From Seed to Seed:
Plant Science for K-8 Educators

Categorizing relationships

Plants that you grow in the classroom might live on their own, isolated in a planter, but in nature-and in our gardens-plants form close relationships with other organisms. These relationships are often complex and many-layered. Slugs and snails, for example, are the nemesis of many gardeners, devouring favorite plants seemingly overnight. Yet these disdained creatures help in the critical task of breaking down organic matter and returning vital nutrients to the soil.

In this section we are going to look at some examples of relationships between plants and other organisms. We'll start "small," with plant-microbe relationships, then move to plant-insect, plant-plant, and, finally, plant-animal relationships. Although it's somewhat artificial to isolate and analyze individual relationships like this, the exercise should help us, in the end, view the garden more holistically-"whole"-istically-by making us more aware of the many individual players and connections.

As usual, let's begin by defining a few terms. You probably know by now that scientists like to categorize and name things! Here are some useful terms and definitions. (It is not important for you or your students to memorize these; it is more important to understand the variety of possible relationships.)

When two (or more) different organisms live in close association, they are said to be living in symbiosis (Gr. syn = together with, bios = life).

If both organisms benefit from the association, the relationship is called mutualism, or mutualistic symbiosis. Mycorrhizae are mutualistic associations between fungi and the roots of almost all plants.

If one organism lives on or in another living organism, from which it derives nutrients, it is called a parasite, and the provider is called the host. Mistletoe is an example of a parasite.

If one plant grows on the body of another plant, but is not parasitic, it is called an epiphyte.


If one organism benefits, and the other remains unaffected, the relationship is called commensalism. The relationship between a rainforest tree and its epiphytes is one example of commensalism.

If an organism fills its nutrient needs from dead and decaying organic matter, it is called a saprophyte.

Another important concept when talking about relationships among organisms is the specificity of the relationship. This refersto how "choosy" an organism is when it comes to forming a relationship. Some relationships aren't very specific at all. Some ivies, for example, will climb up any type of tree-the species of tree doesn't matter. On the other hand, relationships involving very close, cell-to-cell contact tend to be very specific; that is, the two organisms involved must be fully compatible. For example, certain strains of nitrogen-fixing bacteria will form root nodules on specific types of legumes, but not on others.

Now we'll begin our look as some relationships. As you read these isolated accounts, however, keep in mind that the natural world is made up of many complex systems and relationships.