Xylem tracheids are pointed, elongated xylem cells, the simplest of which have continuous primary cell walls and lignified secondary wall thickenings in the form of rings, hoops, or reticulate networks.
honestly. adv. 1. in an honest manner. 2. (intensifier): I honestly don't believe it. interj.
Sincerely is the adverb, meaning in a sincere or genuine way. Yours is a special noun, in this construction, meaning 'belonging' to you. The entire phrase is, merely, a formulaic manner in which to conclude a letter.