Fig. 1.

Thallus of Chara australis and charasomes in internodal cells. a Simplified schematic drawing of a Chara thallus. The apical cell (AC) gives rise to a regular series of cylindrical internodal cells (I) and groups of nodal cells (gray). Nodal cells may divide further and produce side branches (shoots of unlimited growth like that of the main axis; not shown) and branchlets (shoots of limited growth). Nodes incubated in darkness produce upward growing protonemata and downward growing rhizoids. Protonema and rhizoid internodal cells investigated in this study are marked by asterisks. Non-dividing cone-shaped cells are called stipulodes (long arrow) in nodes of the main axis or bract cells (short arrow) in nodes of branchlets. b Light micrograph of an internodal cell incubated in phenol red. Pink color indicates alkaline pH. The arrows point to the intersections of chloroplast-free neutral lines located at the upper and lower surface of the cells. c Cortex of internodal cells containing helical files of stationary chloroplasts (red autofluorescent) and FM1-43-stained charasomes (green fluorescent). Note absence of charasomes at the neutral line (NL). d Side view of charasomes (FM1-43-fluorescent) between chloroplasts (red autofluorescent). e The green fluorescent FM1-43-stained plasma membrane at the cross wall (arrow; optical cross-section) between a nodal (N) and an internodal cell (I) is smooth (compare with (d)). In the internodal cell charasomes are seen at the longitudinal wall and between the chloroplasts (empty spaces; asterisk). Plasmodesmata are out of focus. f Two intertwining basal protonema internodal cells (arrows). g Red fluorescent chloroplasts and green fluorescent, FM1-43-stained punctate charasomes in a basal protonema internodal cell. The arrow points to FM1-43-stained epiphytes. h Proximal internodal cells of rhizoids (arrows) after 2 weeks of exposure to light. i–k Charasomes in a proximal rhizoid internodal cell. CLSM image of FM1-43-stained fluorescent charasomes (i) between starch-containing chloroplasts visualized by DIC in the merged image (j). The electron micrograph (k) is a slightly tangential section showing charasomes (asterisks) along the cell wall (CW) and mitochondria (M). Bars = 1 mm (a, b, f, h), 20 μm (e, i, j), 10 μm (c, d, g), and 500 nm (k)