Figure 1.

Diagram showing two hypotheses for gravitropic sensing in a horizontal apical cell of the moss C. purpureus. The thick lines show the outer limits of the relevant masses. A, Intracellular mass shown as amyloplasts that sediment (gray fill), e.g. which might compress (arrows) a receptor. n, Nucleus. B, Entire cell (cell membrane shown as thick line) may compress and/or pull on (arrows) a receptor located outside the cell membrane. According to this model, a pressure differential between the top and the bottom of the cell can be detected even in the presence of high turgor (Staves et al., 1997a). The gravity vector is toward the bottom of the figure.