How many of us have not enjoyed eating a mango? Making a tiny hole at the top (near the stalk), biting it in a way that you get the juice first and then the pulp, and soaking the fingers until they’re sticky and orange in colour…that is cavemen style for you. Some people, who are more conscious of their clothes and manners, would gently cut a mango into two sweet fleshy side-pieces and a sour seedy core. They would arrange the cut-pieces of their fruit on a shiny plate, sit down at a table and enjoy them one tiny piece at a time using a fork. They might even throw away the seedy bit into a bin without even seeing what it is. I am afraid, that is not a good idea. Scraping the soury-bits attached to the mango seed by your teeth and tongue is a lasting and rewarding experience. I like the cavemen style of eating a mango because if you are following that style, it is likely that you are in a cave (Oh!) or perhaps standing next to a bathroom-sink (Oh no) or better, walking in a forest somewhere, where you are likely to throw the seed away into the wilderness. That seed has all the chances of germinating and becoming a new tree. In that way, the fruit would have served its purpose and of course you would be satisfied too for having eaten a wonderful fruit. Forgive me. I forgot to add that if I were alone eating a mango in a forest, there is also a good chance that a tiger or a cheetah would see a mango in me and having heard what they are capable of, they are unlikely to leave any ‘seeds’ for the future. Alas! bones cannot clone themselves. We will ignore the meat-eaters for the moment. We are interested in the large herbivores though.
That was a long-winded introduction for a subject which might soon go somewhere else. Never mind. We will give it a try. A mango or an avocado as you know has one large seed in it. There are many fruits in the plant kingdom that are very large, fleshy, sweet and single seeded. Sounds yummy. Yep! they are yummy because they are successfully dispersed (for the sake of the tree’s future generation) by big animals such as ourselves and other mammals. I don’t think we humans ever tried swallowing seeds of avocado or mango. We would certainly choke to death. But an elephant or a bear would be more than happy to eat the whole fruit as a single serving and so survived the trees bearing fleshy fruits with single large indigestible seeds.
Not all plants follow the large single seed strategy. In plants that do not produce fleshy fruits, seeds are often produced in thousands and are very tiny (look at orchids for example). They do not depend on living dispersal agents. The seeds get smaller as their numbers become large. There is a clear trade-off. Similarly, there are many fleshy fruits that have smaller and more number of seeds per fruit. Tomato and kiwi fruit come to mind. The smaller seeds mean that small animals and birds can carry them (in their stomach) to greater distances. Plants need resources (which are limited) to invest in fleshy coats around their seeds. If a fruit is too seedy, the herbivore has to find more fruits and spend more energy. Therefore there must be enough of fleshy part in a fruit to keep the dispersal agents interested.
The thing I want us to focus on is not just the number of seeds in a fruit but also the way the seeds are arranged within the fleshy space surrounding them. A single seed of a mango or an avocado does not have a space crisis within the fleshy coat. But, when we look at multiseeded fleshy fruits, the seeds appear to be arranged in an orderly fashion. Often the seeds stay together either in single or multiple compartments (placentation) depending on the structure of the flower (mainly ovaries) and the fleshy coat covers them as a single entity. No doubt that the symmetry in seed arrangement is a result of the way in which a large group of flowers (inflorescence) would be arranged. If we start to think that orderliness is a feature of multiplicity (for classical objects), a large multiseeded fleshy fruit that grows on a climbing cactus (Hylocereus) makes us think otherwise. The dragon fruit as it is called is a tropical fruit of South American origin. It is a large fleshy fruit (a berry) with hundreds of seeds. In both tomato and kiwi fruit, the seeds are arranged in some sort of symmetric orderliness. In dragon fruit, the seeds are arranged in a random fashion. I have been thinking about the dragon fruit for many weeks and I sometimes feel that the fleshy fruits represent possible geometries of our infinite universe in a finite way. The dragon fruit seems to have settled into a geometry that reflects our own universe. If each seed is a galaxy, the dragon fruit is their finite universe and the galaxies seem to have dispersed unevenly in their space. At least in this case, we know that the fabric of fleshy space cannot expand any further.