Pardon my lack of knowledge, but I thought a sheath was a covering on the outside of a stem (looking at my cattleya) Is that right? Does it mean something else?
The sheath is the part of the plant - from new growth - that covers the spike. It splits open when the inflorescence pokes through allowing the flowers to open later. You can see it there on the right hand side leaf.
Look at the 'V' shaped roots, top right. The root tips look like they're pointing to the sheath. In fact here's a close up.The sheath is emerging from the leaf base.