Plant structure and photosynthesis

In the circle of the microscope, elongated cells packed together like bricks, dotted with small green grains. — Elodea cells seen under the microscope (images by Kelvinsong, 2013). Kelvinsong, CC BY-SA 3.0, source

Humans do not see plants.

This is not a figure of speech but a phenomenon that has been studied, named and measured — and it concerns this site as much as its readers.

An issue flagged as early as 1919

In 1919, as general biology courses were being set up, some teachers made an observation: in practice, the words “biology” and “zoology” were being treated as synonyms.

Botany played only a minor part in them.

The phenomenon was named “zoo-chauvinism” in the 1980s. Then, in 1998, two American botanists gave it the name by which it is known today — and defined it.¹

It is the inability to see or notice the plants in one’s own environment. This leads, they say, to the inability to recognise their importance, the inability to appreciate their distinctive features, and a ranking of plants as inferior to animals.

Their two articles have since been cited hundreds of times.

Two important corrections

We began this page with a simple hypothesis: humans do not see plants.

The literature corrected us on two points, and it is important to say so.

First point: it is too general. The researcher who has worked most on this subject makes clear that the phenomenon differs across cultures, that most of the research has been carried out in European and North American societies, and that Indigenous peoples generally show far greater attention to plants.²

In other words: a result obtained in one place had been stated as if it held everywhere. This is exactly the flaw that this site has been tracking from the start — and it is the authors themselves who point it out.

Second point: it is not an inability. A review of twenty years of literature concludes that, in high-income countries, it is a lack of attention due to increasingly scarce experiences with plants — and not an impediment to perceiving them.³

So it is not that we cannot see plants.

It is that we no longer have any reason to look at them.

This changes everything: an impediment is something one suffers; a lack of habit is something one can correct.

The organs of a plant

A plant has no mouth, no legs and no heart. Yet it solves the same problems as we do — feeding, drinking, breathing, reproducing — with entirely different equipment. Four organs take care of this.

Roots anchor the plant and absorb water and mineral salts from the soil. Their exchange surface is considerable: that of a rye plant, root hairs included, exceeds several hundred square metres.
The stem supports the rest and handles circulation. It contains two distinct networks of conduits, which differ both in what they carry and in the direction they carry it.⁴

The xylem carries water and minerals up from the roots to the leaves. Its cells are dead at maturity: they are empty tubes, and it is this material that, as it accumulates, forms wood.
The phloem carries the sugars made by the leaves back down to the rest of the plant, including the roots. Its cells, by contrast, are alive.⁵

A root does not feed itself: it is fed by the leaves.

Leaves capture light. Their flat shape is no accident: it offers the maximum surface area for the minimum of material. Their underside is pierced by thousands of tiny pores, the stomata, which open to let carbon dioxide in and close to limit water loss.
Flowers, where there are any, handle reproduction, then become fruits and seeds.⁶

These organs are not separate parts: they depend on one another all the time. Roots never see the light and live on sugars coming from above; leaves never touch the ground and live on water coming from below. It is this two-way circulation that makes possible everything this page describes.

Where does the matter of plants come from?

This is the central question, and the answer is deeply counter-intuitive.

When we look at a century-old tree weighing several tonnes, our immediate reflex is to think that all this matter was extracted from the soil by its roots. That idea is wrong.

Van Helmont’s historic experiment

In the seventeenth century, the Flemish physician and chemist Jan Baptist van Helmont carried out a famous experiment, published after his death in 1648:⁷

He planted a young willow weighing 2.3 kg in a pot containing exactly 90.7 kg of oven-dried soil.
For five years, he added nothing but rainwater.
After five years, the willow weighed 76.7 kg (a gain in mass of more than 74 kg).
He put the soil back in the oven and weighed it again: it had lost only about 57 grams.⁸

Van Helmont concluded — wrongly — that the tree’s mass came exclusively from water. He was unaware of the fundamental role of air.⁹

The tree is made of air and light

The dry matter of a plant consists of about 95% carbon, oxygen and hydrogen.¹⁰

The carbon and oxygen come from the carbon dioxide (CO₂) present in the air, taken in through the stomata of the leaves.
The hydrogen comes from the water (H₂O) absorbed by the roots.
The minerals from the soil (nitrogen, phosphorus, potassium) make up only a tiny fraction of the total mass (the 57 grams of Van Helmont’s experiment).

A tree is therefore literally an assembly of air turned into solid matter by the energy of light.

And this mechanism has a name

It is photosynthesis. The word comes from Greek: photo, light, and synthesis, putting together. Putting together by means of light — the name describes exactly what Van Helmont had failed to see.

The equation fits on one line: the plant takes in carbon dioxide through the stomata of its leaves, water through its roots, and light energy; from these it makes sugar, which will become wood, leaf and fruit, and it releases oxygen, for which it has no immediate use.

The order of things matters here. The oxygen we breathe is not the purpose of the operation: it is a waste product. The plant makes sugar; oxygen is what is left on the plate.

Why leaves are green

“Because of chlorophyll” is the usual answer. It does not answer the question: it merely shifts it.

Why, then, is chlorophyll itself green?

A misconception to correct first

We often read that chlorophyll “reflects” green, as if it were choosing to.

That is not what happens. It preferentially absorbs blue and red. Green is simply what is left over — what it captures least well.

There is something more surprising still. Leaves lacking chlorophyll reflect green more efficiently than green leaves. What reflects light in a leaf is thought to be mainly the cellulose of the cell walls — not the pigment.¹¹

An article published in 2020 by Finnish researchers was written specifically to correct this confusion, so widespread is it in textbooks.

And green is not wasted

Another surprise: green is absorbed only about 20 to 30% less efficiently than blue and red. It is not a rejected colour: it is a colour that is captured slightly less well.

Once absorbed, a green photon is not wasted: in fairly strong white light, it even makes the leaf work better than a red photon does. Why? Because green penetrates the leaf more deeply than red and blue, which are stopped in the very first layers. In full light, it is therefore the green photons that power the lower chloroplasts.¹²

If everything were absorbed, a plant would look black. The green we see is the small fraction that comes back out.

Why, then, not capture everything?

The most remarkable point was documented in 2020. It starts from an oddity that no one could explain.

A plant has not one pigment, but two: chlorophyll a and chlorophyll b. Yet their absorption peaks do not fall at the top of the solar spectrum, nor do they fall in the same place. They are offset — from the maximum of available light, and from each other.¹³

This is strange. To capture as much energy as possible, one would place one’s sensors where light is most abundant.

The explanation put forward is as follows. Sunlight flickers constantly: a cloud passes, a leaf moves overhead, the sun goes down. And inside the plant itself, the transfer of energy is irregular.

Two sensors whose peaks are slightly offset, and placed where the spectrum changes fastest, respond differently to the same fluctuation. What one loses, the other gains. The variations cancel each other out, and the power coming out of the system remains stable.¹⁴

The researchers call this cancelling the noise. Their model, applied to three very different organisms — green plants, purple bacteria, bacteria living under water — recovers the actually observed peaks every time.¹⁵

Three different light environments, one and the same rule, three correct predictions. That is what makes the explanation sound.

So it is not the maximum that is sought. It is the stability of what comes out.¹⁶

A reversal that is useful well beyond botany: what captures the most is not always what is selected. Regularity can be worth more than performance.

It can be measured

There is now a validated measuring instrument for this phenomenon, built around four components:¹⁷

The attention paid to plants in the everyday environment;
Attitude and empathy towards the plant world;
The level of basic biological knowledge;
The relative interest given to plants compared with animals.

This is what definitively distinguishes the phenomenon, as studied, from a mere shared impression.

Note 1

in 1998 we decided to introduce a new term, plant blindness, to the US biology education literature (1998a). […] Subsequently, we defined plant blindness as: the inability to see or notice the plants in one's own environment

Wandersee, James H. and Schussler, Elisabeth E., “Preventing Plant Blindness”, The American Biology Teacher, vol. 61, no. 2, 1999, p. 82-86 ; et “Toward a Theory of Plant Blindness”, Plant Science Bulletin, vol. 47, 2001, p. 2-9