rounding medium triggers graviperception), graviorientation should be abolished in
neutral-buoyancy experiments, whereas an intracellular gravisensor, such as a starch
statolith-based system, should still be functional under these conditions. Recent experi-
ments have shown that gravitropism in this moss takes place in media that are denser than
the cytoplasm of the apical cell (Sack et al. 2001). This result provides evidence that grav-
ity sensing in Ceratodonstrictly relies on sedimentation of intracellular masses rather
than on the mass of the entire protoplast. Like in Chararhizoids and protonemata, the
statolith-based sensory system is not complemented by alternative mechanism.
7.8 Susception in the Statolith-based System of Loxodes
The ciliate Loxodes(Figure 7.1) maintained its positive gravitaxis when the density of the
external medium was in the same range as or even higher than the density of its cyto-
plasm (1.03 g/cm^3 ). As gravikinesis of Loxodes was slightly reduced under isodensity
conditions, Neugebauer et al. (1998) proposed that in Loxodesan intracellular gravisens-
ing mechanism is complemented by a protoplast pressure-based mechanism (Figure 7.4).
By analogy to the situation in Chara, Loxodes uses BaSO 4 as the statolith material.
Loxodespossesses 5 to 25 so-called Müller organelles (Müller vesicles), which are 7- to
10-μm-wide vacuoles containing a body of BaSO 4 (3–3.5 μm in diameter) fixed to a mod-
CHAPTER 7 SINGLE-CELL GRAVITROPISM AND GRAVITAXIS 149
Figure 7.4. Models of graviperception in different protists. Ca2+- and K+-mechanoreceptor channels are lo-
cated in specific areas of the plasma membrane (ant = anterior cell pole). Receptor channels are activated by
the mechanical gravity-induced load of the protoplast (direction of gravity indicated by arrows). In Loxodes,
specialized gravireceptor organelles (the Müller organelles) complement gravity sensing by using a statolith-
based perception mechanism. Activation of the receptor channels affects the membrane potential and regu-
lates the activity of motion organelles: cilia in Parameciumand Loxodes(not shown) and a single flagellum
in Euglena.
Modified from Hemmersbach and Bräucker (2002).