A plant is a system

Here's the situation this module is for. Something's off with a plant and you don't know what. You look it up, and you get a list: water weekly, bright indirect light, well-draining soil. You've been doing all of that. The list has nothing left to tell you.

That's the limit of rules. They hold until they don't, and then you need to know what the plant is actually doing, because every rule worth following is downstream of that. So we start there, and it turns out to be simpler than you'd think.

A plant runs on four inputs: light, water, air, and nutrients. It takes those in and uses them to make its own food. That's the whole stack.

Animals can't do this. They have to find food elsewhere. Plants are autotrophs: they make their own from sunlight and simple chemicals. So when you set up a houseplant, you're not "feeding it" the way you'd feed a pet. You're setting up the conditions under which it can feed itself.

Three things drive the system:

1. Light hits the leaves. Energy comes in.
2. Water and dissolved nutrients come up from the roots.
3. The plant uses the captured light energy to combine that water with CO₂ from the air, producing sugars. That's its food and the raw material for everything it builds.

That third step is photosynthesis, the engine of the whole plant. Everything else exists to support it: roots pull water up, stems move things around, leaves arrange toward light, flowers reach for pollinators, seeds travel outward.

Fig. 1. A flowering plant as a dissection plate. Left: exterior. Right: the cutaway revealing stem, flower, and root structure.