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I am currently designing the ecosystem for a planet within my world building project. The quick rundown is that it's tidally locked with its host star and is a very dry planet. I was thinking that one strategy for plants to gain water was through condensing water on their leaves or other surfaces. However I am unsure what shapes these plants might evolve to condense as much water as possible while being relatively efficient to make.

But also keep in mind that since the sun is always in the same place plants similar to trees will evolve to make a flat wall of leaves and grass evolved to look more like leaves all pointed towards the sun.

So with plants like this how might they evolve to condense water?

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  • $\begingroup$ Condensing water or conserving water? $\endgroup$ Commented Jan 3, 2023 at 23:26
  • $\begingroup$ Indeed @TheresaKay :). If we look at cactuses 🌵, they're working a lot on the latter because it's very much a matter of not losing the few you earned. Hint for condensation : look at cactus's spines :). $\endgroup$ Commented Jan 3, 2023 at 23:45
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    $\begingroup$ @Tortliena My mind went to cactuses too. Actually currently working on an answer to the OP's question related to cacti. It's true that water condenses on the cactus's spines, but what makes them capable of lasting so long without water is storing (aka conserving) it. I think the OP would get better answers for asking how plants can conserve large amounts of water, rather than how water could condense on the surfaces. $\endgroup$ Commented Jan 3, 2023 at 23:50

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Koosh. Then sphere. And back.

koosh and sphere

To condense water your plant wants to maximize surface area while allowing airflow. A kooshball could be this. Another alternative is a ramifying structure like a tumbleweed. At night humid air flows between the branches, slows and condenses. The branches also facilitate conduction of captured droplets to the center of the plant and down to the root.

The benefit of the koosh shape is that it is easily reversible. When the sun comes out, extruded branches are drawn back into the spherical body. The sphere minimizes surface area and evaporative losses. The sphere is the daytime / drytime shape.

At night, cool water from the deep taproot is pumped up to again extrude the koosh arms. They are thus cool relative to the environment which facilitates condensation of passing moisture.

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  • $\begingroup$ On a tidally locked planet there wouldn't be a night... but it's a good point that condensation can rely on changes in the dew point due to temps. $\endgroup$
    – RedM
    Commented Jan 4, 2023 at 9:42
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They are similar to cacti...

Cacti live in hot desert climates. In dry conditions, they ideally require water only every 4-6 weeks. Desert cacti can go for up to 2 years without water. Three features in particular help them achieve this feat:

They store water in their stems

Some cacti have thick, hollow stems. They can store water in these stems. The stems have a waxy coating (called a cuticle) which prevents the water from evaporating.

They have needle-like leaves

These spines are great for keeping thirsty predators away. But they also have some added benefits: they collect water from wet air or fog, which is then transported to the stem via trichomes on the epidermis. The needle-like structure also reduces surface area, meaning that water loss is reduced. In addition to conserving water by minimizing evaporation, the spines serve as shade for the plant, keeping it cooler.

They have specialized roots

Cacti have specialized roots which contain parenchyma, collapsible water storage cells. The cells get larger as water is absorbed, and shrink when the water is released from storage for the purpose of photosynthesis.

...But not entirely

Cacti have an additional method for conserving water which wouldn't apply to your plants. They use CAM photosynthesis, a unique process which means their pores only open to exchange gases at night. This is a time when the cactus is cooler and less water would be lost to transpiration. If your sun is tidally locked, I would assume there is no difference between day and night, so this method would not work.

However, it's reasonable to assume that they'd evolve stems with storage capabilities, spine-like leaves, and roots capable of collecting large amounts of water.

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A tidally locked planet would provide some interesting niches. If the sun is always in the same location then, like you said, there is some chance to derive a benefit from this. One side of the plant would be in full illumination and hotter. It would also have a shaded portion that would be cooler. The shaded portion wouldn't need any chlorophyll (or at least much less). Some plants in desserts have little translucent windows that they use to channel light down to lower portions of the fat leaves, that are usually underground where it is cooler.

Some kind of temperature/humidity gradient could be formed by very large plants that could be used to channel warm air into cooler sections. Kind of like how termites can control the temperature and humidity of their hives by having chimneys and vents.

Much of the weather on this planet is driven by the day/night, lunar and seasonal cycles. A tidally locked planet could have much less dynamic weather due to the lack of seasonal and day/night cycles but would have much more variations in location based climate. Directly facing the sun would be eternal noon (scorching) while 90 deg offset would be a constant sunrise/sunset where there would be fierce competition to rise above the plants in front. Vertical canopies would form with large bands of shade between them.

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