Sunrise 7:24: Winter coats on cacti

It was in the 30s again this morning as the female Dawn Jogger headed out into the dark across the Stanford campus. There wasn't enough light to see the heavy frost on the fields, but she could feel it. The grounds crew was ready for the cold, covering some of the more vulnerable plants in the Arizona Cactus Garden. There's been significant restoration on this part of Stanford's Arboreum, first planted n the late 1880s, since the DJs have been jogging this route. Given the number of cacti and succulents, why these particular plants and not others...