Early Quenya
sirpe
noun. stem, stalk, slender tube, etc.
telko
noun. stem
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A noun appearing as ᴱQ. sirpe “stem, stalk (slender tube, etc.)” in the Qenya Lexicon of the 1910s under the early root ᴱ√SIRIP (QL/84).
Neo-Quenya: As this early root does not conflict with any later forms, I would retain this word as ᴺQ. sirpë “stalk, slender tube”. However, I adapt the early root as ᴺ√THIRIP “stalk” to preserve more Gnomish forms, so I would assume the s in ᴺQ. sirpë is from older þ. Furthermore, for “stem” I would use the later word Q. telco (LotR/1118).