The Collaborative International Dictionary
Merithallus
Merithal \Mer"i*thal\, Merithallus \Mer`i*thal"lus\, n. [NL. merithallus, fr. Gr. ?, or ?, a part + ? a young shoot.] (Bot.) Same as Internode.
Wiktionary
merithallus
n. (context botany English) An internode.