Algae:
where they fit into the 2-Kingdom System
The plant and animal kingdoms have enjoyed a long
tradition in formal taxonomy. Since the beginning of biology,
organisms have been classified as either plants or animals
because we humans have developed the bias that only these two
kingdoms exist in life. We live in a terrestrial, macroscopic
world where we rarely encounter organisms that fail to fit neatly
in the plant-animal dichotomy. Therefore observational and
subjective characters were used for clear-cut classification.
Important microscopic traits were overlooked, and as a result,
weak criteria were used to justify placements of organisms as
either plants or animals.
Algae, like the bacteria and fungi, were often assigned to the plant kingdom. Properties for including the algae into the plant kingdom were their ability to make their own food by photosynthesis, their structural similarity to land plants and the fact that the larger forms were observed to be sedentary. Eukaryotic unicellular organisms with chloroplasts were also called plants. Bacteria fell under the plant banner because of their rigid cell walls that behaved similarly to cellulose-type cell walls of land plants. Although fungi are not photosynthetic and have little in common structurally with plants, their inability to move about supported their classification as plants.
Unicellular creatures that move and
ingest food - protozoa - were called animals. Microbes such as
Euglena that move but are photosynthetic
were claimed by both botanists and zoologists, and showed up in
the taxonomies of both the plant and animal kingdoms.
Clearly, the 2-Kingdom System was riddled with conceptual and operational problems. Technological advances, such as refined microscopy further proved that the system was oversimplified and therefore not widely acceptable.
