There is a scraggily plant that grows along New England streams that occasionally, in a fit of mischief, I have invited a companion to pull from the ground. OUCH! Along the stems of the plant are a zillion tiny downward-pointing barbs. The name of the plant: tearthumb.
Obviously, the barbs did not evolve for human mischief. They are there to help the plant scramble upwards on the backs of more upright neighbors. It's all about getting on top, up there in the sunlight. Thicker stems might have helped, but tearthumb opted for promiscuous sprawl and grappling hooks.
Here on my window sill my morning glory plants are winding their way up their pole, counterclockwise. They too have opted for help from their more rigid neighbors. In this case, I have provided a convenient neighbor in the form of a bamboo stick.
So up they go. As seedlings, they simply wave their uppermost tendril around in a lazy counterclockwise circle, like a cowboy spinning his lasso. If the tendril touches an upright, it twines around with a gentle embrace. No more flop and sprawl. The morning glory plant now has a backbone and grows towards the sun.
Watching all of this, it would seem that the plant has a mind of its own, a kind of foresight and willfulness. But it's all in the genes. Asymmetrical proteins. Differential growth that is exquisitely sensitive to touch. Charles Darwin knew nothing of DNA, but he was endlessly interested in twining plants. He produced two books on the subject: The Movements and Habits of Climbing Plants (1875), and The Power of Movement in Plants (1880). He wrote that it always pleased him "to exalt members of the botanical world in the scale of organized beings." Tearthumb, morning glories, and Darwin's hops and peas are no dummies.