From the memoir “Lab Girl” by Hope Jahren:

A leaf grows by enlarging the string of cells located along a central vein; single cells on the perimeter eventually decide independently when to stop dividing. From this tip, smaller veins develop, eventually completing the network at the stem; thus the overall maturation proceeds from tip to base. Once the most daring portion of the leaf is complete, the plant puts horse before cart and begins to slide sugar back down and in, down to where it will be used to make more root, which will be used to bring up more water, which will be used to expand new leaves, which will pull back more sugar, and in this manner four hundred million years have passed.

Every once in a while a plant gets an idea to make a new leaf that changes everything. The spines on a cholla cactus are barbed like a fishhook, sharp and tough enough to puncture the leathery skin of a tortoise. They also reduce airflow across the cactus’s surface, thereby reducing evaporation. They provide meager shade for the stem and a surface upon which to condense dew. The spines are actually the leaves of the cactus; the green portion is its swollen stem.

Probably within just the last ten million years, a plant had a new idea, and instead of spreading its leaf out, it shaped it into a spine, such as those we find today on the cholla cactus. It was this new idea that allowed a new kind of plant to grow preposterously large and live long in a dry place where it was also the only green thing around to eat for miles—an absurdly inconceivable success. One new idea allowed the plant to see a new world and draw sweetness out of a whole new sky.