That's the leaf-mold... chemically identical with the earth-blanket they grow under in the wild state.
9 seconds sound clip from the Back for Christmas (Suspense) classic radio play.
You can hear this line at 00:02:38 in the radio play.
Quote context
[...]
- In their wild state they bloom under damp masses of leaf-mold.
- The South American Indians call them devil-flowers because they appear to bloom under the ground.
- Whom else it will interest, I can't imagine.
- Oh! What's that terrible smell?
- I do want to get these started before we close the house.
- Unless you call digging a hole in the cellar making arrangements. I certainly don't.
- Sometimes I think you're going soft in the head, Hubert.
[...]