The paragraph is as follows;
The young man followed her up the stairs. A faint light from no particular source mitigated the shadows of the halls. They trod noiselessly upon a stair carpet that its own loom would have forsworn. It seemed to have become vegetable; to have degenerated in that rank, sunless air to lush lichen or spreading moss that grew in patches to the staircase and was viscid under the foot like organic matter.
What does this phrase really mean?
Thanks in advance.