An Rh antigen; the Rh blood group; Rh incompatibility.
- Stauridium productum, portion of the colony magnified; p, polyp; rh, hydrorhiza.
The primitive cell sends colorless tubelets (rhizoids, rh.) into the mud on which it grows.
The basal cell has less chlorophyll than the others, and is expanded and fixed firmly to the rock on which the plant grows by the basal surface, rh, thus forming a rudimentary rhizoid.