Re: Why can't protists be classified as plants or animals?

Hi Tee!

If you want a quick answer, go to the last three paragraphs. If you want to learn something, hike up your attention span and read on!

Your confusion may stem from too much admiration for the concept of classification. Classification is just a way that we try to get things organized so that we can understand them better. There are no laws about it. Classification is not a court order.

Classifications change all the time as we get more understanding about what kinds of organisms exist and better explanations for how organisms are related. Generally, we hope that our classifications will ultimately show how all organisms are genetically related. That's the ideal toward which most systematists (biological taxonomists; that is, scientists who do research to classify organisms) work.

Some aspects of classification are purely useful and quite arbitrary about relationships. For example, the category of 'tree.' Trees come in all kinds of plants. Being a tree does not show that plants are more related to one another. It's kind of like being red, or being large.

For example, pine trees, oak trees, and palm trees are all trees, but the are only distantly related to one another. Palm trees are monocots (you might want to look that up) while oak trees are dicots (dictionary handy?). Pines are not even flowering plants, as are both palms and oaks. Pine trees are conifers, cone-bearing gymnosperms (conifers = cone bearers; gymno-sperm = naked seed, as in no flowers).

Sometimes for the sake of identification (as in an identification key) we separate plants into woody plants (trees and shrubs) and herbaceous plants (plants without wood in the stem). In a good identification key there are always two choices at each decision point. These are called 'couplets.' The beginning of an identification key for plants might have couplets like this:

1a. plants woody, forming a tree, shrub, or climbing vine - go to couplet 2
1b. plants herbaceous, climbing, spreading or growing erect - go to couplet 3

2a. plants grow as trees, having a single main stem - go to couplet 4
2b. Plants grow as shrubs, having multiple stems from a common base or as climbing vines - go to couplet 5

3a. plants climbing - see HERBACEOUS VINES
3b. plants spreading or growing erect - go to couplet 6

4a. trees with compound leaves, each leaf consisting of two or more separate leaflets - go to couplet 11
4b. trees with simple leaves, leaves may be deeply incised (cut between veins) but with no leaflets - go to couplet 12

5a. shrubs or woody vines with compound leaves, each leaf consisting of two or more separate leaflets - go to couplet 13
5b. shrubs or woody vines with with simple leaves, leaves may be deeply incised (cut between veins) but with no leaflets - go to couplet 14

The point is that these characteristics are not used here to show relationships among the plants but simply as an easy way to tell one plant from another. Many plant families have members that fit in all the categories above, tress, shrubs, herbaceous plants, plants with compound leaves, and plants with simple leaves.

To test your understanding, think about what you might find at couplet 6. What difference would be highlighted at couplet 13? What distinction do you think couplet 14 would bring out?

Did you notice that it might have been just as easy to start the key with the type of leaf (simple or compound) and then go to the type of growth, tree, shrub, vine, etc.? That's the arbitrary nature of a key. A key is an identification classification. It's not meant to show relationships. It's just the opinion of the key writer about how best to identify particular plants. My personal favorite key would start out with "climbing or not climbing." I would then separate each category into woody and herbaceous.

To finally get to the point, there was a time when the difference between plants and animals was the most important difference, both for relationships and for identification. But now, among the Protista, we find organisms that don't fit neatly into our preconceived ideas about what plants and animals are. Just as if we might find a plant that was part woody and part herbaceous (they do exist), or one that grows sometimes like a tree and sometimes like a shrub (there are many such, and they wreak havoc with keys).

Would it make much sense for two biologists to argue about whether such a plant was woody or herbaceous? A tree or a shrub? No. It is much better to see that they are not either and to understand that.

So it is with the protists. They don't fit neatly into the 'plant - animal' dualistic pattern. Does that mean we should force them to? Should our classifications reflect what we understand about the organisms or should the organisms be forced into our invented systems of classification? That's the question.