Parkinsonia florida

A saguaro can't start life in the open. The seedling is a green bead a few millimeters across, and it will cook in full sun, freeze on a clear winter night, or get eaten by nearly anything that comes past.

What it needs is a nurse — usually a blue palo verde or an ironwood. The seed germinates in the litter under the canopy, where there's shade through the afternoon, a few degrees of frost protection on cold nights, and cover from rodents and birds. It grows there for decades. At ten years old it may be an inch tall, still completely hidden.

Then the arrangement turns. As the saguaro's shallow root system spreads, it becomes very efficient at taking water in the top few inches of soil — the same water the palo verde is using. The tree that made the cactus possible tends to decline as the cactus matures, and often dies before it. What you're left with is a large saguaro standing where a tree used to be, sometimes with the grey wood still on the ground beside it.

Palo verde is Arizona's state tree — the designation covers the genus, blue and foothill alike. The bark is green because it photosynthesizes, useful for a tree that spends much of the year with no leaves at all.

Look at any saguaro on a hillside and ask what raised it.