To understand how kelp became members of the Protista, one must delve into the convoluted and messy history of systematics. Early attempts at organizing life into a classification system usually separated all organisms as either plants or animals. According to this grouping, kelp and other algae were considered to be very primitive plant forms, along with fungi, molds, and many other types of organisms. However, as taxonomists understanding of these various organisms extended beyond their outward morphology and into their physiology, reproduction, behavior, histology, cellular construction, etc., it quickly became apparent that many of these plants differed in striking, fundamental ways. For example, while a lot of the organisms were clearly photosynthetic and autotrophic, some were obviously heterotrophic. The advent of the microscope opened up a whole new world of ambiguity, revealing countless single-celled organisms that often defied clear classification: many cells were observed that were certainly photosynthetic, but which moved around like animals.
In response to these advances in understanding, many taxonomists began pushing for a third grouping, which usually consisted of all single-celled organisms, but was frequently proposed to include all manner of ambiguous multi-cellular organisms as well. Various names were proposed for this third group, with Protista being one among many in these early schemes. In short, the Protista was proposed to represent a hodgepodge of anything that wasnt clearly a plant or an animal.
Given their clearly photosynthetic nature and often leafy forms, it is somewhat surprising that most of the algae were actually among the earliest organisms to be removed from the Plantae, along with the single-celled organisms, the fungi, and the molds. The various aquatic plants had always been viewed as quite inferior and primitive compared to the higher plants. While green algae were considered a probable direct ancestor of these higher green plants, and therefore sometimes kept in with the Plantae, the brown and red algae were very early viewed as something different, and classified accordingly.
As biological understanding and classification systems became more advanced, additional kingdoms eventually emerged, and lower taxonomic groupings became tighter and more defined. The last few decades have seen a new flurry of classification debate, as a whole new field of molecular investigation has produced new perspectives that have threatened (and in some cases entirely revolutionized) longstanding views of the reigning systematic schemes. Through all of this, though, the algae have remained among the protists.
There is actually good reason for this. One of the defining characteristics of the Protista is that, unlike animals or plants, its members do not contain more than one clearly differentiated functional tissues. Kelp, for all their outward complexity and internal structure, are not considered to possess more than one clearly defined tissue type. This being the case, they cannot be considered plants, and for this and other reasons they clearly arent animals or fungi either.
This leads to what is perhaps a more cynical, but frequently argued, view (or even complaint): Protists are a group of organisms that are defined not as much by commonality but as by exclusion: a protist is simply something that is not an animal, or a plant, or a fungus, or a prokaryote. So, much as it was in its first conception, the kingdom Protista in many regards may still represent the scrap pile of taxonomy-- it is where all of the misfits are thrown.
Of course, that can change.
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