In Wordsworth's poem, "The World is Too Much With Us," it is probable that Triton's horn is "wreathed" with: a) apple b) trout c) seaweed
In William Wordworth's poem, "The World is Too Much With Us," the reader is bombarded with images of nature. This is not a surprise as Wordsworth was one of the two first-generation Romantics, alongside Samuel Taylor Coleridge.
The Romantic poets are known to have had a great deal of influence on other writers, including American authors Emerson and Thoreau. One of the main focuses of all Romantic writing was on a reverence for, and a return to, nature. Wordsworth's references to nature are seen with words like: Nature, sea, moon, winds, and sleeping flowers. He also uses nature as a metaphor for life, in that as the seasons quickly pass from spring to fall to winter, so do our lives, leaving youth behind quickly to face old age, and eventually, death.
Wordsworth references Triton, coming out of the sea (more nature imagery) who is:
[a] sea god, often depicted as trumpeting on a shell.
With this in mind, Triton's horn would be "wreathed" or covered as with a wreath or vines, etc., with seaweed. It, also, is a plant, like that which makes up a wreath, or like vines or moss, and it comes from the sea.