THE PROTONEMA OF CHARA-FRAGILIS DESV - REGENERATIVE FORMATION, PHOTOMORPHOGENESIS, AND GRAVITROPISM

When exposed to constant white light for four weeks, isolated nodes of Chara fragilis Desv. regenerated side branches, rhizoids, and multice llular protonemata, the latter being similar to those germinated from oospores. When kept in darkness the nodes developed protonemata exclus ively. These were single-celled, colourless, and tip-growing and, with the light microscope, they looked like rhizoids. Upon exposure to blu e light, but not to red or far-red, the growth rates of the protonemat a rapidly declined, the cell apices swelled, and the nucleus migrated acropetally. Within 24 h the cells went through the first of a series of divisions resulting in the formation of multicellular protonemata. When returned to darkness after a blue light pulse of 5 h the cell div isions proceeded normally, but the protonemata showed etiolated growth . While growth of the internode was drastically promoted, the developm ent of the multicellular apex and the lateral initial were suppressed. Both uni- and multicellular etiolating protonemata showed negative gr avitropism but were phototropically insensitive. It is argued that the single-celled protonema is an organ specialized for the penetration o f mud covering the nodes or oospores of Chara and thus serves to searc h for light, comparable to etiolated hypocotyls and stems in seedlings of higher plants.