Awoody plant is aplant that produceswood as its structural tissue and thus has a hard stem.[1] In cold climates, woody plants further survivewinter ordry season above ground, as opposed toherbaceous plants that die back to the ground untilspring.[2]
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Woody plants are usuallytrees,shrubs, orlianas. These are usuallyperennial plants[3] whose stems and larger roots are reinforced with wood produced from secondaryxylem. The main stem, larger branches, and roots of these plants are usually covered by a layer ofbark. Wood is a structuraltissue that allows woody plants to grow from above ground stems year after year, thus making some woody plants the largest and tallestterrestrial plants.[3]
Woody plants, likeherbaceous perennials, typically have adormant period of the year when growth does not take place. This occurs intemperate andcontinental due to freezing temperatures and lack of daylight during the winter months.[4] Meanwhile, dormancy insubtropical andtropical climates is due to the dry season; when lowprecipitation limits water available for growth.[5] The dormant period will be accompanied byabscission (if the plant isdeciduous).[6]Evergreen plants do not lose all their leaves at once (they instead shed them gradually over thegrowing season), however growth virtually halts during the dormant season. Many woody plants native to the subtropics and tropics areevergreen due to year-round warm temperatures and rainfall.[7] However, in many regions with atropical savanna climate or amonsoon subtropical climate, a lengthydry season precludesevergreen vegetation, instead promoting the predominance ofdeciduous trees.[8]
During thefall months, eachstem in a deciduous plant cuts off theflow of nutrients andwater to the leaves. This causes them to change colors as thechlorophyll in theleaves breaks down.[9] Special cells are formed that sever the connection between the leaf and stem, so that it will easily detach. Evergreen plants do not shed their leaves, merely go into a state of low activity during the dormant season (in order to acclimate tocold temperatures orlow rainfall).[10] Duringspring, theroots begin sendingnutrients back up to thecanopy.[11]
When thegrowing season resumes, either with warm weather or the wet season, the plant will break bud by sending out new leaf or flower growth. This is accompanied by growth of new stems from buds on the previous season's wood. In colder climates, most stem growth occurs during spring and early summer. When the dormant season begins, the new growth hardens off and becomes woody. Once this happens, the stem will never grow in length again, however it will keep expanding in diameter for the rest of the plant's life.
Most woody plants native to colder climates have distinctgrowth rings produced by each year's production of new vascular tissue. Only the outer handful of rings contain living tissue (thecambium,xylem,phloem, andsapwood). Inner layers have heartwood, dead tissue that serves merely as structural support.
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Stem growth primarily occurs out of theterminal bud on the tip of the stem.Axillary buds are suppressed by the terminal bud and produce less growth, unless it isremoved by human or natural action. Without a terminal bud, the side buds will have nothing to suppress them and begin rapidly sending out growth, if cut duringspring. By latesummer and earlyautumn, most active growth for the season has ceased and pruning a stem will result in little or no new growth. Winter buds are formed when thedormant season begins. Depending on the plant, these buds contain either newleaf growth, newflowers, or both.
Terminal buds have a stronger dominance onconifers thanbroadleaf plants, thus conifers will normally grow a single straighttrunk without forking or large side or lateral branches.
As a woody plant grows, it will often lose lower leaves andbranches as they become shaded out by thecanopy (biology). If a given stem is producing an insufficient amount of energy for the plant, the roots will "abort" it by cutting off the flow ofwater andnutrients, causing it to gradually die.
Below ground, theroot system expands eachgrowing season in much the same manner as thestems. The roots grow in length and send out smaller lateral roots. At the end of the growing season, the newly grown roots become woody and cease future length expansion, but will continue to expand in diameter. However, unlike the above-ground portion of the plant, the root system continues to grow, although at a slower rate, throughout the dormant season. Incold-weather climates, root growth will continue as long as temperatures are above 2 °C (36 °F).
Wood is primarily composed of xylem cells withcell walls made ofcellulose andlignin. Xylem is avascular tissue which moves water and nutrients from the roots to the leaves. Most woody plants form new layers of woody tissue each year, and so increase their stem diameter from year to year, with new wood deposited on the inner side of avascular cambium layer located immediately beneath the bark. However, in somemonocotyledons such aspalms anddracaenas, the wood is formed in bundles scattered through the interior of the trunk. Stem diameter increases continuously throughout the growing season and halts during the dormant period.[12]
The symbol for a woody plant, based onSpecies Plantarum byLinnaeus is
, which is also theastronomical symbol for the planetSaturn.[13]
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