Secondary growth

This article is about the type of plant growth. For secondary-growth forests, seeSecondary forest.

Secondary growth thickens thestemandroots,typically making themwoody.Obstructions such as this metal post and stubs of limbs can be engulfed.

Inbotany,secondary growthis the growth that results fromcell divisionin thecambiaor lateralmeristemsand that causes thestemsandrootsto thicken, whileprimary growthis growth that occurs as a result of cell division at the tips of stems and roots, causing them to elongate, and gives rise to primary tissue. Secondary growth occurs in mostseed plants,butmonocotsusually lack secondary growth. If they do have secondary growth, it differs from the typical pattern of other seed plants.

The formation of secondary vascular tissues from the cambium is a characteristic feature ofdicotyledonsandgymnosperms.In certain monocots, the vascular tissues are also increased after the primary growth is completed but the cambium of these plants is of a different nature. In the livingpteridophytesthis feature is extremely rare, only occurring inIsoetes.

Lateral meristems

Diagram of secondary growth in atreeshowing idealised vertical and horizontal sections. Newwoodis added in each growth season by the lateral meristems, thecork cambiumandvascular cambium.

In manyvascular plants,secondary growth is the result of the activity of the two lateral meristems, thecork cambiumandvascular cambium.Arising fromlateralmeristems, secondary growth increases the width of the plant root or stem, rather than its length. As long as the lateral meristems continue to produce new cells, the stem or root will continue to grow in diameter. Inwoody plants,this process produceswood,and shapes the plant into atreewith a thickened trunk.

Because this growth usually ruptures theepidermisof the stem or roots, plants with secondary growth usually also develop acork cambium.The cork cambium gives rise tothickened cork cellsto protect the surface of the plant and reduce water loss. If this is kept up over many years, this process may produce a layer of cork. In the case of thecork oakit will yield harvestablecork.

In nonwoody plants

Secondary growth also occurs in many nonwoody plants, e.g.tomato,potatotuber,carrottaprootandsweet potatotuberous root.A few long-lived leaves also have secondary growth.

Abnormal secondary growth

Palms(in the pictureRoystonea regiapalm) increase their trunk diameter by means of division and enlargement of parenchyma tissue, without a real secondary growth (production of secondary vascular strands—secondary xylem and phloem tissues—with the consequent enlargement of the canopy and root system).

Abnormal secondary growthdoes not follow the pattern of a single vascular cambium producingxylemto the inside andphloemto the outside as in ancestral lignophytes. Somedicotshave anomalous secondary growth, e.g. inBougainvilleaa series of cambia arise outside the oldest phloem.

Ancestralmonocotslost their secondary growth and their stele has changed in a way it could not be recovered without major changes that are very unlikely to occur. Monocots either have no secondary growth, as is the ancestral case, or they have an "anomalous secondary growth" of some type, or, in the case of palms, they enlarge their diameter in what is called a sort of secondary growth or not depending on the definition given to the term.Palmtrees increase their trunk diameter due to division and enlargement ofparenchymacells, which is termed "primary gigantism"because there is no production of secondary xylem and phloem tissues,or sometimes "diffuse secondary growth".In some other monocot stems as inYuccaandDracaenawith anomalous secondary growth, a cambium forms, but it produces vascular bundles and parenchyma internally and just parenchyma externally. Some monocot stems increase in diameter due to the activity of a primary thickening meristem, which is derived from theapicalmeristem.

See also

• Bark
• Cambium (botany)
• Root
• Stem
• Tylosis
• Dendrochronology
• Herbchronology
• Anatomy