91ÖÆƬ³§

We were hiking in a dry stream in the Maze District of Canyonlands, following a sinuous path of sand through a landscape of rock when we found a single isolated plant, flowering. This dwarf lupine, Lupinus pusillus, only 3 inches tall, had a full column of flowers. 

I did not notice then but when I returned home and processed the photo, the leaves and flowers had clumps of sand clinging to them, and the stem and branches were densely packed with sand.

Sand adheres to the leaves, stem and flowers of a dwarf lupine, deterring herbivores in Canyonlands National Park. For larger image, click . Photo by Jeff Mitton.

Plants have a variety of defenses, including thorns, stinging hairs, oils that irritate skin, chemicals that can play havoc with digestion, heartbeat and neurological functions. Some plants produce phytoliths — "plant stones" made of silicon dioxide crystals that are sharp and hard, making leaves and stems unpleasant to chew — while other produce raphides, calcium oxalate crystals that form needles in leaves and stems. 

Herbivores generally avoid leaves and stems defended by phytoliths and raphides. Some species of grasses accumulate high levels of silica, up to 6 percent of the dry weight of the plant, that abrade and irritate herbivore mouthparts and reduce palatability and digestibility of leaves. 

91ÖÆƬ³§ 20 years ago, ecologists coined a new word — psammophory, meaning "sand carrying" — to describe plants that accumulate sand on their surfaces. While these plants do not synthesize phytoliths, they accumulate the same defense with either glands that produce sticky exudates or trichomes, structures on leaves that ensnare sand particles. Sand is mostly quartz, or silicon dioxide, the same hard, gritty, insoluble material in phytoliths.

A recent paper by Eric LoPresti and Richard Karban at the University of California at Davis listed more than 200 species of psammophorous plants and specifically tested the hypothesis that a layer of sand defends plants against herbivory. They manipulated levels of sand on leaves of sand verbena, Abronia latifolia, a plant whose common name indicates that it not only grows in sandy sites, but also that wind-blown sand adheres to its leaves.

They manipulated levels of sand on the leaves of plants growing naturally in the field. They chose adjacent pairs of stems, leaving one untouched as a control, and treated the other to either remove or add sand. Sand was removed by gently wiping the leaves with a moist sponge. 

A comparison of the control and reduced sand leaves showed that removing sand doubled the damage by a variety of insect and snail herbivores. Leaves with added sand had less damage than the control group. Clearly, snail and insect herbivory is reduced by increasing levels of sand on leaves and stems.

I enjoy eating clams, scallops, oysters and mussels, but I hate it when I bite down and hear and feel the crunch of sand between my teeth — that is not only unpleasant, but it cannot be good for your teeth. Along the same lines, I was made uncomfortable by a display of tooth wear and damage in the Ancients that lived in Mesa Verde. 

Grinding corn with stone implements necessarily introduced minute stone fragments to corn meal, and decades of minor crunches chipped and eroded tooth enamel. I have no doubt that other mammals such as mice, voles, cottontails, jackrabbits and deer would be deterred from nibbling leaves and stems sprinkled with sand.

Caterpillars that ingest sandy leaves absorb less nutrition from their meals, so they are less active and smaller than those who find leaves without sand. Consequently, natural selection would favor insects that learn to avoid sandy plants. 

Defending oneself with wind-blown sand armor would have several costs. The glands, sticky substances and trichomes would require energy to develop and maintain. Furthermore, sand shades leaves and stems and probably reduces photosynthetic rates. 

I am reminded of a study that compared photosynthetic rates in intact cacti with cacti whose needles had been removed; cactus spines reduced photosynthetic rates by 15 percent. Every defense has costs.