Shakespeare's Symphonies: "MacBeth" as Wagnerian Opera
Take the last one first. Early in the play, King Duncan richly rewards Macbeth and Banquo for the part they played in the recent battle and says, "I have begun to plant thee, and will labor to make thee full of growing." Although it is Banquo who replies, "There if I grow, the harvest is your own," it is Macbeth with whom the motif is associated. So when the play is nearing its crisis and some one comments "Macbeth is ripe for shaking," the tree metaphor which has lain dormant in our consciousness is jarred. And, of course, when Birnam Wood does indeed come to Dunsinane, the motif reaches its climax just as "Now cracks a noble heart" is the climax of the disease motif in "Hamlet."
In a similar way, the motif of "clothing" comes in early when Banquo comments about Macbeth's silence after hearing that the second of the witches' predictions have come true: "New honors come upon him, like our strange garments, cleave not to their mould but with the aid of use." Later, when urged by his wife to kill Duncan, he argues "I have bought golden opinions from all sorts of people, which should be worn now in their newest gloss, not cast aside so soon." Towards the climax, a minor character comments, "He cannot buckle his distemper'd cause within the belt of rule." This triggers the final clothing statement: "Now does he feel his title hang loose about him, like a giant's robe upon a dwarfish thief."
Without beleaguering the obvious, as fared the tree, so fares the clothing.
There are two kinds of blood in "Macbeth." Lady Macbeth says she will "gild" the faces of the dead King's grooms, because his blood was golden. But the rest of the blood and the word is mentioned 23 times in the play, with "bloody" clocking in at 14 is humanly red. We first see a "bleeding Captain" in the second scene and from then on the motif comes thick and fast until it reaches a peak at Macbeth's famous statement that "I am in blood stepp'd in so far that, should I wade no more, returning were as tedious as go o'er." That ocean he mentioned turning from green to red in an earlier soliloquy has truly become an ocean of real blood. And of course Lady Macbeth cannot wash the supernatural blood from her hands as she sleepwalks.
So just as there is metaphorical blood and actual blood flowing all through the script, each mention of it is in some new context, very much like the handling of the Fate motif in Bizet's "Carmen." In the Prelude to that opera, it is heard Andante moderato in 3/4 time. When Carmen makes her first appearance, we hear a twisty version of Fate Allegro moderato in 4/4 time. After the Habanera when she tosses the flower at Don Jose, the theme is played Molto expressivo in 3/4 time. And the final curtain closes on the very same theme played again Andante moderato in 3/4 time. And since it has showed up all through the work in different keys and tempos, we feel (musically at least) that it has resolved itself satisfactorily.
Since any comparison of music to literature can be tenuous at best, I will approach the entire subject briefly from another point of view. We have in music a very popular format known as Theme and Variation, the most notable examples of which are Bach's "Goldberg Variations" and Beethoven's "Diabelli Variations." The basic idea is to keep something constant (the original melody, for example) and do wonderful things with the chord progressions, the value of the notes, the harmonies, what you will.
So from this point of view, we can say that a great poet like Shakespeare can introduce a theme "baby" shall we shall say and have Lady Macbeth say she would have dashed the brains out of her own child. Then Macbeth can speak of cherubim, see bloody babies at the witches' cauldron, slaughter Macduff's children, and so on. But all the while, the basic concept of "baby" or "blood" or "disease" or "madness" is kept constant in the audience's mind. Listen to the final bars of "Gotterdammerung " for a superb use of bringing a host of musical themes to a logical and satisfying conclusion.
By the way, probably the most fascinating theme of them all to follow through a work is that of "nothing" in "King Lear"! But let us consider that for another day.