: How can I explain my world if the character is technologically not yet capable of understanding it? One feature of my world is a plant that lives in a magmaous (rather than "volcanic") cave.

At a medieval technology level, everything is magic. There was absolutely zero understanding of plant and animal breeding. The role of sex in reproduction was known, of course, but no-one knew when female humans and animals were fertile, nor how a foetus developed, nor the best way to manage birth. People knew that seeds grew into things, but had no idea of how. People thought plants derived all their nutrition from the soil, and had no knowledge at all of photosynthesis, nitrogen fixing, or anything like that.

So there's a plant which grows only in caves. Is it more amazing to a medieval mind than a plant which grows only in forest clearings, or only in full sunlight, or only in a few rare areas of the country, or only in rivers, or only in the sea? Or the mushroom Phallus Impudicus? At most, your medieval guy might meet a monk who knew his Pliny (or whatever your Roman equivalent is) and surprise the monk with a new fact. Otherwise the medieval mind will just accept it as something unusual that exists, without further comment.

IMO, attempting an explanation would be utterly anachronistic and would destroy suspension of disbelief. If you really do want to explain it within the story, as a minimum you need a Renaissance environment with substantial scientific research going on. You simply can't get there with a medieval world.

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Something I really liked about the Hitchhiker Guide to the Galaxy was the way it inserted small introductions to each chapter telling small stories or facts about the book's universe.

The writer than referenced that information to make a joke during the chapter that the reader would not be able to understand if he didn't read about it in the introduction.

You could use the same technique to talk about infrared plants and then let the reader link the dots when you introduce the plant in your world.

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In general people will reach for the terms and concepts they're familiar with to explain phenomenon they aren't familiar with. Your adventurer would likely know that plants need soil, water, sunlight, and warmth (not much grows in winter) to grow. Use this framework to fill in the blanks.

What is a term for infrared light that your character would know and matches what they know plants need? Heat! Since this plant has evolved in an environment that essentially always supplies a constant amount of food (heat/warmth) without fail perhaps this plant doesn't store very much reserve energy. As such your adventurer can notice that as they take the plant away from the heat source it starts to wilt, and when they put it back next to a heat source it recovers. By linking the plant's well-being to heat you convey that IR is an important source of nutrients. While all readers might not be able to connect the dots to IR photosynthesis the explanation is consistent with the truth.

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I think you may be looking at this as a character problem, but it is just as much a world-building problem. The basis of your question makes me think that this type of plant is unusual, but that just makes your problem here much more difficult. Developing your world further is the best way for it to make more sense, both for the readers and the characters.

The easiest thing in your case is to just make the plant a more common thing, perhaps by including more magma caves, so that more people know of this plant. This means that the more exposure to it in your world, the less discoveries need to be made about it. All of the questions a reader would have can be answered before the problem presents itself.

You also have to make sure that characters understand it in the context of their own world, whilst translating it to an understanding of our world for the reader. If it's medieval then, like you say, they're unlikely to know about IR radiation. But they would have their own justification for it, like that it's actually heat that causes the plants to grow. They might even call it "fireweed" or something.

You could even take it further than this, and have people in your world understand it to a level where they actually get the "how" science of it, just not the "why". Humans understood that plants needed the sun long before they knew what photosynthesis or chlorophyll was.

Maybe they take seeds or entire plants of fireweed from where they grow naturally, and replant them inside sheds and keep a fire lit in order to farm their own. Of course this would mean they need a use for it, such as food, or a herb/spice, or for medicinal purposes. The reader can then make the connection on their own, without the characters ever needing to have any deeper understanding of it themselves.

I don't know if this is useful for your story, maybe the entire point is that they are only just discovering this plant, in which case to make it more understandable in your world, you will need to make it more justifiable for it to exist in the first place. Introduce different types of exotic plants into your story that also don't grow as plants on our planet do, and this allows a smaller leap for the reader to understand it from a modern science-based context.

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In addition to excellent answers given previously I would point out very, VERY important bit, which may bite you down the way. Instead of thinking HOW to introduce the concept, you might to ponder for a moment WHY instead...

Depending on exactly how advanced the medieval period you're looking at (Early, High, Late?) you may think on how discovery of INVISIBLE part of light spectrum would impact the scientific development of the world you're building.

Light theory, color theory, optics (reflection, diffusion, vision) were all known in Ancient Greece. However, IR is definite proof of EM spectrum of light and this might be - some say - the start of the modern technological revolution that goes on to this day. From IR you go to UV, then X-ray and so on, all in a blink of an eye compared to scientific progress before the 19th century (IR, UV and X are all within 100 years of each other). We all know how X-ray impacted the modern world. Are you prepared for that?

Oh, and even if UV is not needed, it plays part in plant life cycle, especially when talking about edible plants. You may have to think on that, too.

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Let your character stumble upon the book of a monk, that analyzed the flower. I guess he tried to remove the flower from the cave, expose it to sunlight, try growing other flowers in the cave, etc.

Then you can summarize the findings of that monk, leaving your character puzzling over what that implies. You can do that before or after your character finds the flower for himself.

You can also introduce a series of books by that monk, on various different topics, allowing you to have a recurring way of explaining other strange things in your world.

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Which is more valuable or necessary to you?

that the reader be given the mechanical details
that those details be codified in publication
Actually, I mention this one for the sake of completeness. It isn't quite pertinent here â€¦
that the aforementioned details be available for a reader to discover
that someone in your story appreciates or recognizes the details

There is some overlap early in the exposition, but the optimal set of approaches for each of those are rather divergent.

reader should know

This is good for the experience of the reader, listener, or viewer because understanding something helps bind a story together. Your purpose is not to clobber them over the head with the thing, but to expose it and to make it obvious for those who are able or who wish to recognize it.

For your situation, the best method is, frankly, probably to contrive a few co-incident events.
Actually, this doesn't seem likely to be a concern for your situation — it does best with ethical, emotional, or metaphysical drama, — and so I'll explain it with the characters need to know section.

codification

I'm probably not the only one who thinks like this: You've devised something which you think is especially clever or insightful, and you'd like to have it written and saved for the sake of posterity. Something like that.

As I stated above: I'll not discuss this here.

reader can discover

The important thing here is not that it be obvious. The reader who seeks the answer shall discover it; those who do not would not benefit were they to be given it anyways. More mystical mumbo jumbo, et cetera.
The pleasure taken by a peruser of your stories comes from discovering the detail, not from being given it. Your best way to achieve that is to drop hints, here and there — rather like the crafty crook smugly satisfied with their own work.
Works best in larger stories spanning multiple publications; it also works with interactive forms of storytelling, i.e. role-playing games or whatnot strongly in the storytelling corner of a Threefold model.

How exactly is it done, though? Well, think of the Blind Men and the Elephant parable: you know how it works, even if your characters do not.

characters need to know

Maybe they don't quite understand exactly how it works, but they do get some idea.

This is most likely to be what you need for your problem: so, this vegetation exists somewhere which is likely to be quite hot, yes? Your characters will probably need to devise something to protect themselves from the heat. All you need to do is to contrive a few fortunate happenstances whereby they discover that those same protective garments or shields also serve to weaken the vegetation, and — bingo!
The protagonists don't need to understand the details; indeed, you can have one or two of them suggest hypotheses which you would understand to be incomplete or even incorrect. This ties in with the reader can discover class: also, it features a good dosage of the unreliable narrators and all that jazz.

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Pretty much what Tom said, and you could give the reader enough to pick up on a hint that the plants are affected by infra red radiation by introducing a temporary source.

Someone could light a fire in the cave, and a plant that was looking limp could look more healthy. Someone could stroke the leaf (or other part) of the plant, and the part they touched could change colour. The plants could appear to lean towards people as they pass, or petals could open when a source of heat comes near.

That way, the characters don't need to know what's going on in scientific terms, but a reader will probably be able to come up with some theories.

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If this detail is important enough that readers should get it, you can have your characters make guesses at the truth close enough that (at least some) readers can connect the dots, while the characters can't and remain puzzled.

For example, two especially educated explorers in your world could discuss the plants, and how all other plants they know need sunlight to survive but these don't. One of them could notice that magma gives off dim light when very hot. He might wonder if maybe when it cools down and appears as rock it still gives off light, just so dim a human cannot see it. Or one of them could wonder if there might be some kind of light that humans cannot see, connecting it (falsely) to animals that can see in the dark such as bats.

Give enough hints that it "clicks" for the reader, but the characters, missing some science facts that are taught in schools today, can't complete the thought and leave it as a puzzle for later generations to solve.

Bonus: This makes the reader feel smart.

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