Borrowing someone else's trunk
Why Do Vines Climb?
A tree spends years building a trunk to reach the light. A vine skips that, and climbs one that already exists.
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Step 1 of 5
Under a forest, almost no light reaches the floor.
The leaves overhead take nearly all of it. A seedling on the ground is trying to live on the scraps that get through.
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Step 2 of 5
A tree's answer is to build a trunk, which is very expensive.
Wood costs years of growth and most of what the tree makes. All that effort produces no food at all — it only holds leaves up.
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Step 3 of 5
A vine simply does not build one.
Its stem is thin and floppy, and it spends what it saves on growing long and fast and making leaves. Something else does the holding up.
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Step 4 of 5
The climbing itself is done with grabbers.
Some vines coil a whole shoot round a support. Some send out springy tendrils. Some grip flat walls with sticky pads or short clinging roots.
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Step 5 of 5
The trade is that a vine needs somebody else standing there.
In the open, a vine sprawls along the ground and gets nowhere. And a heavy vine can shade out or drag down the tree it climbed.
A tendril searching for a support waves slowly in circles until it touches something, and then coils within an hour.
The short version
Vines climb because building a trunk is enormously expensive, so they grow thin and fast, grab a support somebody else built, and put everything into leaves.
Try it yourself
Find a climbing plant and look at how it holds on — coiled stem, curly tendril or sticky pad. Each one is a different solution.