VEGETATIVE AND ASEXUAL REPRODUCTION
Binary Fission or Cellular Bisection
It is the simplest form of reproduction; the parent organism divides into two equal parts, each
having the same hereditary information as the parent. In unicellular algae, cell division may be
longitudinal as in
Euglena (Euglenophyta) (Figure 1.16) or transverse. The growth of the population
follows a typical curve consisting of a lag phase, an exponential or log phase, and a stationary
or plateau phase, where increase in density is leveled off. In multicellular algae or in
algal colonies this process eventually leads to the growth of the individual.
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FIGURE 1.16 Cell division in Euglena sp.
(Bar: 10 µm) |
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FIGURE 1.17 Zoospores of Chlamydomonas sp. within the parental cell wall. (Bar: 10 µm) |
Zoospore, Aplanospore, and Autospore
Zoospores are flagellate motile spores that may be produced within a parental vegetative cell as in
Chlamydomonas (Chlorophyta) (Figure 1.17). Aplanospores are aflagellate spores that begin their
development within the parent cell wall before being released; these cells can develop into
zoospores. Autospores are aflagellate daughter cells that will be released from the ruptured wall
of the original parent cell. They are almost perfect replicas of the vegetative cells that produce
them and lack the capacity to develop in zoospores. Examples of autospore forming genera are
Nannochloropsis (Heterokontophyta) and Chlorella (Chlorophyta). Spores may be produced
within ordinary vegetative cells or within specialized cells or structures called sporangia.
Autocolony Formation
In this reproductive mode, when the coenobium/colony enters the reproductive phase, each cell
within the colony can produce a new colony similar to the one to which it belongs. Cell division
no longer produces unicellular individuals but multicellular groups, a sort of embryonic colony
that differs from the parent in cell size but not in cell number. This mode characterizes green
algae such as
Volvox (Chlorophyta) and
Pediastrum (Chlorophyta). In
Volvox division
is restricted to a series of cells which produce a hollow sphere within the parent colony,
and with each mitosis each cell becomes smaller. The new colony everts, its cells form flagella
at their apical poles, which is released by the rupture of the parent sphere. In
Pediastrum the protoplast of some cells of the colony undergoes divisions to form biflagellate
zoospores. These are not liberated but aggregate to form a new colony within the parent
cell wall.
