Abstract
The giant internodal cells of Chara corallina exhibit a well-documented gravity-induced polarity of cytoplasmic streaming. In order to determine whether these cells also exhibit a gravitropism, we grew Chara in small containers that could be reoriented with respect to the vector of gravity and monitored their growth with time-lapse video. We find that Chara internodal cells are negatively gravitropic and that the internodal cells curve such that growth is reoriented to be parallel to the vector of gravity. We also find that the presence of the apical internodal cell is vital to a gravitropic response. Growing Chara in a dense medium so as to yield a neutrally buoyant protoplast eliminates the gravitropic response. These results along with the rapid rate of cytoplasmic streaming and the lack of any visible sedimenting particles in the internodal cells indicate that the statolith model for gravisensing cannot be invoked to explain gravitropism of Chara "shoots." However, the gravitational pressures model of gravitropism is able to explain these data.