Why aren't algae in the plant kingdom?

Question

I know they are in the kingdom Protista, but why?

Answer 1

Recently scientists declared Plants are only terrestrial (water lillies only use water for support they are not aquatic). Since algae are aquatic they are thrown into the protista kingdom, which is basically a kingdom of animals that don't fit anywhere else.

Also some algae have different chlorophyll/cell wall compositions as plants.

Answer 2

Algae do not develop from a multicellular embryo into a more complex form like plants. They retain a very simple  form when mature even if multicellular. Algae develop no specialized tissues that support or transport fluids. Algae have no roots stems or leaves. The cell wall chemistry contains different components like agar or alginic acids mixed with cellulose and the the form of food reserves in cells differ from that of plants.

Algal plastids do not differentiate into many types as plant plastids do. Algal plastids contain pyrenoids to fix CO2 into starch. These are specialized membrane bound protein functions within the algae's plastids. Plant plastids have diverged into specialized functions such as amyloplasts to store starch.

Plants can be aquatic like the  flowering plant Wolffia borealis and show all the specialized development of a plant. It has flowers and emergent floating leaves. Aquatic plants can obtain nutrients from both their substrate and the water they live in. Plants need more nutrients and carbon than the algae and have the means to acquire them. Aquatic plants have small or fine roots since they merely anchor and transport nutrients and CO2 gases with no support needed to hold the plant up.  Leaf stomata remain open so are not functional. Only flowers break the surface in some aquatic plants.

Answer 3

most Algae grow in water, where as plants grow above ground.

Algae are also missing the basic parts that most plants have.