Structure of Thallus in Algae

Occurrence and Distribution
Structure of Thallus
  Unicells and Unicell Colonial Algae
  Filamentous Algae
  Siphonous Algae
  Parenchymatous and Pseudoparenchymatous Algae

Examples of the distinctive morphological characteristics within different divisions are summarized in Table 1.3.

Unicells and Unicell Colonial Algae
Many algae are solitary cells, unicells with or without flagella, hence motile or non-motile. Nannochloropsis (Heterokontophyta) (Figure 1.1) is an example of a non-motile unicell, while

Ochromonas (Heterokontophyta) (Figure 1.2) is an example of motile unicell. Other algae exist as aggregates of several single cells held together loosely or in a highly organized fashion, the colony. In these types of aggregates, the cell number is indefinite, growth occurs by cell division of its components, there is no division of labor, and each cell can survive on its own. Hydrurus (Heterokontophyta) (Figure 1.3) forms long and bushy non-motile colonies with cells evenly distributed throughout a gelatinous matrix, while Synura (Heterokontophyta) (Figure 1.4) forms freeswimming colonies composed of cells held together by their elongated posterior ends. When the number and arrangement of cells are determined at the time of origin and remain and constant during the life span of the individual colony, colony is termed coenobium. Volvox (Chlorophyta) (Figure 1.5) with its spherical colonies composed of up to 50,000 cells is an example of motile coenobium, and Pediastrum (Chlorophyta) (Figure 1.6) with its flat colonies of cells characterized by spiny protuberances is an example of non-motile coenobium.

Transmission electron micrograph of   Ochromonas sp., motile unicell.
FIGURE 1.1 Transmission electron micrograph of
Nannochloropsis sp., non-motile unicell.
(Bar: 0.5 µm.)
  FIGURE 1.2 Ochromonas sp., motile unicell.
(Bar: 4 µm.)
Non-motile colony of   Free-swimming colony of
FIGURE 1.3 Non-motile colony of
Hydrurus foetidus.
  FIGURE 1.4 Free-swimming colony of
Synura uvella.
Motile coenobium of Volvox   Non-motile coenobium of Pediastrum
FIGURE 1.5 Motile coenobium of Volvox
  FIGURE 1.6 Non-motile coenobium of Pediastrum