Saturday, October 1, 2011

Rubber - Fruit and Seed

Rubber Tree (Hevea brasiliensis)
We know a lot about rubber tree, and I have seen many rubber seeds and pods strewn along the trails in Bukit Timah.  Not only are the evidence everywhere, but occasionally, one can hear the noise of "explosion" of the seed pods, before you hear the soft thud on the ground as the seeds and pods land!

Then I have never give it much thought about rubber seed, and fondly remember
a.   you can tease your friend by rubbing the rubber seed very hard on hard ground, and then applying it to your friend'ss bare skin.
b.   in our primary school, we were introduced to seed dispersion, and one of the method utilises by plants is the "explosive action".  Examples typically given by our teachers were Balsam Plant and Rubber Tree!

After the closure of the Railway Line, I decided to take a walk just like many folks here, and started my journey at Clementi Road.  Walking parallel to a big canal, I came across this fallen tree, with the rubber fruits (that has turned to rubber seed pods) still hanging onto the dead branches.

Then a thought occurred to me, why don't I write about the mechanics of the "explosive action".

Picture 1 - Dried Rubber Seed Pods
Picture 2 - Stages of Fruit to Pod (tri-lobes)
First and foremost, like all fruiting trees, the fruit does have some fleshy inedible flesh covers by green skin. And as it matured, the outer skin started to dry up, and split at certain point, till it turns brownish, and eventually strip away from the Seed Pods. This can be seen from the picture 2 and 3 below.

From picture 2, as the fruit dries, the gap between the lobe increase, until the lobes are being held at two points (at the top and bottom)

Picture 3 - Drying Process causes the skins of the fruits to split.

Picture 4 - Top View of fruit that has split skins
One interesting fact of the Rubber Tree, is that it comes in THREE.  If you observe the leaves, it comes in THREE. Likewise for the fruits, it is Tri-Lobe.
Imagine the fruit hanging on the branch, and the purpose of fruits and seeds are to propagate the species. Therefore, the design of the fruits is to ensure that the seeds that leave the pod, should be away from the parent tree!
With Tri-Lobe, at least 1 lobe will be facing away from the parent tree, and if not 2 lobes.  Thus the probability of propagation is increased.

Therefore, for the explosive action to happen, and to ensure that the seed is flung outward, the design of the Tri-Lobe and the individual seedpod are rather interesting.Now the mechanics of the explosive action, as the fruit and eventually the pod starts to dry, the weakest part of the fruit is actually the seam of a single lobe. However, the 3 lobes are connected, and will stress each other, and the explosion is usually a occurrence of all the 3 lobes, rather than individual lobe from the same pod.
  • Different thickness of the lobe around the seam
  • Connection region between lobes must be thicker than the seam of the lobe
  • Dehydration is the primary factor - which is also the stress inducer.
This drying process (dehydration) will cause the shrinkage of the lobe and is likened to the process of opening a cockle.  However, as each lobe is connected to two other lobes, and in that sense will be held in its place.
Once the stress level across the seam reaches the critical point, the shear stress or splitting pressure moves from the seams of the lobes to the TWO connecting points between lobes.

At the critical moment, and very fast, the final split occurred almost simultaneously in all the six connecting joints, and at the same time the seam of the lobe split open, and by virtue of the seed being connected to the inner wall, the seed is flung outward, awayfrom the centre of the tri-lobe.

Picture 5 - Individual Pod with Seed

Picture 6 - Purposely saw the Seed Pod to separate them.  The Blue marking from the saw, also indicates the connecting parts of the  lobes.  There are 2 connecting points between lobes.

Picture 7 - Notice the whitish colour seed pod, the bigger lobe has extra seam!
To summarise the explosive action,
  1. Different thickness of the lobe.  It is thicker at the side, and thinner at the top and bottom.  This is to facilitate the initial crack propagation. Cracks will occur at the top or bottom, and slowly creep to the centre.
  2. The connecting region is thicker between lobes than at the seam.  This is to ensure the the "in between lobe" fails at the last point.  Otherwise it will be shooting out individual lobe, instead of seed.
  3. All these "tearing" or "shear stress" is the result of dehydration, by design, water will not be fed to the fruit, but slowly dried under the sun.
Another observation, immediately after the lobe has opened and dropped to the ground, both halves can be easily put back (just like cockle shell), but through time, it will continue to deform due to dehydration.

Finally, this is my observation from my collections of fruits, pods, seeds, and experimenting with them, as it is very difficult to catch the explosive actions on film.  And if there is any mistake or correction, kindly feedback to me.

Update 22nd July 2014
Today I chance upon a "freak" rubber fruit.  One that has 4 pods instead of normal 3s.
 I would assume the explosive action will not be as effective as 3, for the angle of throw is now greatly reduced!