In search of allelopathy: an eco-historical view of the investigation of chemical inhibition in California coastal sage scrub and chamise chaparral

Journal of the Torrey Botanical Society, Oct-Dec 2004 by Halsey, Richard W

To produce evidence for his interpretation, Muller (1953) expanded Gray and Bonner's laboratory studies. In addition to watering tomato seeds with Encelia extract, he included a companion experiment with an extract from the other plant Went studied, Ambrosia dumosa (burroweed). His results confirmed the toxicity of Encelia, but revealed an interesting paradox. The extracted chemicals from Ambrosia, the plant harboring large numbers of desert annuals, were more injurious to seedlings than the compounds from Encelia. How could one species producing extremely toxic substances support "a miniature flower garden" beneath its canopy whereas another, less harmful plant successfully inhibit all undergrowth?

Muller proposed a simple explanation for the inconsistency between lab observations and what was actually occurring in the field. Annuals failed to grow under Encelia because of poor soil conditions, not due to chemical toxins. On the other hand, Ambrosia encouraged annuals because of humus-rich soil that was produced from the accumulation of leaves dropping from the plant's low hanging branches. Muller also suggested that toxic chemicals leaching into the ground were irrelevant since soil microorganisms likely destroyed them before having an impact on germination. Gray and Bonner mentioned the denaturing role of microbes in rich soils as well, but considered it unimportant since Encelia only grows in sandy, well-drained locations. "Since shrub-dependent herbs will not grow in the open except upon decaying remains of former shrubs," Muller wrote, "there is no reason to expect them to grow upon soil covered by, but little changed by, Encelia."

In concluding his discussion of results, Muller wrote an eloquent statement reflecting the difficulties in studying plant ecology.

The natural habitat, even in a relatively simple community of the desert, is far too intricate a system of influences and factors, physical and biological, to hope that there may be found a single factor controlling the complicated life of a perennial species. An explanation, when it is arrived at, will be at least as intricate as the situation it seeks to describe.

Muller's paper was dramatically different from Gray and Bonner's in both objective and tone. While Gray and Bonner were following up Went's investigations and narrowly focusing on one species, Muller widened the inquiry in order to disprove a hypothesis. In doing so, he contributed an important component of the scientific process: clarifying the reasons for an observed phenomenon by pointing out other variables effecting experimental results. With the systematic precision of a well-prepared district attorney, Muller presented his case and supported his hypothesis; soil conditions rather than plant toxins determined the distribution of annuals. However, his prosecutorial style was not helpful in convincing others of his viewpoint. By implying Gray and Bonner were overly simplistic and failed to understand the true complexities of the natural world, Muller diverted attention away from the scientific merits of his paper and toward personalities. It revealed an approach he would use many times in the future when responding to those with whom he disagreed, subtly characterizing their viewpoints as less than enlightened.