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.
This root was connected to leaves throughout Tolkien’s life. It did not appear directly in the Qenya or Gnomish lexicons of the 1910s, but ✱ᴱ√LASA “leaf” is implied by ᴱQ. lasse and G. lass “leaf” (QL/51; GL/52). ᴹ√LAS appeared directly in The Etymologies of the 1930s with the derivatives ᴹQ. lasse and N. lhass (Ety/LAS¹), and the root √LAS “leaf” was also mentioned in Tolkien’s later writings (PE17/77; VT39/9).