Tiny Discovery: All Cacti Sprout Leaves

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Aug. 17, 2007 — Most cacti appear to be leafless, but a new study has determined that all of these often prickly plants actually do possess ultra tiny leaves.

The findings, published in the latest International Journal of Plant Sciences, not only add to the fascination surrounding these already unusual plants, but they also may broaden the known functions of leaves.

For curious cacti owners desiring a glimpse of the leaves, caution is advised.

"If people do look closely at their cacti trying to see leaves, they need to be careful of spines close to their eyes," James Mauseth, the paper’s author, warned.

Mauseth, a professor of biological sciences at the University of Austin at Texas, explained to Discovery News that the newly found leaves — best viewed under a microscope — exist at the base of spine clusters.

He made the determination after gathering samples of 147 different cacti species, most of which he obtained in the wild. Mauseth then conducted extensive analysis of the plants and their tissues under high magnification.

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While the leaves "had more or less normal morphology," almost all were missing blades. Just over half had xylem, a tissue that conducts water and nutrients from the roots up through the plant, but no cacti leaf analyzed had just phloem, which is a food-conducting tissue.

Leaf sizes ranged from 30 to 2310 micrometers. A single micrometer is equal to one millionth of a meter or just .00003937 inch, so the documented cactus leaves may very well be the world’s smallest.

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.

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Related Links:

The Cactus Jungle

Cactus and Succulent Society of America

James Mauseth's lab at the University of Texas, Austin