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 draws on light, water, air and mineral nutrients. In this apprenticeship we call them the four inputs. From light, water and the carbon dioxide in air, a plant makes its own food.

Plants are autotrophs: they use light energy to build their own organic molecules, the compounds they make their cells from and store energy in (Michaels et al., 2022). Animals are heterotrophs: for their energy they depend on the sugars that photosynthesizing organisms make (Clark et al., 2018). So when you set up a houseplant, you aren't feeding it the way you'd feed a pet, because it makes its own food. You're setting up the conditions under which it can feed itself.

Three steps drive the system:

1. Light hits the leaves, and 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. Those sugars are its food, and the plant builds its cells from the carbon compounds that photosynthesis makes.

That third step is photosynthesis, the process that makes the plant's food. The other parts keep it supplied: roots take up water and minerals, stems carry water and sugar, and shoots bend toward the light. Flowers and seeds are for reproduction, which Lesson 9 covers.

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