The mental model
Here's the whole plant as one system, drawing on light, water, air and mineral nutrients. Photosynthesis, mostly in the leaves, makes sugar, and the vascular tissue carries water, minerals and food through the plant. A shortage of light, water or carbon dioxide alone can hold back photosynthesis, however plentiful the other two are (Whiting et al., 2016).
Fig. 12. A plant as one system, with water moving up from the roots to the leaves. Every later module is operating instructions for some part of this system.
Read the signal, name the cause
Tap a symptom on the left, then tap the explanation on the right that fits it. Diagnosis is the skill, so you do it, you don't just read it.
The symptom
What's actually happening
Fig. 13. The four inputs, and the later modules that take each one further. Most of the diagnoses you just made trace back to one of them. That's the whole apprenticeship in one map.
Seven things to remember and use
- A plant draws on light, water, air (carbon dioxide for photosynthesis, oxygen for respiration) and mineral nutrients. Start every care decision by asking which of these it serves. Nutrients are the one this module only names; Module VI is Feeding.
- Roots respire. Excess water cuts the oxygen in the soil and damages the fine roots, and in most plants a root hair lasts somewhere between a few days and a few weeks.
- Stems carry water up through xylem, whose conducting cells are no longer alive, pulled by evaporation at the leaves; they carry sugar in more than one direction through living phloem. Root pressure adds a push only at night or in cloudy weather, and only in some plants.
- Leaves are layered factories; stomata trade water loss for CO2 every time they open. Light opens them and water stress closes them. In the one study we found, on tropical tree saplings, stomata narrowed steadily when the air's drying pull rose along with leaf temperature (about 90 to 108 °F, or 32 to 43 °C); a heated winter room is dry air, if much cooler, so the houseplant link is our reasoning.
- Photosynthesis: capture light, then fix carbon into the raw material for sugars. Leaves absorb green light less readily than red and blue, which is why they look green, though in one lettuce study the leaves were calculated to absorb about 81 percent of the green LED light used. Indoors, light is often what runs short.
- Transpiration drives the column; add evaporation from the surface of the mix and you have evapotranspiration. Humidity, heat, and airflow all affect how fast it goes.
- Flowers are the plant's reproductive organs: pollination, fertilization, seeds, fruits. The rest of the apprenticeship is operating instructions for this system.
Sit with one plant, then walk outside
Start seeing the system you just learned. Nothing to fix. Just look, slowly and on purpose.
Part 1 · One houseplant
20 min · indoors
Look at the leaves
Count, arrangement, color, shape. Are they turned toward the light?
Check the roots from outside
Any roots at the drainage holes? Lift the pot. Heavy (wet) or light (dry)?
See it as a system
What's this plant doing well? What might it be struggling with?
Part 2 · One outdoor plant
25 min · outdoors
How does it hold leaves to the light?
Branching pattern, overall shape, how the canopy fills space.
Same system, different scale
How does this plant solve the same problems your houseplant solves: light, water, gas exchange, reproduction?
Apart from mosses and their nonvascular relatives, every plant on your shelf moves water and nutrients through the same kind of internal plumbing (Moore & Bradley, 2022). Once you see the pattern, every plant you meet is legible.
"I've done the field exercise and I understand the foundation."
This marks Module I complete and carries your progress forward. The work is done by your own honest account. No points, no badge, no leaderboard.