Scientific Discovery

It's time to talk science. I am studying the rare tea mangrove (Pelliciera rhizophorae), and trying to understand what makes it rare. Very little is known about the ecology of this plant, so there are a lot of unanswered questions. Leaves of the tea mangrove have little glands on them, and no one knows what they do. Possible uses for the glands are: salt secretion--some of the other mangrove species secrete salt out of specialized glands, allowing them to live in salty environments that most plants can't tolerate; extrafloral nectaries--I'll get back to this; mite domatia—little holes that mites can live in, and in return clean the leaf surface of fungi and parasites; or nothing--random glands on leaves seem to be characteristic of tropical plants so maybe they don't do anything special. See the photo below; look at the white stuff on the leaf.

I think the glands are extrafloral nectaries. The plants are not secreting salt and don’t live in super salty environments anyway. I have looked for mites on many leaves and they aren’t there. Plus, the holes aren’t really big enough, even for mites. When I look at leaves out in the field nothing seems to be coming out of these glands. However, I am currently growing one specimen as a houseplant, and I was surprised to see white goo coming out of the leaf glands! Hooray! They ARE doing something. But what? I started collecting the white stuff, which actually hardens into a little ball, but it will take awhile before I have enough of it to chemically analyze and figure out what it is made of.

The point of extrafloral nectaries is usually to recruit an animal (such as ants) to take part in a mutualism. The plant provides a nutritious substance such as sugar or protein or lipids, and in turn the ants protect the plant, attacking any other herbivores that approach. A classic example of this is the Bullhorn Acacia, which recruits ants with protein-lipid goodies called Beltian bodies, and the ants take care of any threats to the plant, including herbivores and other plants growing near the Acacia! I often see ants on individual trees of the tea mangrove, and ants are even present on the plants I am growing at the lab in Florida. The reason I am convinced that the glands are extrafloral nectaries is that one day I saw an ant crawl around the plant and stuck its head into each gland on each leaf, like it was looking for something to eat! I have seen other obnoxious herbivores hanging out around the leaf glands, and my hands have also become covered in ants when they provoked a leaf for too long.

I probably don’t see the white goo on leaves in the field because the secretion gets used/eaten quickly. I was able to capture it on my houseplant because there are no insects inside to gobble it up. I don’t know if there is one or multiple species of ant that are usually associated with the tea mangrove in the wild, so I’ll have to make some ant collections next time I travel. For now, I’ll focus on collecting leaf liquid!