Malformation of plant tissue or stems commonly manifested as enlargement and flattening. as if fused

Merriam-Webster Online Dictionary
fasciation (noun)
a malformation of plant stems commonly manifested as enlargement and flattening as if several stems were fused
fasciation (Wikipedia)
Wyethia helianthoides or mule's ear wildflower (on right) showing fasciation
A "crested" saguaro cactus (Carnegiea gigantea ), resulting from fasciation, located at Saguaro National Park (West), Arizona, U.S.

Fasciation (pronounced /ˌfæʃiˈʃən/, from the Latin root meaning "band" or "stripe"), also known as cresting, is a relatively rare condition of abnormal growth in vascular plants in which the apical meristem (growing tip), which normally is concentrated around a single point and produces approximately cylindrical tissue, instead becomes elongated perpendicularly to the direction of growth, thus producing flattened, ribbon-like, crested (or "cristate"), or elaborately contorted, tissue. Fasciation may also cause plant parts to increase in weight and volume in some instances. The phenomenon may occur in the stem, root, fruit, or flower head. Some plants are grown and prized aesthetically for their development of fasciation. Any occurrence of fasciation has several possible causes, including hormonal, genetic, bacterial, fungal, viral and environmental causes.

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