John 15:1-6 (Revised English Bible):

I am the true vine, and my Father is the gardener. Any branch of mine that is barren he cuts away; and any fruiting branch he prunes clean, to make it more fruitful still. You are already clean because of the word I have spoken to you. Dwell in me, as I in you. No branch can bear fruit by itself, but only if it remains united with the vine; no more can you bear fruit, unless you remain united with me.

I am the vine; you are the branches. Anyone who dwells in me, as I dwell in him, bears much fruit; apart from me you can do nothing. Anyone who does not dwell in me is thrown away like a withered branch. The withered branches are gathered up, thrown on the fire, and burnt.

I found the photo above (source: Corbis) to be a haunting companion to this verse - the image of barren vines lined up like a row of crosses in the moonlight is quite striking.