Trunk (botany)
Trunks, also called boles, are the stems of woody plants and the main structural element of trees. The woody part of the trunk consists of dead but structurally significant heartwood and living sapwood, which is used for nutrient storage and transport. Separating the wood from the bark is the cambium, from which trunks grow in diameter. Bark is divided between the living inner bark, which transports sugars, and the outer bark, which is a dead protective layer.
The precise cellular makeup of these components differs between non-flowering plants and flowering plants. A variety of specialised cells facilitate the storage of carbohydrates, water, minerals, and transport of water, minerals, and hormones around the plant. Growth is achieved by division of these cells. Vertical growth is generated from the apical meristems, and horizontal growth, from the cambium. Growth is controlled by hormones, which send chemical signals for how and when to grow.
Plants have evolved to both manage and prevent damage from occurring to trunks. Trunks are structured to resist wind forces, through characteristics such as high strength and stiffness, as well as oscillation damping, which involves taking energy, and therefore damage, out of the trunk and into the branches and leaves. If damaged, trunks employ a complex and slow defence mechanism, which starts by creating a barrier to the incoming disease. Eventually, diseased cells are replaced by new, healthy cells, once the threat is contained.
Trunks not only support the extensive ecological function of living trees, but also play a large ecological role when the trees eventually die. Dead trunk matter, termed coarse woody debris, serves many roles including: plant and animal habitat, nutrient cycling, and the transport and control of soil and sediment. Most trees grown outside the tropics can be dated by counting their annual rings. Variations in these rings can provide insights into climate, a field of study called dendroclimatology. Trunks have been in continuous use by humans for thousands of years including in construction, medicine, and a myriad of wood-related products. Culturally, trunks are the subject of symbolism, folk belief, ritual, and feature in art of many mediums.
Occurrence
All vascular plants have both roots and stems. But only gymnosperms, and angiosperms that are both woody and sprout two initial leaves, have trunks. The rest of the angiosperms can be categorised as either herbaceous plants with one initial leaf, like bamboo, or herbaceous plants with two initial leaves, like flax. Neither grow trunks.Structure
Trunks, the stems of woody plants, connect the roots to the upper branches, canopy, and leaves. In general, the trunk of woody plants, which is their most easily identifiable feature, consists of: heartwood, sapwood, cambium, inner bark, outer bark, and the pith. In this way, the xylem, or wood, of the tree is separated from the bark by the cambium, which functions as a lateral meristem. The cambium promotes growth radially. The younger part of the xylem conducts water up from the roots to the leaves. It also acts as storage for food, through the parenchyma, which is made up of ray cells. While only 10% of the sapwood cells are alive, the heartwood, the darker part of the xylem, is completely dead. It proves structural value to the plant. The pith is the most minor feature of the trunk, being a remnant from when the stem was not yet woody. The purpose of producing a trunk is to enable a taller plant, with greater stability.Earlywood and latewood describe the difference in density between wood grown early and later in the growing season. Tree rings, seen when the trunk is viewed in cross-section, are the result of the difference in cambial growth rates during the year. The difference in thickness of the cells of earlywood and latewood is generally responsible for the presence of growth rings. They are most pronounced in conifers and are mostly not annual in equatorial regions. In angiosperms, annual rings are also influenced by the proportion of different cells present in the different regions. This varies between genera, however. The outer annual ring or rings are generally responsible for most of the water transport in trees, to differing degrees.
In gymnosperms
Up to 90% of the xylem of gymnosperms is made up of vertically oriented tracheids, a type of conductive cell, which often overlap one another. To facilitate liquid transfer, the cell walls of tracheids contain pits, and are around 100 times as long as they are wide. They also provide structural strength through their thick cell wall. In the horizontal direction, the most significant component in gymnosperms are wood rays, formed by the cambium. They consist of groups of cells which both store carbohydrates and minerals, but also move water, minerals, and other compounds in the horizontal direction. Ray tracheids and parenchyma, in different combinations, make up the structure of wood rays. Parenchyma chiefly function as nutrient storage, but can also assist in liquid transport to a limited degree. They also supply mechanical strength to the tree.In angiosperms
In angiosperms, the axial direction is dominated by fibres, as well as vessel elements, parenchyma cells and tracheids, as in gymnosperms. The vessel elements are responsible for the majority of water transport and as such are orientated on top of one-another. They range from 1 to 10 m in length and the presence of them can be used to separate hardwoods from softwoods. The structure of fibres is similar to tracheids, but with smaller pits and thicker cell walls. Their main function is structural. Generally, the proportion of axial parenchyma found in angiosperms is greater than that found in gymnosperms. In the horizontal direction, wood rays can be found, as in gymnosperms, however they consist exclusively of parenchyma. In contrast to gymnosperms, the radial water transport is mostly achieved through adjacent axial vessels, or between any axial member through their pits.Bark
The structure of bark consists of a primary phloem, secondary phloem, cortex, periderm and a dead outer layer of rhytidome. This is the case for radial growth caused by the cambium, called secondary growth. In primary growth located at stem tips, however, the secondary phloem and periderm are not grown. Phloem support carbohydrate transport throughout the tree, through a process called translocation. The periderm protects the trunk from mechanical damage and reduces loss of water. Lenticels are small holes in the periderm consisting of porous tissue that allow for gas transfer. This includes transfer of carbon dioxide, oxygen, and water.Growth
There are two types of growth that produce tree trunks: primary growth of stems, and secondary growth through the cambium. Primary growth occurs on the apical meristems through apical dominance, in which buds not at the tip are prevented from growing.Secondary growth occurs in the vascular part of the cambium, in the cambial zone, a layer between 1 and 10 cells thick. Both additive and multiplicative division take place in this zone. In additive division, fusiform initial cells are tangentially divided to produce mother cells for the subsequent production of xylem and phloem cells. In multiplicative division, the same initial cells are divided anticlinally. This is the division responsible for growing the diameter of the trunk.
When trees grow on a lean, it causes an increase in density and cambial growth in the concave section being leaned on. The wood this creates is called reaction wood and is generally undesirable. In angiosperms it is known as tension wood and in gymnosperms as compression wood, as a result of the different strategies employed by the trees.
Hormones
is the hormone responsible for preventing auxiliary buds from growing, thus fostering apical dominance. The exact mechanism and full picture of its contribution, as well as genetic and other factors is not clear.Although all of the major plant growth hormones can be found in the cambium region, auxin exerts a major influence on both divisions that occur in the cambial zone. There is some evidence that gibberellins have an effect on cambial growth in some plants. There is evidence to suggest that exogenous cytokinins both stimulate and do not stimulate cambial growth rates. Abscisic acid has an effect on cambial growth, although it is not clear in what way. Ethylene has been found to contribute to controlling the amount of xylem or phloem cells being produced by trees. Ethephon has been shown to effect the sizes of xylem and phloem cells and cell walls, depending on its concentration. Both Indole-3-acetic acid and ABA have a variable effect on tree trunk growth, depending on the time of year.