You already know that plants are dependent on water. And how often you water a plant in the home will primarily depend on the quality and quantity of light that you give it, though there are other factors that come into account, like the type of plant, the moisture in the air (i.e., humidity), and how well-draining the substrate is, for example.

Some plants, like ferns, need more water than others. If you get a low-maintenance cactus because you’re forgetful at watering, keep in mind that even desert-living plants, like most cacti and succulents, eventually need to be watered. When my friend and colleague Allan Schwarz—the architect and forest conservationist in Mozambique whom I mentioned earlier—visited me, he was bemused by my Lithops. These pebble-like succulents sat in khaki-colored coffee mugs on my windowsill. He had never seen a domesticated kaitjie-kloukie, as he called them. (The name, which in the local South African dialect Afrikaans means “kitten’s paw,” is derived from their resemblance to the soft pads of a kitten’s feet.) He told me about the first time he had met these strange plants, while serving in the South African army.

In the eighties, he was stationed in Namaqualand, an inhospitably arid region of Namibia and South Africa. Rainfall in the area is parsimonious, but during his time there, a rare but welcoming rain had passed through; the next morning in between trainings, he happened upon what looked like a smooth, colorful pebble. He went to pick it up, but found it firmly rooted: it was not a pebble after all—but a plant! As he walked on, he saw others, dispersed all across the desert. His gunnery sergeant, Navarre, who happened to be a lawyer based in the South African countryside, had been familiar with Lithops and explained that the little water that had come was enough to make the plant burst forth in the most brilliant displays of color.

Around 80 to 95 percent of a plant is composed of water, which tells you how much plants rely on it for their survival. While Lithops might not need much water, they still need the occasional sprinkle. All plants need moisture. Even Syntrichia caninervis, a moss growing in the desert with tiny, whisker-like fibers at its leaf tips, is designed to harvest fog and direct the droplets to the moss’s leaves. Water helps plants maintain cellular activity, gives them their form in soft tissues, cools them, carries nutrients to them, guides oxygen to their roots, and so much more.

Epiphytic plants, like some Tillandsia—otherwise known as air plants—affix themselves to everything from trees to telephone wires instead of soil but still require atmospheric moisture to feed them foliarly. And what would seem to be a burden to most host plants actually is a benefit: trees with epiphytes enjoy cooler temperatures and up to 20 percent less evaporation compared to trees without—revealing that epiphytes are more friend than freeloader, acting as both a humidifier and air conditioner to the tree that hosts them.

Not only do some plants depend on each other for water but humans also rely on the hydration plants provide. After plants siphon water up through their root zone, they “exhale” it into the atmosphere through their leaves, which ultimately contributes 10 percent of the moisture in our atmosphere—a component of the Earth’s circulatory system.

But in very dry times, plants make do. Though permanently affixed to land, a plant’s roots actively patrol beneath the dark environs for the first sign of rain. In well-functioning ecosystems, certain plants exhibit something known as “hydraulic lift” during periods of drought, whereby roots pull up water from deep soil layers at night and distribute the water to shallower roots in the upper layers of the soil. This has been shown to not only promote growth in the plants doing the heavy lifting but also ameliorate the drought for their neighbors, thereby keeping the area stable and in working order. Most plants have even made friendly pacts with other soil organisms to gain an upper edge, by festooning their roots and root hairs with fungal mycorrhizae, to increase moisture and nutrient retention, or by housing nitrogen-fixing bacteria in their root nodules for increased uptake of nitrogen—one of the more limited nutrients in plant growth, which becomes more bioavailable for plants with the bacteria’s help.

Water’s physiological purpose

Water serves many important physiological purposes in a plant’s life, including aiding growth and metabolism. Just as water on the earth serves as a mode of transportation (think rivers), so does water that moves through a plant, serving as a conduit from soil to sky and back again. As a result, plants are able to convert many of nature’s inorganic elements, which they get from the soil, such as calcium and magnesium, to organic compounds, which we in turn eat as “nutrients” to nourish ourselves. (Leafy greens and legumes are loaded with calcium that is necessary for healthy bones; and nuts, seeds, and greens are good sources of magnesium, which is used in more than 300 biochemical reactions in our bodies.) In fact, about 80 percent of the molecules in a plant are transported into the plant via water, and the remaining 20 percent are made in the plant using those inorganic elements. This conversion from inorganic minerals to organic nutrients is made via water through the root and plant tissue, and it’s all regulated by osmotic pressure, which also helps maintain a plant’s turgidity and keeps a plant upright.

Plants both “breathe” and “sweat”

According to the fossil record, plants have had “pores”—otherwise known as stomata—for hundreds of millions of years. Stomata have a fundamental role in the control of two of the most important plant processes—photosynthesis and transpiration. Most stomata are found on leaves, but they can also be found in fruits, flowers, stems, and even roots, depending on the plant. “Albino” plants—oftentimes the ones bred for collectors—typically have nonfunctional stomata, so it’s important if you’re considering a variegated variety not to get one with too many white leaves, as white leaves compromise both photosynthesis and transpiration, and it’ll be the green leaves in the plant that will sustain the plant. It’s one of the principal reasons why you typically don’t see many variegated mutants in the wild; they’re simply not as fit.

Fully functioning stomata, on the other hand, allow for both the diffusion of carbon dioxide gas from the air and the release of oxygen. But they also allow for transpiration, or the process by which water is transferred from the plant through evaporation via the stomata; transpiring also cools the plant, much in the way sweating cools a human. However, in plants, at least 90 percent of the water loss occurs via transpiration. It’s part of what can make a house filled with houseplants, like mine, more humid.

Some may ask what the purpose is of a plant losing so much water, especially as water is so precious to a plant’s lifecycle. On a macro level, transpiration is an important part of the earth’s water cycle and climate stability—and it maintains the appropriate conditions for a community of plants to survive as a whole. The exchange of water vapor between leaf and atmosphere is enough to affect local climate, and regional and global weather and climate patterns in turn. Individual plants within a forest ecosystem, for instance, are acting in unison to regulate their preferred conditions for survival. That’s part of the reason why it’s common practice indoors to group plants together to help build up humidity for more humid-loving varieties. Plants that are grouped together transpire together.

The importance of the results of transpiration to a natural ecosystem—and in turn its plants—can be summarized by observations made during a trip I took to the Caribbean island of Antigua in 2005. Historically, the tropical isle had a wetter climate, which makes sense, considering it once was one of the most forested islands in the Caribbean. But as the rain forests were cleared to plant sugarcane during the colonial period, the climate became hotter and drier, and it rarely ever rained the way that it had prior to the clearing of the forests.

The reality is that plants create the environment they want to live in. The great Amazonian forest releases water into the air and brings that water back down to earth as rainfall, in order to maintain the appropriate climatic conditions for that forest. The hydraulic lifting of tree roots also waters the other plants around them, creating a stable environment. Cut a chunk of the forest down and that water cycle is broken—not just affecting the forest but also potentially causing drought in other areas of the world, as seen in places like São Paulo and the southern United States, for instance. According to a report released by Antonio Nobre, a researcher in Brazil’s Earth System Science Center and foremost expert on Amazonian climate models, a reduction of 40 percent of the Amazon rain forest could trigger the area to transition to savannah—eventually eliminating intact forests that have not been felled.

Great forests are not the only collections of plants that help control macro- and microclimates. Even a microcosm of mosses on a wooded path is able to slow airflow in order to maintain the necessary moisture to keep it springy and green. Life will work to perpetuate itself in some of the most glorious ways. And more often than not, that life is working in unison with others of its kind—and not of its kind—to continue its existence.

Knowing that plants can change local climate conditions to better suit their needs demonstrates the powerful ways plants create our environments, and how we affect those environments for our plants, and inevitably ourselves. Perhaps we can take a lesson from our seemingly “passive”—yet incredibly proactive—green friends: we too create the community and the world that we want to live in, through our energy, attitude, and everyday actions.

Understanding how and why plants operate in this manner can help you obtain deeper knowledge and a stronger relationship with your plants, as you’ll be able to begin to deduce what they need from you—and see how they adapt to their environment (like a chilly northern exposure), or why they can’t.

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