Editing Developmental biology (section)

=== Growth ===
A [[vascular plant]] begins from a single celled [[zygote]], formed by [[fertilisation]] of an egg cell by a sperm cell. From that point, it begins to divide to form a plant [[embryo]] through the process of [[embryogenesis]]. As this happens, the resulting cells will organize so that one end becomes the first root, while the other end forms the tip of the shoot. In [[seed]] plants, the embryo will develop one or more "seed leaves" ([[cotyledon]]s). By the end of embryogenesis, the young plant will have all the parts necessary to begin its life.

Once the embryo [[germination|germinates]] from its seed or parent plant, it begins to produce additional organs (leaves, stems, and roots) through the process of [[organogenesis]]. New roots grow from root [[meristem]]s located at the tip of the root, and new stems and leaves grow from shoot [[meristem]]s located at the tip of the shoot.<ref>{{cite journal | vauthors = Brand U, Hobe M, Simon R | title = Functional domains in plant shoot meristems | journal = BioEssays | volume = 23 | issue = 2 | pages = 134–41 | date = February 2001 | pmid = 11169586 | doi = 10.1002/1521-1878(200102)23:2<134::AID-BIES1020>3.0.CO;2-3 | s2cid = 5833219 | department = Review }}</ref> Branching occurs when small clumps of cells left behind by the meristem, and which have not yet undergone [[cellular differentiation]] to form a specialized tissue, begin to grow as the tip of a new root or shoot. Growth from any such meristem at the tip of a root or shoot is termed [[primary growth]] and results in the lengthening of that root or shoot. [[Secondary growth]] results in widening of a root or shoot from divisions of cells in a [[Cambium (botany)|cambium]].<ref>{{cite journal | vauthors = Barlow P | title = Patterned cell determination in a plant tissue: the secondary phloem of trees | journal = BioEssays | volume = 27 | issue = 5 | pages = 533–41 | date = May 2005 | pmid = 15832381 | doi = 10.1002/bies.20214 }}</ref>

In addition to growth by [[cell (biology)|cell]] division, a plant may grow through '''cell elongation'''.<ref>{{cite journal | vauthors = Pacifici E, Di Mambro R, Dello Ioio R, Costantino P, Sabatini S | title = Arabidopsis root | journal = The EMBO Journal | volume = 37 | issue = 16 | date = August 2018 | pmid = 30012836 | pmc = 6092616 | doi = 10.15252/embj.201899134 }}</ref> This occurs when individual cells or groups of cells grow longer. Not all plant cells will grow to the same length. When cells on one side of a stem grow longer and faster than cells on the other side, the stem will bend to the side of the slower growing cells as a result. This directional growth can occur via a plant's response to a particular stimulus, such as light ([[phototropism]]), gravity ([[gravitropism]]), water, ([[hydrotropism]]), and physical contact ([[thigmotropism]]).

Plant growth and development are mediated by specific [[plant hormone]]s and plant growth regulators (PGRs) (Ross et al. 1983).<ref name="ross">{{cite book | vauthors = Ross SD, Pharis RP, Binder WD | date = 1983 | chapter = Growth regulators and conifers: their physiology and potential uses in forestry. | pages = 35–78 | veditors = Nickell LG | title = Plant growth regulating chemicals | volume = 2 | publisher = CRC Press | location = Boca Raton, FL }}</ref> Endogenous hormone levels are influenced by plant age, cold hardiness, dormancy, and other metabolic conditions; photoperiod, drought, temperature, and other external environmental conditions; and exogenous sources of PGRs, e.g., externally applied and of rhizospheric origin.