Sentence:  A tree may, for the imagination, present forcibly one of its qualities at a time; it may be a green dome of shade on a hot day, a ladder of retreat for a man from the attentions of a mad bull, a peg on which an apple hangs, a screen for an assassin, a choir for birds; and its own business of spreading out its million pores to the air and propagating its kind, which comes nearest to being its “<WSD>treeness</WSD>,” may be what occupies the artist least and bores him most. 
Meaning of the word marked with <WSD> tags:  The essence of what it means to be a tree; the qualities that make a tree what it is.