Fasciated stems, i.e., those joined abnormally in a flattened structure. It is believed that fasciation is caused by mutation of a single cell in the central meristem (the primary tissue that differentiates into specialized tissues), which results in cells multiplying dramatically, so that the growing point develops an entire "new plant" within the existing plant. What triggers such a mutation in various herbaceous and woody plants is not known, though the environment is believed to play a key role (the mechanism is believed to involve a hormonal response to an environmental stress).