Why is Algae and Seaweed NOT plants?

I guess for the same reason that we are not plants.

Quote:

Algae is a plant… in the past they were all considered to share a common ancestor ( monophyletic ) so were all grouped in the Plant kingdom… today they are thought to have different ancestors ( polyphyletic ) and are placed in several different divisions within the Plant kingdom while unicellular and colonial algae are placed in seperate kingdom Protista ( these would not be plants since they are not multicellular despite the fact they are photosynthetic ).

Keep in mind there are bacteria capable of photosynthesis but these are all placed in the kingdom Monera which contains the bacteria and “Blue Green Algae” ... due primarily to there cell structure ( they are prokaryotic and not eukaryotic like members of the Plant kingdom… and they are also single celled organisms amongst other differences )

It’s also interesting that Fungi also were once placed in the plant Kingdom but now are placed in the Kingdom Fungi.

Source

wiki
Most algae are no longer classified within the Kingdom Plantae.[4][5] The algae comprise several different groups of organisms that produce energy through photosynthesis, each of which arose independently from separate non-photosynthetic ancestors. Most conspicuous among the algae are the seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the green, red, and brown algae. Each of these algal groups also includes various microscopic and single-celled organisms.
The two groups of green algae are the closest relatives of land plants (embryophytes). The first of these groups is the Charophyta (desmids and stoneworts), from which the embryophytes developed.[6][7][8] The sister group to the combined embryophytes and charophytes is the other group of green algae,Chlorophyta, and this more inclusive group is collectively referred to as the green plants or Viridiplantae. The Kingdom Plantae is often taken to mean this monophyletic grouping. With a few exceptions among the green algae, all such forms have cell walls containing cellulose, have chloroplasts containing chlorophylls a and b, and store food in the form of starch. They undergo closed mitosis without centrioles, and typically have mitochondria with flat cristae.
The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. The same is true of two additional groups of algae: the Rhodophyta (red algae) and Glaucophyta. All three groups together are generally believed to have a common origin, and so are classified together in the taxon Archaeplastida. In contrast, most other algae (e.g. heterokonts, haptophytes, dinoflagellates, and euglenids) have chloroplasts with three or four surrounding membranes. They are not close relatives of the green plants, presumably acquiring chloroplasts separately from ingested or symbiotic green and red algae

Kelp or seaweed and algae lack veinous systems and root systems to bring nutrients from the earth to the top of the ‘plant’ They absorb nutrients directly from the water environment where they are.

Shouldn’t the title be:

Why are algae and seaweed NOT plants?

sorry, bad grammer bothers me

What did Kelsey ever do to you? I don’t think he’s bad at all. He’s very good and has won multiple Emmy awards.

Oh, wait a minute…...

I just realized you weren’t referring to Kelsey Grammer but to the OP’s lack of proper sentence construction—which would be grammar. Sorry for the mix-up. I guess I just take things too literally :)