This root did not appear as the basis of “listen” words until The Etymologies of the 1930s, where Tolkien gave ᴹ√LAS “listen” as opposed to ᴹ√LAS “leaf” (Ety/LAS¹, LAS²). One of its derivatives was N. lhewig “ear” (from fossilized dual lhaw). Tolkien apparently wanted to retain this form in his later writings after deciding that initial l was no longer unvoiced in Sindarin, so he coined a variant s-fortified root √SLAS “ear” from which it could still be derived (PE17/62, PE17/77). The unfortified root √LAS “listen” continued to appear, however (PE17/46; PE19/101), as indicated by imperative S. lasto “listen” (LotR/307). Tolkien did speculate that the roots √LAS¹ “leaf” and √LAS² “listen” might ultimately be related, probably because of the similarity of the shape of Elvish ears and the leaves of trees:
> lasse “leaf” (S las); pl. lassi (S lais). It is only applied to certain kinds of leaves, especially those of trees, and would not e.g. be used of leaf of a hyacinth (linque). It is thus possibly related to √LAS “listen”, and S-LAS stem of Elvish words for “ear”: Q hlas, dual hlaru. Sindarin dual lhaw, singular lheweg (PE17/62).
A similar notion appeared in The Etymologies: “Some think this [ᴹ√LAS¹ ‘leaf’] is related to the next [ᴹ√LAS² ‘listen’] and ✱lassē ‘ear’. The Quendian ears were more pointed and leaf-shaped than [?human]” (Ety/LAS¹).
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).