Stoneworts (Charales)

Stonewort (Chara spp.)

Stoneworts (Charales) is an order of macro-algae, nonvascular plants. These algae colonies are arguably the most complex of macro-algae. The family name is Characeae. This order includes the following genera: Chara, Lamprothamnium, Lycnothamnus, Nitella, Nitellopsis and Tolypella. As a group, these plants are often called Stoneworts because they may become encrusted with calcium carbonate (lime) making them rough and stiffer than other submerged plants. The lime will produce an offensive odor when a plant is crushed. There are over 400 species of Stoneworts throughout the world. The Lamprothamnium and Nitellopsis genera are not likely to be found in Maine. All Stoneworts are freshwater plants and all use chlorophyll for photosynthesis.

The Chara genera is very common in Maine freshwater bodies. The photograph above shows a Chara found in French Hill Pond. The main stem, called the axis, has nodes at which  six to 16 stems called branchlets are attached in a whorl. Bract-like appendages called "stipuloids" extend from the axis below and above or just below the points at which the branchlets attach to the axis. The axis and branchlets are hollow and consist of very long algae cells. These algae cells collectively form the "thallus (plural thalli)," that is, the structure or body of a nonvascular, algae plant.

 

Chara Axial Node

The branchlets have protrusions at the nodes and tip called "bract cells." Bract cells form a whorl around the node. The bract cells on the top or adaxial part of the branchlets are longer than the bracts on the bottom or abaxial part of the branchlets. Similar cells on the main axis (main stem) are called "spine cells." These cells are the part of the plant that most likely become encrusted with lime. The bract cells in the photographs on this page appear brown in contrast to the green of the stems.

Chara Abaxial Bract Cells

Some branchlets may have longer bracts called "bracteoles" on the adaxial side that enclose the "oogonium." The oogonium (plural oogonia) is a female reproductive organ in which an oospore develops. An oospore is an organism that will break away and start a new plant. Charales can be monoecious or dioecious. A monoecious plant hosts both female and male sexual organs; whereas, a dioecious plant has either male or female organs. Monoecious plants can be self-fertilizing because they have both sexual organs. A female dioecious plant must be fertilized by a male dioecious plant. Monoecious plants will have a male sexual organ just below the oogonium called the "globule." The globule contains the antheridial filament that will combine with the oogonium to form the oospore. Dioecious plants have the oogonium and globule on separate plants. The globule on a female Chara plant will be replaced by another bract called a "bractlet." This reproductive system is similar to the reproductive systems of higher order plants but is more primitive.

As demonstrated by the Chara genera, the Charales thalli develop forms and methods of sexual reproduction that are developed more highly in vascular plants. As thalli became exposed to drier environment and the atmosphere, they apparently evolved over a period of time into vascular plants to survive and thrive in the new environment. However, the primitive, nonvascular thalli continued to thrive in the wetter environments.

 

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