Why isn't a large piece of kelp considered a plant?

Answer 1

Kelp belongs to the kingdom Chromista. Plants are photosynthetic and store energy in the form of starch. Kelp are also photosynthetic, but produce sugars and not starch. Also, kelp and other chromists have "chlorophyll c" which gives it he characteristic brownish tint. This pigment and others that chromists contain are not found in any "plant."

Chromista

The kingdom Protista has been divided into Chromista and "other protists" The chromista have closer evolutionary ties to plants, animals, and fungi than they do to other protists. They are eukaryotic and autotrophic. This group includes red/brown algae (kelp) and diatoms. The multicellular organisms in Chromista are thought to have evolved their multicellularity independently from other kingdoms. Chromo means color, and although some chromists are colorless, they are characterized by a yellowish/brownish color coming from their version of chlorophyll.

Anatomy of Kelp

The features of kelp look like plant features. Where a plant would have roots, kelp has a "holdfast". The purpose of the holdfast is to attach the kelp to the sea floor. It does not obtain nutrients in the way that plant roots do. The "stipe" or stalk of the kelp is analogous to the stem of plants. The blades of kelp are analogous to leaves. In some kelp, at the base of the kelp blades, there are structures called gas bladders that allow the kelp to float upright.