This is a work in progress... please bear with me, come back and watch it grow? Send constructive suggestions?
Every alga is a plant. The algae (plural of "alga") group is like a "phylum" (next level subdivision) of the plant kingdom. ("Phylum" is in quotes, because botanists don't use that term, but "algae" is one of the sub-groups the plant kingdom is divided into, so I think it is okay to think of it as a phylum!)
The Flat Earth Academy is not trying to be Yet Another Wikipedia, by the way. I strongly encourage you to go to Wikipedia, graze in the fields of near limitless knowledge. But if you work through what the Flat Earth Academy offers, you will, I hope, feel that you are building a framework of basic knowledge. Without the framework, whatever you can manage to retain from Wikipedia browsing will only ever be just so many scraps of paper with little factoids scribbled on them.
While you don't need to become an "expert" on all the algae, you should be aware that the following are algae. You should have a feeling about their common features. Use the Wikipedia articles which will open if new tabs or windows, if you aren't sure of what a groups members are like.
The algae give you an excellent opportunity to think about the Seven Great Challenges of Life. Think about what you know (or learn below!) about algae, and then think about how those characteristics affect how algae reproduce, assimilate, respire, etc.
The phytoplankton are the tiny plants which drift in the waters of the earth. There are billions upon billions of them. They may be tiny, but the total weight of phytoplankton in the seas is greater than the total weight of all the animals in the seas.
If all the whales on the planet suddenly disappeared, and never returned, it would be very sad, and it would make a difference... but they are not nearly as "important" as the phytoplankton.
If all the phytoplankton disappeared, many, many things would starve to death quite quickly. Not long after that, the things that eat the things that eat the phytoplankton would starve. No phytoplankton, (almost) nothing else. A few things eat the larger plants which also grow in the sea. But most things eat phytoplankton, or things that eat phytoplankton.
You've probably never seen it, but do you now see how important it is? And phytoplankton are alga. At times, the sheer abundance of phytoplankton in a body of water can be sufficient to change its color, as in the "red tides" of which you may have heard.
Besides the teeny tiny algae that are at the mercy of the ocean currents (i.e. the phytoplankton we just covered), there are many seaweed algae... and "pond weeds" and "river weeds". Some are simple things, little more than a "green slime" on rocks. Others have small "bubbles" of gas in their tissues to make the "leaves" stand up, to get better access to the light.
The "sequoias" of the seaweeds, the kelps, can grow to as much as 80m.
Algae do not have vascular systems. In other words, no "little tubes" running through their bodies. Nothing like our circulatory (blood) system.
Unlike, say a tree, their bodies have nothing like bones or the chemical that makes a tree trunk stiff. (Lignin).
Some seaweeds grow in beautiful feathery fan, or other attractive shapes.
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