The Secret Life Of Mistletoe (When It’s Not Christmas)

IRA FLATOW: Next up, this time of the year, it’s not uncommon to see little sprigs of greenery hanging in someone’s doorway. It’s probably mistletoe, the holiday decoration and inspires paramours standing beneath it to kiss. But as it turns out, our notion of indoors during the Christmas season is a lot, well, cheerier than the reality of mistletoe in the wild. You see, the plant that prompts that kiss is actually a parasite here to tell us more about this plant and what it’s up to in other 11 months of the year is my next guest, Josh Der, assistant professor of biological sciences Cal State of Fullerton. Welcome to Science Friday.

JOSH DER: Thanks for having me.

IRA FLATOW: Now I consider myself a bit of a plant geek, but I was even surprised to hear about parasitic plants. I didn’t know there was such a thing.

JOSH DER: Yeah, parasitic plants are fascinating.

IRA FLATOW: Tell us about what makes this plant a parasitic plant. What makes mistletoe a parasite?

JOSH DER: Well, so missletoes attach onto the branches of other shrubs and trees, and they steal mostly water, but some also steal nutrients and sugars, and they rely on their hosts in order to complete their lifecycle. They’ve got really specialized, modified roots in order to help them attach and get into the host’s vascular tissue, and they also have specialized dispersal mechanisms to get the seeds to the next tree. You can imagine, it’s hard to get from one tree to another without some help.

IRA FLATOW: You know, you make this sound like it’s a scary plant to be standing under.

JOSH DER: It’s not really. These parasites do steal water and nutrients from their hosts, but they don’t usually damage the trees enough to kill them, unless the infestation is bad. And they’re not going to really hurt you unless you eat them. A lot of them are poisonous.

IRA FLATOW: What other parasite plants might I be familiar with?

JOSH DER: Well, the biggest flower is also a parasitic plant, rafflesia. It’s the corpse flower, and that grows inside of its host. You may also be familiar with dodder, which is a parasitic plant that looks like orange Silly String on shrubs. We also have root parasites like sandalwood. So sandalwood is a parasitic plant, which we eat.

IRA FLATOW: Let me just remind everybody that I’m Ira Flatow. This is Science Friday from WNYC Studios and go to the question that– I love to grow plants. I grow orchids and other kinds of plants, a couple plants. How tough would it be for me to grow my own mistletoe at home?

JOSH DER: It’d be pretty challenging. You’d first need to have a suitable host, and once you’ve got a host, you’ll need to establish an infection, and you can do that. You can put seeds of a mistletoe onto the branches of your tree. But mistletoes, at least the ones we use in our decorations, grow very slowly. They take several years to establish an infection. And then if you’re going to be harvesting it, you’re going to not want to destroy the mistletoe by taking all of it.

IRA FLATOW: Right. Tell us how mistletoe got associated, this plant that– you used the word infection associated with it because it’s a parasite– how it could get involved with being a loving thing if it’s a parasite?

JOSH DER: Yeah, that’s a great question. Mistletoe has a long history in mythology and lore. It’s featured in stories from Norse mythology and Greek mythology, and the Druids revered it as a sacred plant. It traces its history as a Christmas decoration back to pagan rites in pre-Christian Europe, and it was one of the few green things available in the winter. And so people would bring it in as a reminder of spring, and it became associated with fertility. And for that reason, it is also used in that tradition you mentioned at the start of this segment, kissing under the mistletoe.

IRA FLATOW: How did you get started with studying mistletoe? It’s not something I would think, you know– you did your PhD thesis on this, did you not?

JOSH DER: I worked on missletoes for my master’s, but I was interested in missletoes and parasitic plants because of their specialization and how they have this alternative life history of stealing resources, but also being really important ecologically. And so I’ve continued to work on it since getting my PhD, and I studied–

IRA FLATOW: Did you find your favorite? Do you have a favorite one?

JOSH DER: Sure. So I really like dwarf missletoes. They have a very specialized seed dispersal mechanism, where they launched their seeds out of the fruit all on their own, and those seeds then attach into the host, and the mistletoe actually grows for several years inside of the host, much like an alien infesting someone, and when they’re ready to reproduce, they burst out of the stem, and they’ve got these small, tiny flowers, and then they make their fruits and launch those fruits to the next tree.

IRA FLATOW: Launch it pretty far, pretty fast?

JOSH DER: They can launch up to 30 meters or so.

IRA FLATOW: Have you ever seen one bursting out of the tree?

JOSH DER: I have. Sometimes when they’re just right, you can, like, tap them a little bit, and you can get them to launch the seeds.

IRA FLATOW: Wow. No wonder you’re so interested in it. It’s fascinating. I wish you good luck on your career studying mistletoe.

JOSH DER: Thank you.

IRA FLATOW: Josh Durr, assistant professor of biological sciences at Cal State of Fullerton.