Proskauer and Berman1 have described a technique for culturing green organisms such as filamentous algae and moss protonema on an agar substrate containing activated charcoal which may simulate conditions found in nature. They ascribed the resulting morphological changes primarily to a decrease in the amount of light transmitted by the blackened agar, and considered their technique a simulation of natural soil conditions. We have performed similar experiments which also show that moss development is altered on charcoal–agar, but which require another interpretation.