How the bean got its twist

Legumes are among the most diverse and successful families of plants in the world. In the Neotropical rainforests, the dominant tree species are mostly legumes (unlike in Southeast Asia, where dipterocarps predominate).

Their defining feature are their eponymous fruits, which appear to have a myriad ways of breaking up to release the seeds within. Some drop to the ground and shatter, like the tubular pods of Cassia:

Broken Cassia fistula pod exposing seeds

... while others twist and contort as they dry out to present seeds to dispersers, like Acacia:

Dried up and twisted Acacia auriculiformis seed pods

Legumes are certainly not the only plants with structures that change their shapes as they desiccate. They're not the only organisms to do so, either (think of mushrooms with caps that curl upwards as they mature to release their spores). However, their ubiquity and convenient size makes them a good subject for study and experimentation.

A team of physicists and mathematicians from Israel have recently figured out the rules behind the seed pod's twist, using a legume Bauhinia variegata as the model (paper in Science behind paywall). The basic idea is that the pod wall is made of an anisotropic material, that is, its material properties are not uniform but depend on the direction that it's being manipulated in. In this case, the anisotropy results from the orientation of fibres in the wall. Expansion or shrinkage tend to happen transversely to the aligned fibres. It's also a composite material, being made from at least two layers of wall sandwiched together. These two layers, however, have their fibres aligned in different orientations. As a result, when they dry out, they want to shrink in different directions. This conflict results in a deformation of the wall that produces a helical pattern.

A rare nut

So, the other day we were at the East Coast Park beach playing with the sand when Josh picks up a "brain", well a hard fruit the size of a lime that look much like a brain. It took me a while before I did a double take and realised, my goodness, he had picked up a rare legume... a legume is the fruit of a member of the bean family.. That family of plants basically contains the peanut, soya beans, rain trees, in essence anything that has a pod, to be more accurate, a legume. the legume is a fruit that has 2 valves that will split along a line and reveal beans, which essentially are seeds. A pod is more layman and can be applied loosely to other fruits.

Anyway, what josh had picked up was a rare native of Singapore called Cynometra ramiflora L. var. ramiflora, its an inhabitant of back mangroves, which basically means, somewhere behind the mangroves where the ground isn't so muddy anymore and resembles an inland forest.

Why rare? When I revised the group for the Flora of Singapore, I listed it as "probably extinct", following a previous status accorded to it by other botanists. After a few years, someone else spotted it along one of the offshore islands and so it became extant in Singapore with at least one or two mature individual. So it was really great that a few more years later, Josh simply picked up the legume from the strand line along the east coast park beach.


Here's Josh presenting the rare legume. He had inadvertently picked it up and brought it to the at first unappreciative local authority on the Caesalpinioid legumes of Singapore.


This is Matt contemplating the size of the fruit. Its hard and corky so definitely buoyant. I am not sure where it came from though but at least it is here.

I looked around the strand line for more of the fruit and found another. How serendipitous.


Here's my botanical illustration of the species, from Loo, A.H.B. & H.T.W. Tan, 1997. The Angiosperm Flora of Singapore Part 6 --Caesalpiniaceae. Gardens' Bulletin, Singapore, 49: 55–106.