By Soham Kacker
On a recent trip to South India, my botanist mind reeled at the turn of every street corner, which seemed to offer up all kinds of tropical treasures. Towering jackfruit trees Artocarpus heterophyllus with their trunks laden with stout fruit; dainty bilimbi trees Averrhoa bilimbi, their canopy of downy, pinnate leaves protecting the fruit ripening along their branches; umbrella-like Ficus canopies teeming with birds and fruit bats; and finally, the exquisite flowers of the cannonball tree Couroupita guianensis filling a quiet corner of a local park with a sweet, heady perfume. My father remarked how odd it was that the bilimbi bore fruit directly from its trunk, and suddenly all the diverse trees I had observed revealed a common feature: cauliflory. I began to wonder how these trees from different continents and botanical families had all evolved this one distinctive feature – and whether it served the same purpose for each species.
The cannonball tree Couroupita guianensis sports exquisite flowers and perfectly spherical fruit directly on the trunk. Photo: Soham Kacker.
A botanical term coming from the Latin words for ‘stem’ and ‘flower’, cauliflory refers to a plant species, which flowers and fruits directly from its trunk or woody branches, as opposed to flowering from tender, apical growth like in most plants. Over a hundred plant species from some 15 different families show cauliflory, and they are almost exclusively restricted to the tropics. This offers the first clue as to why this behaviour evolved. Tropical rainforest biomes are distinctly vertically stratified, which means starkly different animals and insects occupy the canopy, the mid-layers, and the understory of a forest. By flowering from the stem, cauliflorous plants open themselves to interactions with a completely different set of animals, such as insects, reptiles, bats, rodents and primates that can inhabit all, or part of the trunk, and can aid in pollination or seed dispersal. Interestingly, trunk-dwelling animals are not a specialised group, so cauliflory actively encourages generalist interactions – a trend opposite to many hyper-specific tropical species such as in orchid pollination. By promoting generalist behaviour, cauliflory increases the number of species a plant can interact with, effectively increasing its allies in the fight for survival. Therefore, the bilimbi’s dark, musky flowers can be pollinated by flies and gnats in the understory, and Ficus species can recruit ground-dwelling ant species to guard their developing fruit against predators.
There is another trend in cauliflorous species, and that is of almost impractically large and heavy fruit. Take a look at a massive jackfruit, or the weapon-like fruits of the cannonball and calabash Crescentia cujete trees and the pattern becomes apparent. Ecologists have suggested that evolving to be cauliflorous may allow a plant to grow larger fruit, which have direct access to water and nutrients through the trunk and could not otherwise be supported on slender branches. This provides the advantage of being able to recruit larger mammals such as peccaries, tapir, elephants, and monkeys to disperse seeds. It also means investing more resources in fewer offspring, to ensure better chances for their survival. However, it has also been suggested that having large fruits in the first place could have acted as a driver for plants to evolve cauliflory – a theory that reverses the cause and effect of the phenomenon. In either case, cauliflory is a unique adaptation that evolved in response to an extremely challenging environment which became so successful, that it evolved repeatedly across plant lineages – a classic example of convergent evolution. Many ecologically, agriculturally and culturally significant species are cauliflorous: jackfruit, durian, cacao, starfruit… the list goes on. These species underscore the timeless evolutionary processes that have shaped the plants we enjoy, and often take for granted today.
Over a hundred plant species from some 15 different families show cauliflory, including the dainty bilimbi Averrhoa bilimbi. Photo: Soham Kacker.
Sharma, Santosh Kumar.’Cauliflory and the Cannonball Tree.’ Science Reporter, June 2011, pp. 53–55., https://doi.org/https://www.researchgate.net/publication/335137859_Cauliflory_and_Cannonball_Tree. Warren, John M., et al. ‘Reproductive Allocation and Pollinator Distributions in Cauliflorus Trees in Trinidad.’ Journal of Tropical Ecology, Vol. 13, No. 3, 1997, pp. 337–345., https://doi.org/10.1017/s0266467400010543.
Soham Kacker is passionate about plants and has apprenticed at the Auroville Botanical Gardens and the Aravalli Biodiversity Park. Based in New Delhi, he is currently a research student at Ashoka University, focusing on plant ecology and conservation.