The Colonial Algae.

The term colonial as used here applies to algae in which cells resembling free-swimming unicells form groups which may be large and elaborately interconnected as in Volvox, or smaller and relatively simple as in Synura.
The filamentous algae, not normally thought of as colonial, are included in a later section of their own.

  Synura, Scenedesmus.

  Volvox.

A typical Volvox colony is a hollow sphere of mucilage having 500 or more biflagellate algal cells fairly equally spaced around its outer surface. The combined beating of their flagellae causes the colony to glide slowly through the water, rolling as it goes. Large colonies can reach a diameter of a millimetre or so, and are easily visible to the naked eye. They are famously one of the great sights that can be seen with a microscope, especially in darkfield illumination.

They reproduce asexually by a process involving a group of surface cells which form a plate on the inside of the parent sphere. The plate becomes concave and eventually forms a sphere attatched internally to the parent sphere. Before it becomes a free-swimming daughter colony, it turns completely inside-out, so that the flagellae of the individual cells are now on the outside of the newly formed colony.
Eventually, the mature parent colony ruptures, allowing the daughters, many of which will have third generation colonies developing within them, to escape.

Sexual reproduction also occurs, and will be added to these notes in a later update. In the meantime, check this diagram of Volvox aureus, which illustrates the sexual stages.

	Picture captions anticlockwise from upper left. All pictures in this sequence are at a magnification of about x60. A mature Volvox colony containing eight daughter colonies. A rupture has occurred in the wall of the parent colony, and the daughters begin to make their way out. The picture may look explosive, but the process takes place over a period of minutes. Volvox don't go anywhere in a hurry. Parent Volvox colony and three daughter colonies. Parent Volvox colony and single daughter colony. The ruptured parent colony will sink to the bottom of the pond, having lost any power of coordinated motion, but the individual flagellate cells will continue to live until they meet their end as a meal for a passing worm or snail.