Creativity in Hypothesis Generation: A Story about Leaves

A hypothesis needs to be testable. It does not need to be right. Almost any statement you could make, if testable, is a hypothesis. Silly hypotheses, which are very likely to be wrong, are a waste of time, but that is the only thing wrong with them.

Sometimes scientists focus on their own narrow research so much that they cannot dig themselves out of it unless they have a great deal of creativity. There are many examples of creative scientists. Today I give you Tom Givnish, now at the University of Wisconsin.

Some plants have mottled leaves. That is, their chlorophyll is patchily distributed in the leaf, rather than being a solid shade of green. An explanation for this is not possible within just the realm of photosynthetic physiology. You have probably seen a lot of mottled leaves. They may have patches or lines of white rather than being a solid green. Plant breeders cherish these plants, because they are decorative and people like to plant them in their yards. But occasionally you will see a stem with solid green leaves emerge on a plant that “is supposed to be” mottled. These are the stems that the horticulturalist cuts off and throws away.

But why should a plant out in nature have mottled leaves? Givnish came up with a creative answer. We cannot know if it is true unless we rerun the clock of evolution and watch mottled leaves evolve, or prevent them from evolving. But we can analyze their patterns and suggest believable answers. This is what Givnish did in an article in Functional Ecology in 1990.

He noticed that leaves that were evergreen in an otherwise deciduous environment are more likely to be mottled than other leaves. In particular, plants with mottled leaves are frequently evergreen plants near the ground in deciduous forests. Givnish used examples from North America, but I saw an example in a forest in Alsace, France: the Italian arum Arum italicum.

One thing that is strikingly obvious about evergreen herbs in a deciduous forest is that it is almost impossible for them to hide. They would therefore be an almost irresistible target for any herbivores in the forest (that had not migrated or hibernated). Givnish suggests that the plants, by producing mottled leaves, are hiding. It is camouflage.

It might seem easier for the leaves to just die back to the ground, the way most of the perennial herbs of the forest floor do. But there is an advantage to the plant to have leaves that are active in the winter and early spring. On the rare (and increasingly less rare) days that are warm in the winter or early spring, these plants can carry out a lot of photosynthesis in the bright sunlight that is not yet blocked by tree leaves that may not open for weeks or even months. In spring ephemerals, almost their entire year’s worth of food production is in the early spring before the tree leaves expand. How much more so with an evergreen plant on the forest floor.

Leaf mottling, by breaking up the line of vision that might make the leaves obvious, might make poor-sighted herbivores less likely to eat them. This is the same principle on which the camouflage of soldiers and hunters is based.