The basic Sindarin word for “leaf”, derived from the root √LAS (PE17/62, 153; PE22/166). It appeared as both lass and las, but I believe the latter is the suffixal form, the result of the Sindarin sound change whereby final ss shortened in polysyllables (LotR/1115). Its plural form was lais, which is of interest because normally consonant clusters prevent i-intrusion]]; compare nern and resg the plurals of narn and rasg. I am of the opinion that the ss was a particular “weak” cluster and allowed intrusion anyway; see the entry on Sindarin plural nouns for further discussion.
Conceptual Development: G. lass “a leaf” appeared all the way back in the Gnomish Lexicon of the 1910s, but there Tolkien said it was sometimes used for “petal” = G. tethlas (GL/52). In Early Noldorin Word-lists of the 1920s it became ᴱN. lhas “leaf” (PE13/148) and N. lhass “leaf” appeared in The Etymologies of the 1930s derived from primitive ᴹ✶lassē under the root ᴹ√LAS (Ety/LAS¹). These 1920s-30s forms were due to the Noldorin sound change whereby initial l was unvoiced to lh. Tolkien abandoned this sound change in Sindarin of the 1950s and 60s, so that lass “leaf” was restored.
_n. _leaf. >> athelas, las