nounrare, historical Botany
In early theories of plant structure, especially those of C. Gaudichaud-Beaupré (1789–1854): each of three units into which a leaf, or a phyton, was regarded as divisible.
Distinguished as radicular, cauline, and foliar.
Mid 19th century; earliest use found in John Balfour (1808–1884), botanist. Irregularly from ancient Greek μερίς part + thallus, after French mérithalle merithal.