Just before the September 23 science lecture at the Smithsonian Tropical Research Center’s Tupper Auditorium, I was warned that the day’s presentation would be one of unusual importance. The title, “Plant Architecture, a Biological Approach,” seemed mundane enough, and the first part of French scientist Francis Halle’s presentation was an overview of a relatively old branch of biology.
“Tree architecture is at the same time extremely easy and very difficult,” Halle explained. “Naked eyes can do it. Sophisticated equipment isn’t needed,” he added. What makes the field, which has existed for about 150 years, so difficult is the constraints. You can’t work in an herbarium, and you have to draw rather than take photographs, because “on the photo you will see the leaves and architecture is hidden by the leaves.”
While most botanists identify trees by their leaves, flowers and fruit, the plant architecture specialist is interested in trunks, branches and twigs instead. A tree species’ architecture is best studied through specimens that are neither young and undeveloped nor aged and decrepit, and uninjured and unpruned. To study a tree’s architecture, one looks at its branching, whether it is branched or unbranched, continuous or rhythmic, whether the branches go vertical or horizontal, and whether or not they are sexually alternating. There are 22 basic tree architectural models, the four most important ones --- Leeuwenberg, Roux, Troll and Rauh --- named after scientists who described them.
One common feature in a lot of tree architecture, and the starting point for Halle’s starting discovery, is reiteration. That’s when there are several copies of one of the architectural models within a single tree, as when a branch looks like a seedling, but growing out of the tree rather than the ground. (Sometimes leaves look like two-dimensional replicas of a seedling, but often they do not, Halle noted.)
“Environment controls reiteration,” Halle explained. Come to think of it, that seems like an obvious expectation, especially when one considers trees that shed their leaves and then in the next season put out new shoots that turn into twigs and branches. When there is reiteration, “the tree becomes a colony.” Now THAT’S a radical discovery.
“We have made a very strange finding,” he said. “Within one crown of one tree, we can find several genomes, so the genome isn’t stable.”
Halle noted that the colonial model, wherein a tree is genetically more like a stand of genetically distinct if related coral polyps than a single organism, is noticeable among dicots (plants in which there are two seed leaves) rather than monocots (plants that put out a single seed leaf). Genetic testing of the different branches of single temperate oak trees and tropical mango trees, Halle said, has shown that different parts of the same trees have distinct genes.
The French researcher, who made many of his discoveries working on the island of Madagascar, is here to study Panamanian trees to see which are colonial in nature and which are not, to discover the causes and effects of colonial structure, and to inquire into the relationship between plant structure and speciation.
Halle noted some “obviously adaptive” aspects of the colonial structure, pointing out, for example, that some trees live long enough to experience various climatic changes in their lifetimes, so that seasonal differences might stop fruiting on one branch but stimulate it on another one that has different characteristics. It might also create the conditions for pollen from flowers on one part of a tree to fertilize those on a genetically distinct part.
Halle also thinks that plant architecture may have a lot to do with which plants become pests and which become victims. Pests, he noted, reiterate a lot and put out a lot of suckers, while victims don’t tend to reiterate much. “The worst pests,” he said, “have an opportunistic and plastic structure.
At Barro Colorado Island, the Smithsonian has built and maintained databases on tropical tree distribution, upon which a lot of theoretical work about speciation has been based. With Halle’s discovery, a lot of that is going to have to be reexamined in a new light.
Also in this section:
Some trees more like coral colonies than individual organisms
Bad sanitation probably played a role in the SARS outbreak
