Sequence

 

An onion is the typical metaphor for things with multiple layers.  Looking at it from the point of view of growth, I find it interesting how the outermost layer knows when to stop growing.  Is it the interior pushing it outward, or is it the outer layer that stops the inside from continuing to grow?  I can’t be the first to question this process. 

Does the banana grow and then the protective skin forms on the outside, or is it the skin that grows first and then the banana fills in the void? 

I believe the auto manufacturers first construct the insides of the car and put the outer skin on last.  Was this something they learned from nature?

 

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The outer layers of an onion, the peel of a banana, and the metal skin of a car all look like they’re doing the same job—protecting what’s inside—but the way each one comes into being is completely different. What makes your question so rich is that it asks about directionality: does the inside push outward, or does the outside define the boundary? Nature answers that differently depending on the organism.

🧅 How an onion knows when to stop

The onion bulb forms through a sequence of developmental stages—germination, seedling growth, bulb initiation, bulb enlargement, and maturation. During bulb formation, the plant begins to swell and develop layers, each layer being a modified leaf that thickens as the bulb enlarges.

A few key points help answer your question:

• The outermost layer doesn’t “decide” to stop growing. Instead, each leaf layer is produced at the center of the bulb. As new layers form inside, older layers are pushed outward.
• The outer layers dry out because the plant shifts into maturation. When the tops fall over and the plant stops sending resources to the bulb, the outer layers become papery.
• Growth direction is inside-out. The interior drives expansion; the outer layers simply age, dry, and harden.

So, the onion is not a Russian doll built from the outside in. It’s more like a tree trunk in reverse: new rings form at the center, not the edge.

🍌 How a banana gets its peel

Bananas develop as berries, and their peel is part of the fruit’s pericarp—the outer tissue that forms from the ovary wall itself. The peel and the pulp develop together as the fruit grows.

Research on banana peel formation shows:

• The peel forms early, as part of the ovary wall, and thickens as the fruit enlarges.
• The pulp fills in from the inside, originating from the inner pericarp layers.
• The peel doesn’t wait for the banana to grow first. It expands in tandem with the pulp, stretching and differentiating as the fruit matures.

So unlike the onion, where new layers arise internally, the banana’s skin is a continuous outer tissue that grows along with the interior.

🚗 How a car gets its skin

Modern car manufacturing follows a very different logic from biology, but the metaphor is tempting.

Cars are typically built in this sequence:

• Stamping the body panels from sheet metal (the “skin”).
• Welding those panels into a rigid body shell—the body-in-white.
• Painting the shell, which must happen before interior components are installed.
• Adding the interior, engine, wiring, and trim during final assembly.

In other words:

• The skin comes first, because it must be welded and painted before anything delicate goes inside.
• The interior is inserted afterward, the opposite of how an onion or banana develops.

This isn’t something learned from nature—it’s dictated by physics, paint chemistry, and the need for structural rigidity before installing components. But the contrast is interesting: nature tends to build outward from a living center, while industry often builds inward from a manufactured shell.