Plants as Parasites: Connecting to the Host with the Most

I saw a bright-red cluster of what I thought were mushrooms on the forest floor when I was in the Kalakad Mundanthurai Tiger Reserve in Tamil Nadu. One often encounters colourful mushrooms in the forest; they are the fruiting bodies of the fungi which lie buried in the rich, organic decaying leaf matter.

Plants on the other hand are almost always associated with the colour green. Plants have chlorophyll, which is how they convert sunlight into energy — this is what is drilled into us from our schooldays. However, in nature, exceptions are the rule. I came to terms with this fact when back at the field station, my colleagues informed me that the weird thing I had seen was not a mushroom but a rather uncommon plant, Balanophora fungosa. There are currently about 44 known species in the Balanophoraceae family and about 11 species in the genus Balanophora. Members of this genus are found in tropical and temperate parts of Asia, Africa, Madagascar, and Northern Australia. They often look like acorns sticking out of the soil.

The Balanophora fungosa plant I had come across is found in the Western and Eastern Ghats. It is a short herb, growing up to 25 cm tall, though its stalks can spread over a few feet on the forest floor. It is a dioecious plant, i.e. the two sexes grow separately as either male or female plants. The male inflorescence is obovoid (egg-shaped with narrow end at the bottom), measures about 10 cm in length and about 5 cm in diameter. Individual flowers, measuring less than 1 cm in length, are arranged tightly on the stalk, and have a white, powdery pollen. The female inflorescence, however, is globular and measures about 5 cm in diameter. Both are bright red or pale orange and only rarely found growing alongside each other. The leaves look like bracts and lack any veins.

The pleasure of being a parasite

Balanophora was first described in the 1700s. It is a root parasite that has neither chlorophyll nor roots. It gets all its nourishment by setting out specialised structures called “haustoria” which tap into the roots of specific host plants.

Plants use chlorophyll to make energy, using sunlight and other chemicals. But in evergreen forests, where trees tower 100 feet high and hardly any sunlight reaches the ground, a plant’s photosynthetic capability would likely be rendered useless. Perhaps this may explain the evolutionary trajectory of parasitism in this group of plants. There are no clear answers yet, as Balanophora continue to remain understudied.

Parasitism is fairly common among plants and fall into two broad kinds. Holoparasites — plants completely dependent on the host for water and nutrition (like in Balanophora fungosa). And, hemiparasites —plants that are dependent on the host at some stages for nutrition and water, but produce leaves in later life stages (e.g. Mistletoes). In fact, parasitism as an evolutionary strategy has appeared independently nearly 13 times among unrelated plant families. The key innovation that enabled parasitism is the evolution of haustoria, the root-like structures that tap into the root system of specific host trees and draw water and nutrition. No one knows why there is host specificity among such parasitic plants, but it could be due to specific physiological requirements. Currently, there are over 70 host tree species for Balanophora fungosa which include common evergreen trees such as Ficus sp. and Litsea sp. Occasionally, the plant may depend on alien plants like Lantana camara too. Most parasitic plants also have what is called self-incompatibility, a strategy to avoid parasitising themselves. How arrogant of us to think that intelligence is a function of having a brain.

Who pollinates these parasites?
Parasitic plants have a bag of tricks to attract pollinators, ranging from pungent odour to sweet nectar. One of the iconic examples of a plant using smell to attract its pollinators comes from the remote jungles of Southeast Asia. It is a large leathery flower of another parasitic plant called Rafflesia, or corpse flower, because it smells of rotting meat. This tricks the flies who are looking for rotting meat to lay their eggs, and pollen gets deposited on them. When they go to another flower, they perform the act of pollination. The flower wilts within a week, and the cycle continues. This parasitic plant depends on a climber (Tetrastigma sp.) as its host and its large flowers typically measure a few feet across in diameter. Currently there are about 28 species of Rafflesia and they have both male and female flowers on the same plant.

Balanophora fungosa does not have a corpse-like smell, but it may produce mild amounts of nectar which attracts bees, ants, pyralid moths, beetles, and even cockroaches. In India, there are several reports of honeybees (Apis cerana) visiting the flowers. Because the male and female plants are different, the pollinator needs to visit the male and then the female flower to pollinate them. To get them to do this, the female flower likely produces a small amount of nectar. The female flower is small and almost like a spore and the seed is often tiny too. It is suspected that the seeds are dispersed by wind or even by the diverse pollinators which visit the flower. The other possible mechanism of dispersal may be rainfall. Having come back to the city, I scoured the internet for more information about the plant. Information about this fascinating plant is frustratingly sparse. I was back in the field a few weeks later when I encountered the plant again. This time, I had developed a fondness for it. Without even trying to take a photograph, I just sat next to it, lost in thought: What were the odds of the flower being pollinated, the seed getting dispersed, it germinating near a host plant, and surviving to repeat the cycle all over again in the vast expanse of the forest?