Yes, LEMMAS is a Scrabble word!

Letter values in Scrabble US: L 1, E 1, M 3, M 3, A 1, S 1.

Is it a valid word?

LEMMAS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a subsidiary proposition that is assumed to be true in order to prove another proposition
the lower and stouter of the two glumes immediately enclosing the floret in most Gramineae
the heading that indicates the subject of an annotation or a literary composition or a dictionary entry

Also: flowering glume.