<P> Secondary growth also occurs in many nonwoody plants, e.g. tomato, potato tuber, carrot taproot and sweet potato tuberous root . A few long - lived leaves also have secondary growth . </P> <P> Abnormal secondary growth does not follow the pattern of a single vascular cambium producing xylem to the inside and phloem to the outside as in ancestral lignophytes . Some dicots have anomalous secondary growth, e.g. in Bougainvillea a series of cambia arise outside the oldest phloem . </P> <P> Ancestral monocots 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 of parenchyma cells, 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 in Yucca and Dracena 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 . </P>

An example of monocot showing secondary growth in stem is