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.