SOME YEARS AGO, I was walking in Stoke Common (just north of Slough) with my teacher and close friend, the late Professor Robert Harkness. The Common was a wooded area with a variety of trees. Some of them looked very awkward in that their curved or leaning trunks seemed to defy gravity. Yet, the trees did not fall over despite this.

Robert, who was a renowned physiologist, was also a naturalist. Everything natural aroused his interest. As we walked through the woods, he explained that the trees did not topple over because each of them maintained their own centres of gravity as they grew. These centres of gravity must, he considered change constantly during the long lifetimes of the trees. How, he wondered, did the trees grow in such a way that they never became unbalanced and always remained standing?

He never told me the answer. Maybe, he did not know, but ever since that damp grey afternoon with him on Stoke Common, I always look at trees and wonder whether anyone knows the answer to his question. This afternoon, I was walking along the lovely tree-lined path that leads to Kenwood House from its public car park, when I noticed some trees growing on a steep slope lining it. The trees’ roots seemed to be clinging to the slope, hanging on for dear life. Seeing them reminded me of Robert and his wondering about arboreal ‘assessment’ of centres of balance and a fine old friend, who passed away in June 2006.