Algae comes in many varieties, so many that most might not even be aware of whether they are looking at algae or not. Algae can be seen floating on the surface of a stagnant lake. It can be found riding the currents of the ocean. It can even be found anchored to the rocks near the shore. But even though algae looks like a plant, it is not a plant at all. The differences between algae and traditional plants are so immense that algae is under a different class entirely. Plants are categorized under the kingdom plantae while all forms of algae are categorized under the kingdom protista.
The largest difference in algae and other plants is the cellular structure of the two organisms. Algae can be unicellular, multi-cellular or even colonial. Plants, as one normally thinks of them, are only multi-cellular. Even if you think you are looking at a single multi-celled organism, the algae is a colony of single celled organisms that work together as a group. Each cell within the group is entirely self sufficient, while plant cells are specialized in such a way that the plant cannot survive without all cells working in unison. The cellular structure of plants also functions differently than it does in algae. With normal plants, the cells have vascular systems that allow for the dispersing of nutrients to the entire plant. Algae, on the other hand, does not possess such a vascular system, and each cell has to absorb its own nutrients independently of the other cells. Algae cells are also multipurpose since each cell is responsible for all actions that maintain life; plant cells have one purpose and cannot exist by themselves.
Another big difference between algae and plants is where they live. Plants are almost always rooted to the ground and are not mobile unless another organism (like a human) moves them. Although some kinds of algae have holdfasts, which are similar to roots, most are free moving and inhabit the water. Some algae is free-floating and moves along with the current, but other types of algae have tiny hair-like filaments on their surface to allow them to propel themselves, which is not a feature that plants possess.
Reproduction is another area where plants and algae could not be more different. Plants reproduce in many ways but it is a complex process. Often the reproduction process requires the assistance of the wind, birds, bees, insects or other small animals. Some kinds of plants have male and female parts and can pollinate themselves, but generally assistance is necessary. Once pollination takes place, a seed develops and then drops from the parent plant to take root and become a new plant. Algae can reproduce in several different ways, but they are all self-contained. Most algae will release spores into the environment that will become new organisms. Other types of algae can replicate their own cells for the purpose of reproduction or to recreate an organism after injury.
Algae and plants may look deceptively similar, but once one starts to look at the small details they are really quite different organisms. The next time you are walking along a pond or at the beach and see something green, take a moment and consider whether you are looking at algae or not.