Or leave a kiss but in the cup,
And I'll not look for wine.
The thirst that from the soul doth rise
Doth ask a drink divine;
But might I of Jove's nectar sup,
I would not change for thine.
I sent thee late a rosy wreath,
Not so much honouring thee
As giving it a hope, that there
It could not withered be.
But thou thereon didst only breathe,
And sent'st it back to me;
Since when it grows, and smells, I swear,
Not of itself, but thee.
The speaker knows that Celia breathed on the wreath. This had some sort of supernatural effect on the flowers which smell of Celia and not of flowers. 
 The implication is that her spirit still remains with him. Like it or not, he still seems to love her.
Song To Celia