You’ll need the following items to complete this lab session. (The standard kit for this book, available from www.thehomescientist.com, includes the items listed in the first group.)

Biologists study the interactions of species in ecosystems to learn more about each species than they could learn by examining that species in isolation. In many cases, there is little or no direct interaction between species. For example, wolves and grasses neither harm nor benefit each other other than incidentally. (A wolf may damage a grass population by urinating on it; conversely, wolf feces may fertilize another grass population.)

When species do interact directly, one species may benefit to the detriment of the other, a state called parasitism. For example, tapeworms may infect a wolf. The tapeworms benefit; the wolf is harmed. In another type of direct interaction, called commensalism, one species benefits without benefit or harm to another species. For example, squirrels build nests in oak trees. The squirrels benefit by having a secure haven from predators while the oak trees neither benefit nor are harmed by the interaction.

The final type of direct interaction between species is called mutualism, in which each species benefits from the presence of the other. For example, woodpeckers eat insects that infest the bark of trees. The woodpecker benefits from the tree providing a ready source of food, while the tree benefits by the woodpecker keeping down the population of insects that might otherwise damage the tree.

Note that while one type of interaction between species may be commensalist, another type of interaction between those same species may be mutualist. Returning to the oak trees and squirrels provides such an example. The oak trees provide acorns as a food source for the squirrels, benefiting the squirrels. In turn, the squirrels bury acorns over a wide range. Some of those acorns are forgotten and subsequently germinate, benefiting the oak tree. Overall, then, the relationship between oak trees and squirrels is mutualist.

Also note that when more than two species are involved, the type of interaction depends on the species-pairing point of view. From an insect-tree POV, the relationship is parasitic; the insect benefits to the detriment of the tree. From a bird-insect POV, the relationship is also parasitic; the bird benefits to the detriment of the insect. From a bird-tree POV, the relationship is mutualist; both benefit at no detriment to either species.

In this lab session, we’ll examine a mutualist relationship between a plant, the bush lima bean, and a bacterium called rhizobia. Legumes, including the bush lima plant, are commonly infected by rhizobia, which form colonies as nodules in the root structure of the plant. The bacteria benefit by having a nice cozy nodular home amongst the plant’s roots. These nodules do no harm to the plant, and in fact benefit it greatly.

To understand why, it’s important to understand that plant growth requires six macronutrients (arbitrarily defined as those that typically make up 1% or more of the dry weight of the plant) and 10 micronutrients, some of which are present only at sub ppm (parts per million) levels. All 10 of the micronutrients—boron (B), calcium (Ca), chlorine (Cl), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), molybdenum (Mo), sulfur (S), and zinc (Zn)—must be obtained from the soil. Three of the macronutrients—carbon (C), oxygen (O), and hydrogen (H)—are obtained from carbon dioxide gas and water. Two of the remaining three macronutrients—phosphorus (P) and potassium (K)—must be obtained from the soil.

The final macronutrient, nitrogen (N), may be obtained from the soil or from atmospheric nitrogen with the aid of nitrogen-fixing bacteria, such as rhizobia. From a plant’s point of view, nitrogen is both abundant and rare. Nitrogen is abundant in the sense that our atmosphere is about 79% nitrogen gas. Unfortunately, plants cannot use nitrogen gas directly but must obtain it in the form of soluble nitrates in the soil, which may be present in inadequate amounts to sustain plant growth. That’s where nitrogen-fixing bacteria come in. They convert atmospheric nitrogen gas into soluble nitrates, making the nitrogen available to plants.

In this lab session we’ll investigate the growth of the common lima bean, Phaseolus lunatus, under controlled conditions. We’ll grow two plants in vermiculite, which is a sterile, nutrient-free type of soil that provides only support for the plant. We will provide each plant with nutrients in the form of a fertilizer that contains all of the nutrients named above with the exception of nitrogen. We’ll inoculate one of the seeds with rhizobia before planting it, and plant a second seed as a control without rhizobia, and compare the growth of the plants with and without nitrogen-fixing bacteria present.

Q1: Why did we use vermiculite rather than potting soil or ordinary soil from a lawn or garden?

Q2: In the absence of any information about the bacteria, what three hypotheses might you make about the effect of the bacteria on plant growth? Which of these hypotheses is more or less likely? Why?

Q3: What differences did you expect in the two cups? Why? Did your observations confirm or falsify your predictions? Could rhizobia infection of the magic beans account for the extreme size and rapidity of growth of Jack’s beanstalk?

Q4: What are the controlled, independent, and dependent variables in this experiment?

Q5: We used height and mass as a proxy in this experiment, on the reasonable assumption that they would fairly represent the actual question, which is more difficult to answer. What is that question, and why is it more difficult to answer?

Q6: Is the interaction between the plant and bacteria commensalist or mutualist? Explain.