated by weak zones, where the cells have thinner walls abutting thicker walls of the cork cells.
Some species of woody plants form deeper successive periderms that are concentric, or nearly so, around the axis. These stems thereby can produce a series of cylindrical periderms, one inside another.
Most naturalists are aware that the type of bark can be used as a diagnostic feature to identify species. In temperate regions, where trees are winter deciduous, students learn to identify species based on bark (and winter bud) characteristics, and bark types are useful taxonomic characters in other habitats as well.
Often more than one term can be used to describe the bark of a tree or shrub. The young stems often have a different appearance than larger branches, and branches do not resemble the bark found at the base of a trunk. The following are some of the terms used to describe bark.
The majority of woody plants have tannin cells in bark. Tannins are bitter polyterpenes that are rejected by many animals and, if ingested, often interfere with protein digestion. Tannins, as the name suggests, are used for tanning leather. Bark can also have bitter alkaloids.
Secondary phloem is a tissue that can contain either resin ducts or laticifers. Resins are colorless or colored aromatic, combustible, terpene hydrocarbons, such as found in pines and other conifers, and the liquid occurs under pressure in the ducts, which are surrounded by living cells (epithelium) that make the terpenes to be stored in the canals. Laticifers mostly are narrow tubes that produce and contain the isoprene (terpenes), which, when exposed to air, solidify as a long-chain polymer (rubber, latex).
Bark may have pigments that give color to the layers. In some cases, bark is light-colored when initially exposed but darkens when the pigment is oxidized (eucalyptus and sycamore).
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