Chlorophyceae, commonly known as green algae, exhibit an astonishing range of thallus organization, illustrating a diverse evolutionary journey from simple to complex forms.

The simplest forms are unicellular, which can be motile with flagella, like Chlamydomonas, or non-motile, such as Chlorella. These tiny, independent cells perform all life functions.

A step up in complexity are colonial forms, where cells aggregate but often retain their individuality. These can be:

• Motile colonies: Volvox is a classic example, forming a spherical colony of thousands of flagellated cells, often with some division of labor.
• Non-motile colonies: Pediastrum forms flat, star-shaped colonies, and Hydrodictyon (water net) forms large, reticulate colonies.

Further complexity leads to filamentous forms, where cells are arranged end-to-end to form threads. These can be:

• Unbranched filaments: Spirogyra (known for its spiral chloroplasts) and Ulothrix are common examples.
• Branched filaments: Cladophora and Chaetophora show true branching patterns.

A distinct and often spectacular organization is siphonaceous or coenocytic. Here, the thallus consists of a single, large, multinucleate cell without septa, forming complex shapes. Codium (dead man's fingers) exemplifies this, forming a large, lobed, or branched body that is essentially one continuous cytoplasm.

Finally, some Chlorophyceae achieve a parenchymatous organization, resembling true plant tissues. Ulva (sea lettuce) is a prime example, forming a flat, sheet-like thallus that is two cells thick, demonstrating genuine multicellularity with cell division in multiple planes.

This spectrum of thallus organization highlights the evolutionary plasticity within Chlorophyceae, reflecting various adaptations to diverse aquatic environments.