Saguaros and other large cacti of the Sonoran desert develop only slowly from seeds - the seedlings might take 2-3 years to reach a height of only 3 cm. During this time they are very vulnerable to drought in the intense exposure of the sun, so they almost invariably develop in the shade of a tree or shrub - a "nurse plant". For example, the image below shows a young saguaro - about 40 cm tall and probably 15 years old. It has grown up in the shade of a shrub (limber bush).

The following images show further examples of saguaros and another large cactus - the organ pipe cactus - which have developed from seedlings in the shade of shrubs or trees. These nurse plants not only provide shelter from the sun but also provide a soil with at least some organic matter (from fallen leaves etc.) and a richer supply of nitrogen - most of these nurse trees are nitrogen-fixing species, with Rhizobium or related bacteria in their root nodules.

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