View source for Secondary growth

In many [[vascular plants]], '''secondary growth''' is the result of the activity of the [[vascular cambium]]. The latter is a [[meristem]] that divides to produce [[secondary xylem]] [[plant cell|cell]]s on the inside of the meristem (the adaxial side) and [[secondary phloem]] cells on the outside (the abaxial side). This growth increases the girth of the plant [[root]] or [[stem]], rather than its length, hence the phrase "secondary thickening".  As long as the vascular cambium continues to produce new cells, the stem or root will continue to grow in diameter. In woody plants, this process produces [[wood]]. 

Because this growth usually ruptures the [[epidermis (botany)|epidermis]] of the stem or roots, plants with secondary growth usually also develop a [[cork cambium]]. The cork cambium gives rise to [[cork (tissue)|thickened 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 the [[cork oak]] it will yield harvestable [[cork (material)|cork]].  

Secondary growth also occurs in many nonwoody plants, e.g. [[tomato]],<ref>Thompson, N.P. and Heimsch, C. 1964. Stem anatomy and aspects of development in tomato. ''American Journal of Botany'' 51: 7-19. [http://links.jstor.org/sici?sici=0002-9122(196401)51:1%3C7:SAAAOD%3E2.0.CO;2-C]</ref> [[potato]] [[tuber]], [[carrot]] [[taproot]] and [[sweet potato]] [[tuberous root]]. A few long-lived leaves also have secondary growth.<ref>Ewers, F.W. 1982. Secondary growth in needle leaves of ''Pinus longaeva'' (bristlecone pine) and other conifers: Quantitative data. ''American Journal of Botany'' 69: 1552-1559. [http://links.jstor.org/sici?sici=0002-9122(198211%2F12)69%3A10%3C1552%3ASGINLO%3E2.0.CO%3B2-K]</ref>

'''Primary growth''' in roots and stems is growth in length and occurs in all vascular plants. Secondary growth occurs mainly in many [[dicots]] and [[gymnosperms]]. [[Monocots]] usually lack secondary growth. If they do have secondary growth, it differs from that described above.

==Anomalous secondary growth==
'''Anomalous secondary growth''' does not follow the pattern of a single vascular cambium producing xylem to the inside and phloem to the outside. Some dicots have anomalous secondary growth, e.g. in [[bougainvillea]] a series of cambia arise outside the oldest phloem.<ref>Esau, K. and Cheadle, V.I. 1969. Secondary growth in bougainvillea. ''Annals of Botany'' 33: 807-819. [http://aob.oxfordjournals.org/cgi/content/abstract/33/4/807]</ref> Most monocots have no secondary growth or anomalous secondary growth of some type. For example, [[Arecaceae|palm]] trees increase their trunk diameter due to division and enlargement of parenchyma cells, which is termed diffuse secondary growth.<ref>Esau, K. 1977. ''Anatomy of Seed Plants''. New York: Wiley</ref> In some other monocot stems 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 the apical meristem.

==See also==
* [[Bark]]
* [[Cork cambium]]
* [[Periderm]]
* [[Root]]
* [[Stem]]
* [[Vascular cambium]]

==References==
<references/>

[[Category:Plant anatomy]]
[[Category:Plant physiology]]
[[Category:Tissues]]