Previously we have noted that parenthetical commentary appears and disappears as the story grows lighter and darker by turns, and that this in general follows the relationship of Bilbo and then Frodo with the Ring. We have also just seen a very similar dynamic occur with Merry and Pippin in Book Three. Though neither of them ever possesses the Ring, it is nevertheless Saruman's lust to acquire it and Sauron's to regain it that motivates their kidnapping by the orcs, thus directly causing the darker and lighter turns the narrative takes in The Uruk-hai and Treebeard. Indeed Merry and Pippin perceive the role the Ring is playing in their captivity, and with desperate audacity play upon Grishnákh's mistaken belief that they have it, wagering their lives for a chance at escape. So here, too, the Ring is intimately connected to the dynamic at work and the parentheses. Since it transfers so smoothly from Bilbo and Frodo to Merry and Pippin, and, as we shall presently see, to Sam, it should also be evident just how closely concerned with the hobbit voice these asides are.
After the cluster of parentheses in Treebeard a long gap of 155 pages follows (TT 3.iv.483-4.iii.638), empty except for the somewhat knowing comment on the sinister multiple meanings of Orthanc (TT 3.viii.555). An even longer gap of 177 pages before Treebeard (3.iv.465) extends back to The Ring Goes South (FR 2.iii.288), also interrupted only once (2.vi.344). This lack of parenthetical comments elsewhere in Book Three coincides with the general absence of the hobbits from this book despite the crucial role played by Merry and Pippin, a dynamic to be repeated in Book Five. Something similar holds true also in Book Two, where the narrative attends more to the Company as a whole than to the hobbits or Frodo specifically. So darker turns in the narrative connected to the Ring may be the most striking reason for the absence of parentheses, but not the only reason.
In Book 4 parentheses reappear in The Black Gate Is Closed. As I noted in Part 4, in this book Sam begins to carry the burden of the narrative as Frodo becomes increasingly preoccupied by his struggle against the Ring. It is Sam to whom the three parenthesis in The Black Gate Is Closed refer, at least two of which give us Sam's commentary on his own thoughts at the time (TT 4.iii.638, 640), and the third almost certainly does, too (4.iii.647). This last is perhaps the most remarkable since Sam's behavior in the tale here is as lighthearted as his comment on it, recalling Frodo from the darkness of his cares and purpose by this recitation of 'the old fireside rhyme of Oliphaunt' outside the Black Gate of Mordor, and recalling for the reader an earlier such moment where Sam did the same thing in the same way, hands behind his back and all (FR 1.xii.206-208). Consider also the comment in Of Herbs and Stewed Rabbit where we learn that Sam is 'a good cook, even by hobbit reckoning' (TT 4.v.653), an art hobbits 'begin to learn before their letters (which many never reach)'. Here we must remember that Sam was called out by name a full chapter and seventeen years before we met him as a hobbit who had learned his letters. As with his pose while 'speaking poetry', the narrator is using the parentheses to remind the reader of how special Sam is. Not only could he cook the cabbages and potatoes which his Gaffer thought he'd be better off minding, but he knew his letters and poetry and great tales, which repeatedly helped to sustain him on the long road into darkness he and Frodo had to walk. His sense of mission comes from his learning his letters. Sam Gamgee had read all the right books.
These parentheses also further mark the shift we saw earlier with Merry and Pippin, a shift away from Frodo as his hobbit comrades step forward and begin to take up the roles they will play until the end of the book. This is not to say that Frodo is becoming less important. Far from it. But his are now not the only small hands that turn the wheels of the world while the eyes of the great are elsewhere. Sam in particular becomes critically important, and increasingly the story of Frodo's journey is seen through his eyes because Frodo's eyes are elsewhere.