This past week, it was hard to miss evidence of gravity at the Arnold Arboretum while standing by one of our osage orange trees (Maclura pomifera, 471-36*B). The delightful scene, looking a bit like a sea of abandoned tennis balls, can be found in the Centre Street Beds across from the Faulkner Hospital. Osage orange is dioecious so individual plants either produce pollen (male function) or fruits (female function). The very cerebral-looking “fruits” of osage orange are actually not single fruits, but rather are tightly packed multiple fruits that have developed from hundreds of individual flowers in tight clusters.
Of course, gravity is just part of the equation that leads fruits to fall from trees. Indeed, long before a fruit is destined to fall, in fact all the way back in time at a point when fruits are first being initiated from flowers, “abscission zones” with specific modifications of cell types and cell wall chemistry are laid down at a specific point in the axis of a fruit pedicel. While the gravitational pull between Earth and an osage orange multiple fruit remains constant, when the time comes to let go, a cocktail of enzymes helps weaken the abscission zone and the result is a lot of multiple fruits on the ground.
Maclura pomifera is a member of the Moraceae or mulberry family. While this family of flowering plants is largely tropical and subtropical, we do have a few temperate members of this family, all of which have wonderful fruits to observe. It is probably too late this year (I need to check), but next year, keep an eye out for the weirdly orange (multiple) fruits of Cudrania cuspidata (photo here).
-Ned Friedman, Director of the Arnold Arboretum