LB702: How plants water themselves from the air

David Noel
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.

How plants harvest water from the atmosphere
Plants which can survive or thrive in arid conditions have developed two special characteristics which aid this survival, characteristics not always recognized as performing this function. The first is development of water-containing structures (which act as condensation bodies), and the second is development of spiky protrusions (which act as condensation points).

This article is a revision of one published in 2011, entitled "Why do cacti hold a lot of water and have spines?" [1]. This article noted that "It's usually thought that cacti hold water to tide them over in dry periods, but this cannot be the main reason, as they do not shrink much in extended dry periods or swell much when an occasional downpour happens. They may have spines to deter browsers, but is this their only purpose?".

So the usual view is that arid-area plants have evolved to hold stores of water to 'tide them over' during long drought periods. In fact, it appears that the reason they hold water, is that these water-filled structures are quite efficient at extracting water from the air as the temperature cycles through day and night conditions.

Everyone is familiar with "morning dew". As the temperature climbs at daybreak, plants are somewhat colder than the surrounding air, and can condense water from the water vapour which exists in the air. The more water there is held in the cactus stem, the greater its heat capacity, and the better it is at extracting water.

In some areas which are usually very arid over most of the year, such as in Baja California (the long thin peninsular in western Mexico), rainfall is almost non-existent, and so only plants which use this dew-extraction mechanism can survive. Although from quite different families, plants here all tend to look similar in structure, while their relatives from wetter areas have the typical appearance of their particular family.

The amount of water which can be collected is surprisingly high. Figure F1 shows how a cactus stem may be running with water around daybreak.

Fig. LB702-F1. Cactus stem in Baja California. From [1].

Another thing that makes this mechanism work well in arid areas is that these areas have big changes in temperature between day and night, which helps dew formation. Places where the temperatures are very even, often under thick cloud cover, cannot produce much dew.

These arid-area plants all have similar structures. The water-holding parts are swollen and pillow-like (Figure F2). Other parts, such as leaves, are readily shed under stress, so that if no water is available from rain, plants revert to column-like trunks and rounded water containers.

Fig. LB702-F2. "Donkey's Tail" succulent "leaves". From [1].

Other examples of the lobe/column structure in arid-area plants can be seen with the Boabs and with Argentinian Ombu. The latter is the only tree structure found widely in the open pampas of Argentina.

The Boab tree shown in Figure F3 (native of northwest Australia) has the typical swollen trunk of a tree adapted to arid conditions. The interior of the trunk is a spongy material which can hold water well, rather than the solid wood of normal trees.

Fig. LB702-F3. Boab tree in Northwest Australia. From [1].

When the wet season comes, leaves rapidly appear on the sausage-shaped branches and on the thin twigs. As the dry season progresses, leaves, and sometimes the twigs, all fall.

Figure F4 shows the similarly-formed trunks of the Ombu tree found in dry areas of Argentina. Its water-collection property allows it to thrive where normal trees are absent. The material within the trunk is spongy, just like that of the Boab.

Fig. LB702-F4. Trunks of the Ombu Tree from the Pampas of Argentina. From [1].

Figure F5 shows how new leaves form on Ombu branches when weather conditions are favourable. When prolonged dry weather occurs, and the leaves would transpire too much water, they are easily shed (Figure F6).

Fig. LB702-F5. Ombu with wet-season leaves . From [1].

If it sheds all its leaves, a tree looks dead. But when rains arrive, it soon comes to life.

Fig. LB702-F6. Trunks of the Ombu Tree from the Pampas of Argentina. From [1].

Spines on Cacti
While spines on cacti clearly do deter browsing animals, they also function as part of the water-condensing process. Figure F7 shows how droplets of dew on a cactus are initiated at the spine points.

Fig. LB702-F7. Dew forming on cactus spines. From [1].

The value of spines in formation and movement of water droplets has been explained in some 2015 work on "biomimetic cactus spines" [2], that is, artificial structures which copy ones found in nature. There is more mention of these spines at the end of this article, in relation to synthetic cacti in agriculture.

Figure F8 shows how the plant called "Elephant's Foot" (Adenia pechuelii) has been modified by the arid conditions in its native location of Namibia into the typical water-bag-plus-spines form. Surprisingly, this plant is in the Passionflower family, but has little resemblance to the rest of the family..

Fig. LB702-F8. "Elephant's Foot", Namibia From [1].

Plants develop spines to deter browsing animals, but then animals develop mechanisms to cope with the spines. Figure F9 shows one of the giant tortoises of the Galapagos, which feed mainly on prickly-pear cacti. To be able to do this, they have developed very horny mouth parts.

Fig. LB702-F9. Galapagos Tortoise From [1].

The prickly pears, in their turn, have developed a local form with a hard trunk (Figure F10), and have most of their leaf pads up high, out of reach.

Then, to counter this, some of the tortoises have developed shells with an upward curve, which allows them to reach higher up the trunks....

Figure LB702-F10. Prickly Pears in the Galapagos Islands.

Tallness in Cacti
Although the Galapagos prickly pears have developed tall trunks to limit grazing, many other of the plant forms looked at here have quite tall shapes for other reasons.

Most ordinary trees develop height in competing for light with adjacent plants. A young rain-forest sapling, growing among mature trees, will strive desperately to reach the light, and initially may be tall and spindly. Growing in the open, the same tree will grow much shorter and bushier.

Cacti, growing in the open desert, do not have this competition pressure. Instead, their greater height allows them to reach higher layers of moisture-laden air to extract water. The giant Saguaro cacti of the southwest USA and Mexico are examples of this. Some of the other arid-land plants looked at are tall for the same reason.

Fig. LB702-F11. Saguaro cactus. From [1].

Even the giant Redwoods of California use this mechanism. Their great height allows them to extract water at height from the coastal fogs or low clouds which roll in, rather than have to draw it all up from the root zone.

'Synthetic Cacti' in agriculture
Because the water-extraction mechanism of cacti is mechanical, it should be possible to manufacture water-holding "synthetic cacti", complete with artificial spines, to provide self-irrigation to adjacent plants, especially tree seedlings.

These might be made from plastic pipe. Initial installation might use pipes filled from tankers. In areas with some rain, pipes might collect rainfall if surmounted by wide funnels, running through one-way valves, to keep the "Synthe-cactus" topped up.

Biomimetic artificial "cactus spines"
In 2015, some Chinese authors published a study of the use of imitation cactus spines in collecting water from fog [2]. They showed that dew collection and movement, in plants and in artificial dew-collection devices, depended on what they called "hierarchical groove structure". They mention that "research about the cactus has revealed that its water‐collection ability derives from the hierarchical groove structure on the surface of the cactus spine, that can endow itself with a Laplace pressure gradient and wettability gradient".

Fig. LB702-F12. Artificial cactus with spine structures. From [2].

The same study mentions the ability of some desert beetles to collect dew using the hydrophilic and hydrophobic structures of its back. So dew harvesting is used in the animal world, too!

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References and Links

[1]. Why do cacti hold a lot of water and have spines?. .
[2]. Fan Bai et al. Biomimetic “Cactus Spine” with Hierarchical Groove Structure for Efficient Fog Collection. 2015. .

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Commenced conversion from Zombal version ([1]), 2020 Dec 23.
Version 1.0 up on Web, 2021 Jan 2. V. 1.1 with artificial cactus spine reference, 2021 Jan 4.