The basic Quenya word for “leaf”, derived from the root √LAS (PE17/62, 153; VT39/9). This word dates all the way back to the Qenya Lexicon of the 1910s where ᴱQ. lasse “leaf” appeared as its own entry (QL/51). ᴹQ. lasse “leaf” also appeared in The Etymologies of the 1930s under the root ᴹ√LAS (Ety/LAS¹). In the Gnomish Lexicon of the 1910s Tolkien said that lasse meant both “a leaf or petal” (GL/52). After that Tolkien translated it only as “leaf”. In one set of later notes Tolkien said it was even more restricted in meaning, and “only applied to certain kinds of leaves, especially those of trees, and would not e.g. be used of leaf of a hyacinth (linque)” (PE17/62).
Neo-Quenya: Despite Tolkien late declaration, I would use lassë as the general “leaf” word for purposes of Neo-Quenya, though more specialized words may also exist such as linquë “(leaf of a) hyacinth”. I would also use it metaphorically in its Early Qenya sense as the “petal” of a flower where the context is very clear, such as lassi indilo “leaves of a lily” = “lily petals”. But where ambiguous, I would use the neologism ᴺQ. lótelas for “petal”, more literally “flower leaf”.
lassë noun "leaf"; pl. lassi is attested (Nam, RGEO:66, Letters:283, LAS1, LT1:254, VT39:9, Narqelion); gen. lassëo "of a leaf", gen. pl. lassion "of leaves" (earlier lassio) (WJ:407). The word lassë was only applied to certain kinds of leaves, especially those of trees (PE17:62), perhaps particularly _ear-shaped _leaves (cf. the entry _LAS1 _in the Etymologies, where Tolkien comments on the pointed or leaf-shaped Elvish ears and suggests an etymological connection between words for "ear" and "leaf"); see also linquë #3. Compound lasselanta "leaf-fall", used (as was quellë) for the latter part of autumn and the beginning of winter (Appendix D, Letters:428); hence Lasselanta alternative name of October (PM:135). Cf. also lassemista "leaf-grey, grey-leaved" (LotR2:III ch. 4, translated in Letters:224, PE17:62), lassewinta a variant of lasselanta (PM:376). Adj. laicalassë "green as leaves" (PE17:56). See also lillassëa, lantalasselingëa.