ABSTRACT

Protonemata of the mosses Ceratodon, Physcomitrella, Funaria, and Pottia possess apical cells which display a negative gravitropic response in the dark. A mutant of Ceratodon, wrong way response has net downward gravitropism and exhibits upward initial and mitotic reversals. Moss genera shown to have gravitropic protonemata are Physcomitrella, Ceratodon, Funaria, and Pottia. Clearly the upward growth of moss stems and setae is adaptive in positioning leaves for photosynthesis and in elevating the capsule for spore dispersal. Moss protonemata are the only uninucleate tip-growing cells with amyloplast sedimentation and well-documented vigorous gravitropism. Much evidence supports the hypothesis that gravitropic sensing is plastid-based. Amyloplasts have a high density because of their high starch content, but, as indicated, this density only results in sedimentation in specific locations, suggesting that cells with sedimentation are physiologically specialized. Moss protonemata appear to be the only uninucleate tip-growing cells with well-documented vigorous gravitropism and amyloplast sedimentation.