Near the San Andreas Fault in California, I saw cholla cacti for the first time. They can get quite big, and then they start dying off from the bottom up. So while the top can still be some hue of light green, further down it turns brown, then black, and near the ground you may just see the skeleton of the cactus.
It was fascinating to me to see this. How can the top be green if it gets no water or nutrients from the ground?
The skeleton is also quite remarkable to me. I grew up with deciduous trees, in other words, solid wood. To have large structures like these cacti consist of such fibrous material was a revelation to discover.
And here is the plant as a whole. A strange and unusual beauty.