Cacti are better known for their spines, so the question arises, "Which came first, the spine or the leaf?" Mauseth suggested cacti began as regular, leafy plants in the Americas, with many species later evolving spines.
Some cacti, in fact, still have large leaves and stems, such as the pad-like stems on prickly pears, or the big, flat thin leaves found on certain genus Pereskia cacti that look like trees.
Most leaves absorb the sun’s rays, manufacture plant sugars, and help with water absorption and respiration. In the case of microscopic cacti leaves, however, Mauseth thinks something else is at work.
He hypothesizes that cacti need even the tiniest of leaves in order for the plant to establish its axillary buds, which emerge near where leaves attach to the main stem.
"So the plants cannot lose leaves altogether, or they might then not be able to produce buds, or would produce them in the wrong spot," he said, adding that cacti genetically control their leaf size, in part, through release of plant growth hormones, such as auxin.
Peter Lipson is co-owner of the Cactus Jungle, a California nursery that specializes in cacti and succulents.
Lipson was surprised to learn that all cacti have leaves.
"I would have assumed the leaves on most disappeared altogether over the course of evolution, but now it appears that’s not the case," Lipson told Discovery News.
In future, Mauseth hopes other researchers will brave the spines and study cacti further, since they may provide a good model for testing theories about genetic control in plants.
Related Links:
The Cactus Jungle
Cactus and Succulent Society of America
James Mauseth's lab at the University of Texas, Austin
