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Conservators Annual Dinner Meeting
May 1, 2013

Ned Friedman:  "What it Means to be a Tree"

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Dr. Ned Friedman, Director of the Arnold Arboretum and Arnold Professor of Organismic and Evolutionary Biology at Harvard University, was the featured speaker at our Annual Meeting and dinner on May 1.

Early in his talk about “The Evolution of Big,” Dr. Friedman asked the audience what it means to be a tree. He showed a slide of palms and asked the audience whether they are trees. The easy answer? No. Why? Palms do not have a cambium, the layer of undifferentiated cells that many see as the defining feature of a tree. The cambium is the site of cell division and growth that allows a tree to increase in girth; it produces the xylem, the tubes that move water and nutrients from the roots to the rest of the tree, and phloem, the tubes that move sap from the leaves down to the rest of the tree.

Dr. Friedman told his listeners that approximately 350 million years ago there were no trees and then in a span of 15 million years (as he said, just the blink of a geological eye) six different groups of plants separately evolved the ability to increase their girth and to qualify as trees. He described the features of those early trees and then revealed that only one of those groups—the “progymnosperms,” which reproduced by spores—survived to become the trees we see today.

That is when Dr. Friedman’s discussion became more complicated. He explained that those progymnosperms gave rise to seed-producing plants, evolved into flowering plants; then one group broke away and redeveloped into herbaceous plants and re-evolved “small.” He gave evidence that most of our flowering plants still have remnants of a cambium. He even revealed that some herbaceous plants have once again re-evolved woodiness. He left us with the idea that it can be hard to define just what the essence of being a tree really is.

Dr. Friedman’s talk was warmly received, and afterwards, he was surrounded by members wanting to ask questions or to talk with him about their ideas. He invited everyone to visit the Arnold Aboretum on the Arborway in Boston, or online at

Note: Visit the Arboretum’s website to sign up to receive notices of their Tree Mobs. What is a Tree Mob? Their web site describes them as “interactions with scientists or other specialists at the Arnold Arboretum [that] provide another pathway to enjoy and learn in the landscape. Experts share little-known facts about our living plant collection, its relevance today, and its importance to future generations. A Tree Mob may attract a small group or a large gathering— we won’t know until it takes place.…Plan to spend approximately 30 minutes learning about an interesting component of our collection.”

--Beth Wilkinson

Dr. Ned Friedman

Photo by Henry Finch


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