Figure 4–1. Coenobial and colonial forms in representatives of four genera of Volvocaceae family: 1 — Gonium pectorale; 2 — Eudorina elegans; 3 — Pandorina morum; 4 — Volvox aureus (From Life of Plants, vol. 3, 1977).
A higher stage of organization is demonstrated by the colonial forms of
Volvox genus. Their colonies are mucous, spherical, with a diameter up to
2 mm. In their peripheral layer there are a lot of chlamydomonada-like cells
joined together through their mucous side walls. The motion of flagella is
also coordinated. In this case a better expressed specialization of the cells
in the colony is observed. Largest is the share of vegetative cells, which do
not participate in reproduction. Among them there are groups of cells
(connected by cytoplasmic fibres) from which new colonies are formed in
asexual way. At the end of vegetation sexual cells (oogonia and antheridia)
appear. As a result of their fusion new initial colonies arise. The cells are
mutually dependent and incapable of independently existence. Breacking
down the colony leads to their death.
Some of the lower multicellular blue-green algae (Cyanophyta) also
are a community of cells, arranged like fibres of different size. The fibres
can be single or branchy (Fig. 4–2). The cells posses pores, channels and
realize cytoplasmic bridges crossing through the transverse cell walls.
Separating the fibres into sections (segments) or individual cells is one
usual mode of vegetative reproduction. In evolutionary respect the thread-
like structure is interesting as a starting point for the formation of
determinate line (fila) of multicellular organisms.