New organs such as shoots and roots can be induced to form on cultured plant tissues. Such freshly originated organs are said to be adventive or adventitious. The creation of new form and organisation, where previously it was lacking, is termed morphogenesis or organogenesis. Tissues or organs that have the capacity for morphogenesis/organogenesis are said to be morphogenic (morphogenetic) or organogenic (organogenetic). So far it has been possible to obtain the de novo (adventitious) formation of: • shoots (caulogenesis) and roots (rhizogenesis) separately. The formation of leaves adventitiously in vitro usually denotes the presence of a shoot meristem. Sometimes leaves appear without apparent shoot formation: opinions are divided on whether such leaves can have arisen de novo, or whether a shoot meristem must have been present first of all and subsequently failed to develop.
• embryos that are structurally similar to the embryos found in true seeds. Such embryos often develop a region equivalent to the suspensor of zygotic embryos and, unlike shoot or root buds, come to have both a shoot and a root pole. To distinguish them from zygotic or seed embryos, embryos produced from cells or tissues of the plant body are called somatic embryos (or embryoids) and the process leading to their inception is termed embryogenesis. The word 'embryoid' has been especially used when it has been unclear whether the embryo-like structures seen in cultures were truly the somatic equivalent of zygotic embryos. Somatic embryogenesis is now such a widely observed and documented event that somatic embryo has become the preferred term.
• flowers, flower initials or perianth parts. The formation of flowers or floral parts is rare, occurring only under special circumstances and is not relevant to plant propagation.
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