Inbotany,secondary growth is the growth that results fromcell division in thecambia or lateralmeristems and that causes thestems androots to thicken, whileprimary growth is 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, butmonocots usually 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 ofdicotyledons andgymnosperms. 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 livingpteridophytes this feature is extremely rare, only occurring inIsoetes.
In manyvascular plants, secondary growth is the result of the activity of the two lateral meristems, thecork cambium andvascular cambium. Arising fromlateral meristems, 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 atree with a thickened trunk.[citation needed]
Because this growth usually ruptures theepidermis of the stem or roots, plants with secondary growth usually also develop acork cambium. The cork cambium gives rise tothickened cork cells to 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 oak it will yield harvestablecork.[citation needed]
Secondary growth also occurs in many nonwoody plants, e.g.tomato,[1]potatotuber,carrottaproot andsweet potatotuberous root. A few long-lived leaves also have secondary growth.[2]
Abnormal secondary growth does not follow the pattern of a single vascular cambium producingxylem to the inside andphloem to the outside as in ancestral lignophytes. Somedicots have anomalous secondary growth, e.g. inBougainvillea a series of cambia arise outside the oldest phloem.[4]
Ancestralmonocots lost 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.Palm trees increase their trunk diameter due to division and enlargement ofparenchyma cells, which is termed "primary gigantism"[3] because there is no production of secondary xylem and phloem tissues,[3][5] or sometimes "diffuse secondary growth".[6] In some other monocot stems as inYucca andDracaena with 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 theapical meristem.[7]
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