Fog collecting spines
One of the characteristics features of cacti are their spines. These are modified dead leaves. The prickly hard ones are generally seen as a mechanical defence against herbivores. But some cacti have more porous flexible spines. A group of international researchers wondered if they could be involved in collecting fog water.
Cacti live in dry arid places, where rain is infrequent and sparce. But nocturnal fogs do occur. That fog would be an excellent water source if the cacti could harvest it. So, the researchers set out to investigate if the cacti did so.
They focused on the Turbinicarpus alonsoi, a cactus species native to central Mexico with porous, flexible spines who adorn the top of the cactus like messy hairdo. To catch the cacti in the act, the researchers filmed the cacti when they exposed them to fog. Watching the film they noticed that the bended spines slowly straightened up in the fog.
Harmonica cell walls
This straightening was a result of the water the spine absorbed. But that was only the start, the researchers noticed that once the spine was straightened, gravity was directing the collected water to the base of the spine.
Subsequently the researchers looked more closely at what makes the spines straighten up. When the spines are dry the cell walls of the cells which make up the spine are kind of folded, like a harmonica, but only over the width of the spine and not so much in the lengthwise direction. This folding of the cell walls in only one direction causes the spine to bend. However, when in foggy conditions the spines absorb the water, the cells swell up, which force the folded cell walls to stretch. This, in turn, causes the straightening up.
Now, once the spines are maximally absorbed, any extra captured water runs off down the spines. This water doesn’t however enter the living tissue at the base of the spine. The researchers noticed that the tissue there was actually water impermeable. Instead, the water runs down the cacti towards the roots, those eventually take up the water. This has, according to the researchers, an extra benefit of improving the uptake of soil nutrients.
Maybe main purpose
Fog water harvesting appears not to be just a function of the porous flexible cacti spines studied in this study. Previous studies show that the non-porous and stiff spines, those that are often thought in context of defence against herbivores, can also harvest fog water. As cacti generally grow in dry places, this makes you wonder if the fog water function of the spines came first and the herbivore defence capability second.
In addition, to explaining how cacti manage to find enough water so they can survive, this study also might as an inspiration of how to design fog water harvesting devices.
Literature
Huss, J.C., Box, F., Zhang, Q., Grömmer, M.A., Antreich, S.J., Ovee, T.A., Louf, J.-F., Schönenberger, J., Williams, D.G., Gierlinger, N., Liu, M. and Hultine, K.R. (2026), Humidity-driven shape morphing enhances fog harvesting in porous cactus spines. New Phytol. https://doi.org/10.1111/nph.71648
